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Pectoral limb musculature and shouldergirdler structure in fish and tetrapods.

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PECTORAL LIMB MUSCULATURE AND SHOULDERGIRDLE STRUCTURE IN FISH AND TETRAPODS
ALFRED S . ROMER
U n i v e r s i t y a n d Bellevue H o s p i t a l Medical College
TEN FIQURES
Despite the very large literature bearing on the limb skeleton, few comparisons have been made between the musculature of the fish fin and that of the tetrapod limb. The reason
is obvious. Until recently little has been known of the
osteology of primitive tetrapods, and nothing at all of their
myology.
Recent advances in palaeontology, however, have partially
Fridged this gap. Numerous papers, by Case and Williston
among others, contain a good account of the permocarboniferous vertebrate faunas and have served to reveal a type of
animal in which the girdle and limb structure is much more
primitive than in any living land form. Renewed study of
still earlier carboniferous fossils promises to bridge many
cf the remaining gaps in our knowledge of the osteological
relationships of fish and tetrapods.
Given a knowledge of the osteology of the primitive limb,
and the transitions from it to modern types, many facts
concerning its musculature may be deduced. Watson in
1917 outlined the history of the muscles as well as the bones
of the pectoral limb; in 1922 I discussed the history of the
extremities; Miner, in a paper now in press, restores the
pectoral limb of the permocarboniferous amphibian Eryops.
I have already shown ('22) that the primitive tetrapod limb
musciilature appears to have been arranged in opposed dorsal
(or dorsomedial) and ventral (or ventrolateral) groups,
119
120
ALFRED S. ROMER
and suggested that these groups might be homologous with
the two opposed muscle masses of the fish fin. I n the present
paper I propose to give the evidence for this view which
bears upon the pectoral appendage. I n this connection the
structure of the shoulder-girdle requires consideration also.
Palaeontology shows that tetrapods are in all probability
descended from the extinct rhipidistian Crossopterygii
(Baur, '96 ; Gregory, '12, '15 ; Watson, '13 ; Broom, '13 ;
Bryant, '19, etc.). This group differed considerably in fin
structure from any living bony fish, and consequently we
cannot hope to find much evidence f o r muscular homologies
in the distal part of the fin. But in the distal part of the
tetrapod limb the opposed sets of muscles are as easily separable as they are in fish; the difficulties lie in the homologies
of the shoulder region. Here the probabilities regarding the
history of skeleton and muscle may be estimated with a considerabIe degree of confidence.
The conditions encountered, respectively, in the tetrapod
and fish will be discussed in order: 1) shoulder-girdle;
2) musculature; 3) areas of muscular attachment to the
shoulder-girdle ; 4) innervat'ions.
The forms listed below are those which have been chiefly
used in this study. A number of teleosts were dissected, but
the conditions encountered may be referred back to those
found in the Holostei.
Primitive tetrapods : Diadectes, Eryops, Eogyrinus (all
extinct).
Dipnoi : Ceratodus.
Crossopterygii : Polypterus.
Choridrostei : Scaphirhynchus, Polyodon.
Holostei: Lepidosteus, Amia.
Of the conditions existing in these forms, those of Polypterus would be expected to resemble the tetrapod condition more closely than others, as both Polypterus and the
group from which the tetrapods are probably derived may be
included in the Crossopterygii in a broad sense. The Dipiioi
represent an early offshoot from the same stock. The
FISH AND TETBAPOD PECTOBAL LIMBS
121
Chondrostei and Holostei represent stages leading to the
teleosts, but retaining many characters of the primitive bony
fish.
I wish t o thank Dr. W. I<. Gregory and Dr. H. D. Senior
f o r the interest they have taken in this paper. To Doctor
Gregory, as Curator of Comparative Anatomy, American
Museum of Natural History, I am indebted for specimens
of Ceratodus, Polypterus, and Amia, as well as for much
other material assistance. Dr. R. J. Terry, of Washington
University, St. Louis, kindly sent me specimens of Polyodon.
The United States Bureau of Fisheries furnished me with
a table at the Woods Hole station during the summer
of 1922.
SHOULDER-GIRDLE
I n all forms above the cartilaginous fish this is always
separable into two parts, the primary girdle (scapulocoracoid) and dermal girdle.
The evolution of the latter is now rather completely known
(fig. 1). Reading the series in reverse order, it is represented
in most mammals only by the clavicle. I n the monotremes,
as Ornithorhynchus, a median ventral element, the interclavicle, is present as well. A similar condition is found in
the higher mammal-like reptiles. I n the lower members of
this group, a splint-like bone is found on the anterior edge
of the scapula. This is the last remnant of the cleithrum,
which is usually larger in primitive reptiles and amphibians,
and in many forms caps the top of the scapula, as in Diadectes
and Eryops.
