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The percentage of water in the brain of the smooth dog-fish Mustelus canis.

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THE PERCENTAGE OF WATER I N THE BRAIN OF T H E
SMOOTH DOG-FISH, MUSTELUS CANIS
GEORGE G. SCOTT
Department of Natural History, College of City of New York
Donaldsonl has shown that in the albino rat between birth
and maturity the percentage of water in the brain diminishes
from 87.8 per cent to 77.5 per cent. He calls attention to the
fact generally known that the human brain at birth contains a
greater percentage of water than at maturity and from the investigations of Weisbach and Koch he obtains as the percentage
of water in the human encephalon the following: birth, 88.3 per
cent; two years, 81.1 per cent; five years, 79.2 per cent; twentyfive years (mature), 77.0 per cent. Donaldson further says:
We reach the interesting conclusion that probably in all mammals we
shall find approximately the same range in the percentage of water
between birth and maturity and that the loss of water in them occurs
in the same manner but that the time required for each successive step
is determined by the intensity of the growth process characteristic for
each species.
The present author in 1910 had obtained the percentage of
water in the brain of a few smooth dopfish but at the suggestion
of Dr. Donaldson, has collected further data on this subject in
order to see whether the above law holds true for the elasmobranchs, which occupy a place at the base of the vertebrate
ladder.
The author collected the following data at theBiologica1 Laboratory of the United States Bureau of Fisheries at Woods Hole,
Massachusetts. He is greatly indebted to the Bureau of Fisheries for the material and facilities furnished him.
The data are not as complete as they might be but since they
illustrate a difference between the elasmobranchs and the mam'DonaIdaon, H. H., Jour. Comp. Neur., vol. 20, no. 2, p. 119, April, 1910.
55
56
GEORGE G. SCOTT
mals, this paper is presented at this time. The percentage of
water in the brain of ninety-seven smooth dog-fish, Mustelus
canis, was obtained. These were obtained from the laboratory
trap in Buzzard's Bay and the brain tissue was removed on the
same day that the fishes were brought into the laboratory. I n
TABLE 1
Showing the percentage of water i n the brain of the dog-fish, Mustelus canis, of i n creasing body length. Sex, male.
- -
.-
BRAIN
WEIQHT
NUMBER
-~
LENGTH
BRAIN
WEIQHT
WATER IN
BR4IN
cm.
grams
per cent
~
cm.
1
2
3
39
42
42
4
44
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
45
52
55
56
57
60
____
60
61
64
65
65
65
65
66
67
67
67
69
69
70
70
70
grams
1.39
1.42
1.48
1.41
1.51
1.98
2.08
2.24
2.26
2.24
2.33
2.35
2.50
2.13
2.67
2.80
2.61
2.59
2.79
2.91
3.35
2.94
3.17
2.77
3.05
2.94
__
.
79
79
I
I
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
70
70
72
74
75
75
75
76
77
77
77
79
79
80
80
81
81
82
82
82
82
83
85
90
91
2.60
2.61
3.17
3.36
3.18
3.20
3.20
3.14
3.52
3.55
5.77
3.44
3.41
3.45
3.33
3.78
4.16
3.38
3.25
3.65
3.65
3.47
3.60
4.06
3.89
79
79
81
78
78
78
78
78
79
80
85
74
78
75
80
80
81
79
74
78
78
80
80
81
79
..
.
each case the following technique was employed. The sex,
length and weight of each specimen was first recorded, then the
brain case was opened. The olfactory tracts were severed close
to the forebrain, a transverse cut made at the pos,terior margin
of the fourth ventricle, the cranial nerves severed and the brain
57
WATER I N BRAIN OF DOG-FISH
carefully placed on clean filter paper. A longitudinal cut was
then made through the brain and each half very carefully turned
over on the filter paper until no further cerebral fluid was absorbed. The brain tissue was then placed in a watch crystal
and its weight determined. It was next placed in a desiccator
over sulphuric acid. The desiccator was made a partial vacuum.
TABLE 2
Shoun'ng the percentage of water i n the brain of the smooth dog-fish, Mustelus canis,
of increasing body length. Sex, female.