Paired cleithra and clavicles are also found in the more
primitive bony fish, as Dipnoi, Crossopterygii and Chondrostei, although the interclavicle, which is probably an expanded
median ventral scale developed in relation to the pectoralis
muscle (Gregory, '15) is not found in fish. There are two
conspicuous differences, however : 1) The fish clavicle and
cleithrum are usually broad plates covering much. of the
122
ALFRED S. ROMER
primary girdle externally; 2) the shoulder-girdle in fish is
attached to the head by the posttemporal bone. These differences are completely bridged over by Eogyrinus, a carboniferous embolomerous amphibian which will be described in a
HOMO
ORXITKOREYNCHL'S
ER YOPS
\
EOG PRlNNUS
POLYE'TERLX
TLEOST
Fig. 1 The dermal shoulder-girdle in tetrapods and bony fish (the primary
girdle hatched. I n the more primitive bony fish (as Polypterus) the dermal girdle
includes a large cleithrum and clavicle; the latter is lost in more 'modern' fish.
I n primitive tetrapods (as Eogyrinus) a n interclavicle is added. I n higher tetrapods the dermal elements are reduced in size and number, and the connection
with the head, present in fish and Eogyrinus, disappears.
forthcoming paper by Prof. D. M. S. Watson; he has kindly
permitted me to figure the shoulder-girdle. I n this form
the cleithrum and clavicle are widely expanded and cover
much of the area over which the primary girdle is exposed
in Eryops. Further, a supracleithral element with an articular surface for the posttemporal has been discovered. Hence
FISH A N D TETRAPOD P E C T O R A L LIMBS
123
we may now only differentiate between tetrapod and fish
dermal girdles by the presence or absence of the interclavicle.
I n the Holostei and Teleostei the clavicle has been lost, the
so-called clavicle being the cleithrum (Gegenbaur, '95).
POLPPTERW
glenoid forumen
a
LEP'PIDOSTELE
SCA P/IIRi?YNCHUS
Fig. 2 External views of the shoulder-girdle in primitive tetrapods and bony
fish. Cartilage stippled. The restoration of a primitive tetrapod differs from
Eogyrinus only in the addition of two foramina. The supraglenoid buttress is
identifiable throughout the series, although visible only from the inner side in
the higher bony fish (fig. 3 ) . The arrow emerging from it in all forms except
Polypterus indicates the external opening of the supraglenoid foramen. The
buttress is hollowed out into a muscle canal in most higher bony fish. The
coracoid plate, in which an arrow indicates the external opening of the coracoid
foramen, lies below the glenoid. A scapular blade is shown in Eryops; in all
others this region is covered by the dermal girdle. The glenoid foramen and
its apparent fish homologue is iudicated.
The primary shoulder-girdle of a permocarboniferous tetrapod, such as Eryops (figs. 2 and 3)' may be taken as a
starting-point f o r comparison with the primary girdle of
fish. The evolution of the modern shoulder-girdles from
this primitive type has been summarized by Watson ('17)
124
ALFRED S. ROMEE
and by the writer ( ’22), while the primitive tetrapod girdle
shows points of resemblance even to that of the modern
t.eleost. This is true more particularly of the ‘mesocoracoid’
region, as first suggested to the writer by Doctor Gregory.
Fig. 3 Inner views of the shoulder-girdles shown in figure 2. Cartilage
stippled. Arrows indicate the course of the supraglenoid (above) and coracoid
foramina (below) from the subscapular fosaa t o the exterior. The scapula is
reduced in all except Eryops. The supraglenoid buttress is shown on the inner
surface in Lepidosteus and Amia. Fenestrae for the ventrolateral muscles
appear in Scaphirhynchus and Lepidosteus ; dorsal fenestrae for muscles are
found in these and Amia; in the last-mentioned the upper fenestra does not
coincide with the subscapular fossa. The formation of the mesocoracoid arch
of Holostei and Teleostei is brought about by a narrowing of the space between
the inner and outer openings of the dorsal muscle canal; that is, between
subscapular f ossa and supraglenoid region.
The primary girdle, or scapulocoracoid, of Eryops, as of
most land vertebrates of early periods, consists essentially
of, 1) a coracoid plate below the glenoid; 2) a triangular
supraglenoid buttress lying above the glenoid, leading above
FISH AND TETBAPOD PECTORAL LIMBS
125
to the inner surface of the girdle; 3) a scapular blade, lying
above the coracoid plate and anterior to the supraglenoid
buttress, and bounded anteriorly by the dermal girdle. The
inner surface of the girdle was for the most part a smooth
sweep of bone, broken by two openings, a small glenoid foramen and a large subscapular fossa from which the supraglenoid and coracoid foramina took their origin. The glenoid
and coracoid foramina opened externally on to the coracoid
plate ; the supraglenoid foramen opened in the supraglenoid
butt re ss.
The coracoid foramen is found in many existing Amphibia
and reptiles, and transmits the supracoracoid (suprascapusirpracletdhnr
EOO YRINUS
Fig. 4 Right shoulder-girdle of Eogyrinus.
supposed supraglenoid foramen.
After Watson.
Arrow indicates
lar) nerve. The supraglenoid foramen is not positively
identifiable in recent tetrapods, unless it be a foramen transmitting a blood-vessel in some lizards, reported by Williston.
Its apparent homologue in fish transmits the nerve which
supplies a large part of the dorsomedial musculature. The
glenoid foramen is unknown in recent land forms; certain
bony fish have an apparently homologous foramen which
t.ransmits a small nerve.
The scapulocoracoid of Eogyrinus (fig. 4) shows, 1)a small
coracoid plate anterior and ventral to the ill-developed
glenoid; 2) a dorsal supraglenoid region. But the large cleithrum covers the region which in Eryops is occupied by the
scapular blade. I have previously ('22) attempted to shorn
that the scapular blade replaces the fish cleithrum function-
126
ATAPRED S. ROMER
ally; this replacement had iiot yet taken place in Eogyrinus.
This is correlated with the fact that the scapular region did
not extend far dorsally, the scapulocoracoid merely attaching
itself anteriorly to the cleithrum and clavicle for support.
The foramina typical of most primitive scapulocoracoids
are not well represented in Eogyrinus. The supraglenoid
foramen is doubtfully present; n o glenoid or coracoid foramina have been identified. This, however, can scarcely be
considered a primitive feature. The three foramina are
found in all primitive reptiles, later labyrinthodont amphibians and in other members of the group to which Eogyrinus belongs (as Cricotus). It is generally agreed that
these lines can have had no ancestor above the most primitive
Embolomeri. The absence, then, of scapulocoracoidal foramina in Eogyrinus appears to be a secondary condition; and
a restoration of a primitive tetrapod differing from Eogyrinus in the presence of these foramina, but agreeing in all
other essentials, seems to be a justifiable step.
The primary girdle of the primitive tetrapod, then, may
be considered as consisting essentially of, 1) coracoid plate
and, 2 ) supraglenoid buttress, with their associated foramina.
These structures may be compared with similar ones in
bony fish.
A flat surface below and anterior t o the glenoid may easily
be identified as the coracoid plate in Polypterus (and Ceratodus). Of the two foramina which open on to the primitive
tetrapod coracoid plate, the coracoid foramen is found in
Polypterus, while the glenoid foramen is absent. I n Ceratodus neither foramen is present. I n the Chondrostei, as
Scaphirhynchus and Polyodon, the coracoid plate has been
hollowed out into a muscle canal. Into this, as shown by the
lower arrow head in the figure, an opening passes from the
dorsal muscle canal, to be described later. This opening
contains a nerve corresponding to the supracoracoid, and
the opening is hence homologous with the coracoid foramen.
Posteriorly, a nerve foramen in the position of the tetrapod
glenoid foramen opens beneath the glenoid. Internally, the
FISH A N D TETRAPOD PECTORAL LIMBS
127
ventral muscle canal has fenestrated the coracoid region so
that the canal is visible from the inner aspect. This fenestration of a solid plate in the region of a muscular origin is
quite comparable to fenestrations sjmilarly situated with
regard to musculature in the temporal region of the amniotes
(Gregory and Adams, '15), the coracoid plate of lizards, and
the pubo-ischium in many land vertebrates.
I n the Holostei the coracoid plate is more typical in appearance and not hollowed out to such a marked extent, although
a fenestra may be seen at the anterior edge of the plate in
Lepidosteus. A coracoid foramen is found in both Lepidosteus and Amia; the glenoid foramen is found in the former
only.
The coracoid plate of teleosts is essentially similar to that
of the Holostei and is fenestrated to a variable degree. It
contains an opening comparable to the coracoid foramen, but,
as far a8 I am aware, none homologous to the glenoid
foramen.
The supraglenoid buttress of Polypterus is comparable in
position to that of Eogyrinus, except that Polypterus has no
supraglenoid foramen. The buttress leads internally up on
to the inner side of the cleithrum.
The supraglenoid buttress in Ceratodus consists of an area
above and anterior t o the glenoid, located almost entirely on
the outer aspect. It contains no supraglenoid foramen, and
leads to the outer, rather than the inner surface of the cleithrum, as contrasted with Eogyrinus. This is correlated with
the fact that the Ceratodus cleithrum does not possess a broad
outer surface as is usual in the bony fish.
I n the Chondrostei (as Scaphirhynchus and Polyodon) the
supraglenoid buttress is hollowed out into a large muscle
canal, which follows the course of the supraglenoid foramen
anteriorly and inward, its anterior end (which is equivalent
to the subscapular fossa) being visible on the inner aspect.
Since the fossa is included in the canal, the opening of the
coracoid foramen (large in this case) is found in the floor
of the canal.
THE ANATOMICAL RECORD, VOL. 27, N O .