NUMBER
LENGTH
BRAIN
WEIGHT
//
KATER IN
BRAIN
1
NUMBER
LENGTH
cm.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
grams
42
44
45
56
60
62
62
62
62
62
62
62
66
66
67
69
69
70
71
72
74
74
75
-
__
1.48
1.33
1.47
2.20
2.26
2.40
2.48
2.43
2.24
2.53
2.73
2.92
2.65
3.20
2.73
2.99
2.55
3.33
2.92
3.25
3.22
3.31
3.49
per cent
78
78
78
77
79
78
80
8o
77
78
79
80
76
81
79
~-
BRAIN
WEIGHT
RATER I N
BRAIN
grams
per cent
__
~
'1
cm.
I
~
I
I
i
iI
I
1'
'
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
75
75
75
75
77
77
79
79
80
81
81
82
82
82
89
90
90
92
96
97
99
104
10.5
3.27
3.35
3.02
3.63
3.48
3.88
3.58
3.51
3.24
3.29
3.27
2.85
3.56
3.50
3.82
4.22
3.74
4.26
4.11
4.28
4.58
4.45
4.49
76
79
83
79
82
84
75
80
75
81
84
80
79
78
78
80
80
78
77
79
77
76
78
~~
-
The brain tissue was then dried to a constant weight and the
percentage of water computed. Since in this problem the percentage of water only was desired, the same great care to get
every trace of brain tissue was not as necessary as in the case
where the exact weight of the brain at various ages was to be
investigated.
58
GEORGE G. SCOTT
The change in the weight of the brain of Mustelus canis and
of increasing body weight has been carefully worked out by Kellicott ('08) whose paper will be referred to later. Tables 1 to 5
show the results obtained. Tables 1 and 2 show the percentage
of water in the brain of smooth dog-fishes of increasing body
TABLE 3
Showing the percentage of water in the brain o j the smooth dog-jkh, Mustelus canis, of
increasing body weight. Sex, male.
NUMBER
WEIGHT
BRAIN
WEIGHT
grams
grams
~
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
218
264
280
311
326
420
451
560
560
575
591
653
669
684
715
746
746
746
775
840
933
933
995
995
995
1042
1.39
1.48
1.42
1.51
1.41
1.98
2.08
2.26
2.61
2.24
2.33
2.35
2.24
2.91
2.50
2.13
2.67
3.35
3.71
3.06
2.59
3.17
2.77
2.94
2.60
2.94
per cent
77
77
77
79
78
79
79
80
79
74
77
77
grams
27
30
31
32
33
34
35
36
37
38
80
79
82
74
79
81
81
80
80
79
75
77
79
80
41
42
43
44
45
46
47
48
I
50
51
i
1057
1057
1057
1088
1120
1244
1306
1337
1368
1399
1399
1462
1462
1462
1555
1586
1648
1679
1679
1773
1773
1990
2021
2053
2379
grams
~
3.05
3.55
2.80 I
5.77
2.79 1
3.20
3.18
3.20 i
3.14 i
3.36 I
4.16
3.44
3.38
3.33 I
3.41
3.78 '
2.8
3.25 I
3.52
3.45
3.47
4.06
3.65
3.65 I
3.89
~
~
~
~
~
~
~
~
~
,
i
~
~
~
I
per cent
77
80
79
85
81
78
78
78
78
78
81
74
79
80
78
80
80
74
79
75
80
81
78
77
79
length, m a k and females respectively. Tables 3 and 4 show the
percentage of water in the brain of fishes arranged according to
increasing body weight instead of length.
Donaldson found that at different body weights the male
brain contains a greater percentage of water than the female
59
WATER I N BRAIN OF DOG-FISH
brain in the case of the albino rat. Not only is this sex difference true of the percentage of water, but as is commonly known,
the male brain actually weighs more than the female brain in
relation to the weight of the body. Kellicott,2.on the other hand,
found that in the smooth dog-fish there was no sex difference as t o
total brain weight. He says, “It is not possible to distinguish
TABLE 4
Showing the percentage of water i n the brain of the dog-jkh, Mustelus canis, of increasi: body WI ht. - S e z , Female.