2
128
ALFRED S. ROMER
In Lepidosteus the supraglenoid buttress is also developed
into a muscle canal as in Polyodon, but the external opening
is not visible from the outer aspect of the girdle as in
Scaphirhynchus. This is also true of Amia and the more
primitive teleosts. I n Lepidosteus also, although the dorsal
muscle canal opens on to the inner surface of the girdle
anteriorly, it has nothing in common with the subscapular
fossa. The nerve foramen apparently homologous with the
subscapular fossa is located above the inner opening of the
canal. In contrast with the Chondrostei, the entire supraglenoid buttress is not included within the canal ; beyond the
outer opening of the canal, a smooth surface above leads on
to the inner surface of the cleithrum, as is the case in Polypterus and also, probably, in the most primitive tetrapods.
As usually figured, the nerve opening in Amia equivalent
to the subscapular fossa is separate from the inner opening
of the muscle canal ; but in the specimen dissected and figured
here, the two coincide.
Apart from the absence of ossification, the supraglenoid
region in Amia is similar to that of teleosts. The slender
bridge of bone that remains in Amia between the inner and
outer openings of the dorsal muscle canal represents the
mesocoracoid region, or ' Xpangenstiick ' (Gegenbaur ) , of
te!eos ts.
The inner surface of the primitive tetrapod shoulder-girdle
was, as we have seen, a continuous surface, broken only by
the subscapular fossa and the small inner opening of the
glenoid foramen. In Ceratodus the inner surface is unbroken; in Polypterus the only opening is the inner end of
the coracoid foramen (partly homologous with the subscapnlar fossa).
In the Chondrostei, Holostei, and Teleostei the continuity
of the inner surface of the shoulder-girdle is greatly interrupted. The dorsal muscle canal always fenestrates the
primary girdle and, as in many teleosts, the mesocoracoid
bridge may even be thus destroyed altogether. I n the
Chondrostei and Lcpidosteus the ventral musculature also
FISH A N D TETRAPOD PECTORAL LIMBS
129
causes a fenestration, absent in Amia, but present in a varied
manner in many teleosts. The inner opening of the glenoid
foramen is present in Chondrostei and Lepidosteus.
Gegenbaur pointed out in 1865 that the coracoid and supraglenoid foramina could be homologized throughout the fish ;
the openings, a single internal and two external, one above
and one below the glenoid, being traceable in almost all
groups. Owing t o lack of knowledge of the fossil forms,
a satisfactory comparison with tetrapods was impossible at
that time.
The homologue of the glenoid foramen of vertebrates appears to be absent in Teleostei, Amia, Polypterus, and
Ceratodus; that of the supraglenoid is absent in Polypterus
and Ceratodus ; and that of the coracoid, as well, in the latter.
The absence of two of these foramina in Polypterus, and
of all of them in Ceratodus, appears to be a secondary condition. The identification by Gegenbaur of the supraglenoid
and coracoid foramina in the cartilaginous fish and the
opinion of Bruas as to the universal presence of nerves piercing the girdle in these forms appear to be correct. The
glenoid foramen in the Chondrosteus and Lepidosteus lies
close t o the posterior margin of the girdle; a slight reduction
here would cause its loss. Again, in Polypterus, the coracoid
foramen is close to the posterior edge of the girdle; a slight
reduction in this region, which has apparently occurred in
Ceratodus, would occasion the loss of this foramen as well.
Similar conditions have probably occasioned the loss of the
supraglenoid foramen in Polypterus and Ceratodus.
Any attempt to compare the centers of ossification of the
primary girdle would appear to be unprofitable. I n many
primitive tetrapods, if not all (Watson, '17, etc.), there is
but one center of ossification, the scapular. As contrasted
with this, there are none in Ceratodus, the Chondrostei and
Amia, one in Lepidosteus, two in Polypterus, and typically
three in the more generalized Teleostei.
The type of shoulder-girdle from which that of existing
bony fish and tetrapods may have been derived may be characterized as follows :
130
ALFRED S. ROMER
Dermal girdle consisting of paired expanded clavicles and
cleithra attached to the skull by supracleithral and posttemporal elements. Paired primary girdles, which consist of a
coracoid plate ventrally, with the outer openings of coracoid
and glenoid foramina, and of a supraglenoid buttress, with
the outer opening of the supraglenoid foramen. The buttress
leads dorsally to the inner surface of the girdle. The primary
girdles have unfenestrated inner surfaces which are broken
only by the subscapular fossae and glenoid foramina.
All these conditions are met with in Polypterus and the
Chondrostei collectively. I n the former, two of the foramina
are absent, while in the latter muscle canals replace plate
and buttress.
MUSCLTLATURE
The musculature of the pectoral appendage of primitive
tetrapods and its relationships to that of modern types was
discussed by the writer in 1922. The conditions found in
permocarboniferous forms are well represented by the restorations of Diadectes, a very primitive reptile, given in
figure 5. The muscles appear to be divided into a dorsal (or
dorsomedial) and a ventral (or ventrolateral) group, the
components of which are tabulated below. The grouping
differs from that which might be deduced from the mammalian condition in that the supraspinatus, infraspinatus,
teres minor, and deltoid muscles are considered as ventrolateral. The supraspinatus and infraspinatus are, however,
generally recognized as derivatives of the supracoracoideus
of existing reptiles and amphibians, which is obviously a
ventral muscle. The deltoid and teres minor are not so generally recognized as ventral, since their position is dorsal
in the mammals and the axillary nerve which supplies them
appears to be a dorsal muscle nerve. I n man, for instance,
the axillary and radial are grouped together as arising from
the posterior cord. As seen also from the lateral and superficial aspect the deltoid of Diadectes extends f a r dorsally.
In 1922 I presented evidence to show that the teres minor
is a derivative of the reptilian scapulohumeralis anterior,
FISH AND TETRAPOD PECTORAL LIMBS
131
and that the latter is a ventrolateral muscle. The Eogyrinus
girdle, as discussed below, tends t o show that the deltoid
also was primitively ventrolateral in position. The innervation of these two muscles will be treated in the appropriate
section.
I n a lateral view of Diadectes the ventrolateral musculature
is chiefly in evidence. Proximally the deltoid and pectoralis
wntro-lateral musculature
DEEP LATERAL VIEW
MEDIALVIEW
Fig. 5 Musculature of Diadectes, a primitive reptile. Superficial external
view above t o the left. The greater p a r t of the musculature visible is t h a t of
the ventrolateral group. Proximally, the deltoid a n d pectoralis cover a large
area; they were much reduced in still more primitive forms (Eogyrinus, fig. 6 ) .
Distal t o these appear the brachialis, biceps, a n d coracobrachialis, and still more
distally the flexor musculature of the forearm. The dorsomedial musculature
appears dorsally ltnd internally. On the right, the deltoid and pectoralis a r e
removed, showing the short deep ventrolateral muscles (scapulohumeralis anterior or teres minor, supracoracoideus or supra- a n d infraspinatus, and coracobrachialis). Below, a medial view, showing the subscapularis a n d latissimus
proximally, then the triceps, brachioradialis and muscles of the extensor surface
of the forearm.
are seen to arise from the shoulder-girdle and to be inserted
o n the humerus; upon the removal of these muscles (fig. 5 ,
right side) a large deep muscle mass is found arising from
the coracoid plate, which is the equivalent, apparently, of the
teres minor, supra- and infraspinatus, and coracobrachialis
(scapulohumeralis anterior, supracoracoideus and coracobrachialis). More distally, other elements of the ventrolateral group, the biceps or its equivalent, and the brachialis,
132
ALFRED S. ROMER
are seen extending to the radius and ulna, and still more
distally the great flexor mass arises chiefly from the huge
entepicondyle and covers the ventral surface of the forearm
and manus.
TABLE O F MUSCLE HOMOLOGIES
The probable tetrapod homologzues of t h e muscle masses of the fish pectoral
fin are given in t h e accompanying table. The innervation of t h e veutrolateral muscles i s given in accordance w i t h t h e groupzng used in the body
of t h i s paper
MAHMAL
PRIIITIVE TRTRAPOD
BONY FISH
-
+
Dorsoinedial
musculature
(addnctors)
-
~
.
___
Latissimus
major
Latissimus dorsi
dorsi
+ Teres
Subcoracoscapularis scapulo
huineralis posterior
Subscapularis
Triceps
Triceps
Muscles of the extensor surface
of the forearm and hand
Muscles of the extensor surfaceof the forearm andhand
Deltoid, B
Deltoid, B
Pectoralis, C
Pectoralis, C
Scapuloliumeralis anterior, B
Teres minor, €3
Supracoracoideus, A
Infraspinatus
tus, A
Coracobrachialis, C
Coracobrachialis, C
Biceps, C, or coracoradialis proprius, B
Biceps, C
Brachialis, C
Brachialis, C
Muscles of the flexor aspect of
the forearm and hand, C
Muscles of the flexor aspect
of the forearm and hand, C
~~~
Ventrolateral
musculature
(abductors),
A+B+C
._
~
+ supraspina-
.
The dorsomedial muscle mass is partially seen from the
lateral view, above. The latissimus dorsi, from the trunk,
and the subscapularis (subcoracoscapularis) are seen running
t o the arm. The remaining dorsomedial muscles may best
be seen from the inner aspect. They include the triceps, the
FISH A N D TETRAPOD PECTORAL LIMBS
133
humeroradialis, and the extensor mass which arises from the
ectepicondyle.
The fore limb of Eogyrinus is unknown, but apart from
uncertainty caused by lack of knowledge of the number of
digits, the general arrangement of the musculature was undoubtedly similar to that of Diadectes. One important difference will, however, be noted (fig. 6). The dermal girdle of
Eogyrinus covers the greater part of the outer surface of the
scapulocoracoid. The deltoid, which probably arose from
the posteribr edge of the dermal girdle, and certainly did
not extend over it (the dermal girdle being here a superficial
structure) must have had a more restricted area of origin
(Watson), and could not have extended very far dorsally.
Its position must have been that which might be expected in
the case of a proximodorsally placed ventrolateral muscle.
Thus one of the two main objections to the conception of the
deltoid as a ventrolateral muscle is removed. Its position
would not have been markedly different in such a case from
the dorsoproximal portion of the ventrolateral muscle mass
of Polypterus shown beside it.
I n figures 6 and 7 views of the pectoral fin musculature of
representative bony fish are shown for comparison with the
musculature of the tctrapod pectoral limb. No detailed description of the fish-fin musculature will be given here; the
forms illustrated have been described by Pollard ( '92),
Klaatsch ( 'SS), Braus ( 'OO), Danforth ( '13), and others.
As has been mentioned, no detailed comparisons of the
distal portions of the muscles can be attempted, owing to
the gap between the tetrapod arm and hand elements and
the endoskeletal elements of the fish fin ; comparisons may,
however, be made between the proximal portions of the
muscle masses in the two groups.
The shortening of the muscle masses in the higher bony
fish will be noted. Only in Ceratodus is there a series of
muscles extending down most of the length of the fin which
is comparable with the tetrapod limb musculature. I n Polypterus the musculature is more restricted; in the other 'ganoid'
134
ALFRED S. ROMER
forms pictured, the fin musculature is still further restricted.
This restriction may be correlated with the recognized reduction in bony fish of the endoskeletal fin supports.
/
musculature
1]
L
'
POLYODOiV
LEPID OSTELTS
Fig. 6 Lateral views of the musculature of the pectoral appendage of a
primitive tetrapod (Eogyrinus) a n d bony fish. The musculature seen is mainly
t h a t of the ventrolateral mass; the dorsomedial musculature is seen above and
internal t o it.
135
FISH AND TETRAPOD PECTOBAL LIMBS
I n every case the limb musculature of the bony fish, as has
long been recognized, may clearly be divided into two opposed
masses, working on opposite sides of the fin, and usually
separated by well-marked dorsal and ventral septa. These
two masses, as may be seen from the illustrations, are
_-
id
inilSCIdnfUle
EOG I'RINUS
POL PPTER OS
SCAPHIR HPNCEL'S
CERA TDD U3
medial muscalnlu,a
o m e d ~ a inusculatsrt
l
POL YODON
LEPIDOSTEL'S
Fig. 7 Medial view of the pectoral girdle musculature of the forms shown in
figure 6. The muscles seen a r e mainly those of the dorsomedial mass, the
ventrolateral musculature appearing only a t the lower edge.
136
ALFRED S. BOMER
obviously similar in position to the two muscle groups which
are distinguishable in the primitive tetrapods. The ventrolateral muscle group of the primitive tetrapod is seen chiefly
on the outer aspect, as is the ventrolateral muscle mass of
the fish ; above, and internally, in either case, the dorsomedial
musculature is partly visible. From the medial view, the
dorsomedial musculature is chiefly in evidence both in primitive tetrapods and in fish ; the ventrolateral musculature
shows ventrally. The dorsomedial group arises in part from
the dorsal internal portion of the girdle in the primitive tetrapod, Polypterus and the Holostei. This group of muscles
appears internally and ventrally to the glenoid to any extent
only in cases in which canals through the girdle have been
developed, as in the Chondrostei and Holostei. I n many
teleosts, where the mesocoracoid bridge is lost, the dorsomedial musculature has a wide area of origin from the inner
surface of the girdle, as in Scomber (Allis, '03). This is, as
we have seen, a secondary condition.
The mode of origin of the latissimus dorsi in the primitive
tetrapods is suggested by the fact that in Ceratodus the fibers
of the dorsal muscle arise not only from the girdle itself, but
from the fascia posterior to i t ; a continuation of this tendency
would lead to the formation of a muscle similar to the latissimus. The superficial proximal fibers of the ventrolateral
musculature of Polypterus occupy a position similar t o that
of the deltoid of primitive tetrapods ; as will be noted later,
the innervation of this region is similar in tetrapods and
Polypterus. The proximal portion of the ventrolateral musculature of the Chondrostei and Holostei is more ventral in
position than that of Polypterus or the tetrapods; this, as
well as the restricted distal extent of the muscle mass, is of
interest in connection with the question of innervation.
AREAS O F MUSCULAR ATTACHMENT
The areas of origin from the shoulder-girdle of the two
muscle groups of the tetrapod may now be compared with
the areas of origin of the two muscle masses of the bony
fish (figs. 8 and 9).
FISH A N D TETRAPOD PECTORAL LIMBS
137
I n forms such as Diadectes, the dorsomedial musculature
takes origiii from the supraglenoid buttress and, above it,
from the inner surface of the scapular blade. In a type such
as Eogyrinus the dorsal internal area of origin will of course
include the inner surface of the cleithrum within its Iimits if
the scapula is small. The area of origin does not include the
Fig. 8 External views of the shoulder-girdle of a primitive tetrapod and bony
fish, to show the areas of origin of the appendicular muscles. The origin of
the dorsomedial musculature (stippled) is in general from the supraglenoid
buttress and the inner upper surface of the girdle. The ventrolateral musculature (ruled) takes origin from the coracoid plate and the ventral part of the
dermal girdle. The latter area is often on the inner surface of the cleithrum
in fish, and hence cannot be indicated.
inner ventral part of the girdle, to which only axial muscles
were attached.
Conditions of this nature are encountered in the bony fish
also. I n Polypterus the dorsomedial musculature arises from
the area of the primary girdle above and internal t o the fin
articulation, which has been compared to the supraglenoid
buttress. Internally this area extends dorsally on to the
138
ALFRED S. ROMER
cleithrum. I n Ceratodus the origin is from the supraglenoid
buttress. Owing t o the reduction in the outer part of the
cleithrum as noted above, the cleithral origin is apparently
external rather than internal.
I n Scaphirhynchus and Polyodon the excavation of the
dorsal muscle canal has caused the dorsal musculature to be
confined to the canal, which is believed to be the homologue
of the supraglenoid buttress. I n Lepidosteus and Amia the
musculature occupies the muscle canal, but also extends
upward on to the inner side of the cleithrum. The conditions
in teleosts are similar.
In fish, as in tetrapods, the dorsomedial musculature has
no area of origin from the inner ventral surface of the primary girdle, although the excavation of the muscle canal in
all the higher bony fish causes it to appear on the inner surface
of the girdle.
In Diadectes the ventrolateral musculature arises from
the coracoid plate, and from the posterior edge of the dermal
girdle, the latter area reaching a considerable distance dorsally. I n a still more primitive type, such as Eogyrinus, the
area of origin from the dermal girdle is considerably reduced dorsally.
Polypterus and Ceratodus show similar conditions, the
ventrolateral mass arising from the coracoid plate and the
posterior edge of the cleithrum and clavicle, although these
two areas are not separated from one another, as in tetrapods.
I n the Chondrostei, Holostei, and Teleostei the ventrolateral musculature arises from the coracoid plate and from
the ventral portion of the dermal girdle; the latter area of
origin, however, is from the inner side of the clavicle and
cleithrum and is not visible from the outer surface. In the
Chondrostei the coracoid plate is excavated into a muscle
canal through which the ventral musculature is visible from
the inner surface. It is also visible through the fenestra in
Lepidosteus.
I n both primitive tetrapods and bony fish the area of origin
of the ventrolateral musculature would appear theref ore to
FISH AND TETRAPOD PECTORAL LIMBS
139
be from the coracoid plate and the posterior ventral edge of
the dermal girdle. The area of origin of the dorsomedial
musculature would appear to be from the supraglenoid buttress and from the dorsal internal surface of the girdle.
These conditions may be greatly modified, however, in either
group.
n
POL YOUON
LEPIDOSTEUS
CERA TOU US
Fig. 9 As figure 8, internal views. The area of origin of the dorsomedial
musculature extends upward from the supraglenoid buttress on to the inner
surface of the girdle. The inner ventral surface of the girdle is not utilized
by appendicular muscles except as the muscle canals, dorsal and ventral, have
fenestrated the primary girdle, as in Scaphirhynchus, Polyodon, Lepidosteus, and
Amia, and thus allow the external musculature to appear on the inner side.
INNERVATION
As regards the relations of the nerves to the osseous elements of the shoulder-girdle and limb, the ventrolateral musculature in tetrapods may be regarded as being innervated by
fibers reaching their destination by three different routes
(fig. 10): A ) By passing through the coracoid foramen or,
140
ALFRED S. ROMER
with the reduction of the coracoid in mammals, by crossing
the anterior (superior) edge of the girdle. (Supracoracoid
or suprascapular nerve.) B ) By passing dorsally to the
humerus from behind forwards. (Axillary nerve or homologous elements.) C) By passing ventrally beneath the humerus. (Musculocutaneous, median, and ulnar nerves.)
Nerves following all of these routes may be found in the
various groups of bony fish.
I n Polypterus nerves follow all three of these routes : A) A
nerve recognized by Klaatsch as comparable with the supracoracoid passes through the apparent coracoid foramen to
a part of the ventrolateral musculature. B ) A nerve which
supplies the dorsal and proximal elements of the ventrolateral group passes outward dorsal to the girdle and fin
just as does the tetrapod axillary. C) Nerves, supplying
more ventral and distal elements, pass beneath the fin. (Pollard, '92 ; Klaatsch, '96 ; Braus, '00.)
The recognition in Polypterus of an axillary nerve which
supplies part of the ventrolateral mus~ulature,removes the
last obstacle to the recognition of the deltoid and teres minor
as parts of the ventrolateral musculature. The teres minor
or its equivalent, as I have previously noted ( ' 2 Z ) , is intimately related to the ventral musculature. I n the Chelonia,
monotremes, many placental mammals, and in the human
embryo it is intimately connected with the supraspinatus, a
ventrolateral muscle. The deltoid apparently represents part
of the ventrolateral group of a form such as Eogyrinus. Its
innervation, and that of the teres minor, is comparable with
that of a portion of the rentrolateral group in Polypterus,
a form in which we should expect to find the closest resemblances to tetrapod conditions.
I n Ceratodus only type C is found (Braus, '00). We should
expect this group to be well developed, since it supplies in
tetrapods mainly the more distal muscles of the extremity,
and Ceratodus has this region better developed than any
other modern bony fish. As we have noted, the absence of
the coracoid foramen and consequently of a separate supra-
FISH A N D TETR,APOD PECTORAL LIMBS
141
coracoid nerve (A) is probably a secondary condition. The
reasons f o r the absence of an axillary (B) are not obvious.
I n the Chondrostei, Holostei, and Teleostei (Harrison,
'95; Braus, '00; Allis, '06; Pychlau, '07; Danforth, '13) the
distal musculature is reduced. With this is correlated a reduction in the nerves of type C, which is absent in the Teleostei.
POLYPTERCS
Fig. 1 0 Method of innervation of the ventrolateral musculature through,
A ) a supraeoraeoid (supraseapular) nerve or its equivalent in all types except
Ceratodus; B) a n axillary or equivalent nerve in tetrapods and Polypterus;
C ) equivalents of museulocutaneous, median, and ulnar nerves in all except
teleosts.
Again, since the proximal portion of the ventrolateral
group is more ventral in position, we would expect a lack
of development of the axillary, and the placing of emphasis
on type A, the homologue of the supracoracoid. I n the
Teleostei this nerve has come to be the only source of supply
to the ventrolateral muscles.
Nothing is known as to whether any nerve passed through
the glenoid foramen in primitive tetrapods. The nerve which
142
ALFRED S. ROMER
runs through the apparently homologous foramen in certain
fish innervates part of the ventrolateral musculature, and is
comparable in general to the musculocutaneous, or elements
of that nerve, grouped with type C.
There are no morphological landmarks to give us a comparison of the nerves to the dorsomedial musculature. In
fish which possess it, the supraglenoid foramen, or the muscle
canal replacing it, serves as a means of entrance for nerves
to the dorsomedial musculature; but nothing is known as
to the relation to this foramen of the nerves of the primitive
tetrapods.
SUMMARY
1. The chief features of the tetrapod shoulder-girdle may
be recognized in the bony fish. These include the structure
of the dermal girdle, the coracoid plate, the supraglenoid
buttress, and the glenoid, coracoid, and supraglenoid
foramina.
2. The pectoral limb musculature of tetrapods may be divided into dorsomedial and ventrolateral groups which are
comparable in position to the dorsomedial (adductor) and
ventrolateral (abductor) muscle masses of the fish fin, as
homologized in the appended table,
3. The areas of origin from the shoulder-girdle of the
muscle groups in tetrapods are comparable with those occupied by the corresponding muscle masses of the bony fish.
4. Three paths by which the ventrolateral musculature is
innervated are found both in tetrapods and fish. The presence in Polypterus of a nerve comparable to the axillary tends
to confirm the conclusion, based on other evidence, that the
deltoid and teres minor (as well as supra- and infraspinatus)
belong to the ventrolateral muscle group.
FISH AND TETRAPOD PECTORAL LIMBS
143
LITERATURE CITED
References other than those given below may be found in “ A Bibliography
of Fishes,” Bashford Dean (American Nuseum of Natural History, New
York, 1917).
BRYANT,
W. L. 1919 On the structure of Eusthenopteron. Bull. Buffalo Soc.
Nat. Sci., vol. 13, pp. 1-39.
MINER,R. W. 1924 The pectoral limb of Eryops and other primitive tetrapods.
Bull. Amer. Mas. Nat. Hist. (In press.)
ROMER,A. S. 1922 The locomotor apparatus of certain primitive and mammallike reptiles. Bull. Amer. Mus. Nat. Hist., vol. 46, pp. 517-606.
SHANN,E. W. 1920 The comparative myology of the shoulder-girdle and
pectoral fin in fishes. Trans. Roy. SOC.Edinburgh, vol. 52, part 3,
pp. 531-570.
WATSON,D. M. S. 1917 The evolution of the tetrapod shoulder-girdle and
fore-limb. Jour. Anat., vol. 52, pp. 1-63.
1924 The structure, etc., of the Amphibia. The grade Embolomeri
of the Labyrinthodontia. Phila. Trans. Roy. SOC.London. ( I n press.)
T H E ANATOMICAL RECORD, VOL.
27, NO. 2
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