~ _ _
NUMBER
WEIGHT
BRAIN
WEIQHT
~-
WALTER
NUMBER
~
grams
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
280
295
342
529
560
622
637
653
684
715
746
746
762
809
871
871
902
964
1026
1057
1057
1057
1151
grams
1.48
1.33
1.47
2.20
2.92
2.73
2.26
2.42
2.40
3.20
2.24
2.55
2.48
2.53
2.65
3.35
3.33
2.99
2.92
2.55
3.02
3.63
3.25
per cent
78
78
78
77
80
79
79
80
78
81
77
80
80
78
76
79
76
79
77
79
83
79
77
‘
~~-
._
grams
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
1151
1182
1213
1244
1368
1368
1368
1493
1550
1617
1679
1679
1835
1928
2364
2395
2581
2846
2892
3297
3390
3452
4198
BRAIN
WEIGHT
grams
3.48
3.31
2.73
3.49
3.27
3.50
3.51
3.22
3.58
3.88
3.29
2.85
3.24
3.56
3.74
3.82
4.22
4.58
4.26
4.11
4.45
4.28
4.49
WATER
-
per cent
82
79
79
78
76
78
80
76
75
84
81
80
75
79
80
78
80
77
78
77
76
79
78
between the sexes with respect to brain weight.” But what is
the condition as regards the percentage of water? The average
percentage of water in the brain of the forty-six females recorded
here is 78.6 per cent, while that of the fifty-one males is 78.5 per
cent. There is no sex difference in Mustelus canis as far as the
2
Kellicott, W. E., Am. Jour. Anat., vol. 8, no. 4, p. 207, December, 1908.
60
GEORGE G. SCOTT
percentage content of water in the brain tissue goes. This is in
agreement with the result obtained by Kellicoti;.
But what is the condition as regards the percentage of water
in the brain at different ages?
Since there are no sex differences we can group together the
males and females. Nothing is known of the exact age of the
dog-fish but in general they increase in length and weight as they
grow older. Kellicott; following the methods of Moenkhaus and
Fulton with teleosts, has roughly estimated the ages as shown in
table 5.
TABLE 5
WEIGHT
LENGTH
..-.-..
~~
Birth. . . . . . . . . . . . . . . . . . . . . . . . .
1,year. . . . . . . . . . . . . . . . . . . . . . . .
2 years.. ......................
3 years.. ......................
4 years ........................
5 years.. ......................
. . .
grams
em.
75
300
775
1400
2325
2750
32.5
45 .O
63 .O
78.5
90 .o
99 .o
.
.-
~~
Now since we are ascertaining the relation of .the percentage of
water in the brain to the age and since age is measured by length
and weight, it is necessary t o distribute the specimens concerning which we have records, according to the above schedule.
Applying Kellicott’s criterion we have table 6.
TABLE 6
(A)
I
LENGTH MALE +FEMALE
,
(B)WEIGHT MALE fFEMALE
,
7 + 4=11
22
18 = 40
18 14 = 32
4+ 3 = 7
Of
7 = 7
.
...............
2 years.. . . . . . . . . . . . . . . .
3 years.. . . . . . . . . . . . . . . .
4 years.. . . . . . . . . . . . . . . .
5 years.. . . . . . . . . . . . . . . .
1 year
~
~
+
~
~
6 + 3 = 9
22
15 = 37
20
19 = 39
3 + 4 = 7
o+ 5= 5
____
Total, 97
+
+
.
_.__I
.
+
+
~
I
Total, 97
We thus see that we have about the same distribution by
length as by weight. It will be noted that the medium sized
61
WATER I N BRAIN O F DOG-FISH
2 years.. . . . . . . . . . . . . . .
3 years.. . . . . . . . . . . . . . .I
4 years.
3 years . . . . . . . . . . . . . . . .
I1
..
78.7
78.8
79 .O
77.9
78.5
78.9
79.4
77.4
1
'
.................
~~
78.6
78.8
79.2
77.7
1
~~
~~~~
.
~
-~
SEX
LENGTH
cm.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
27
29
30
30
30
30
31
31
31
31
32
34
34
34
35
36
36
.
.
.
.
.
WEIGHT
_.
grams
140
140
140
109
140
187
109
124
93
155
187
124
124
187
233
155
171
IRAIN WEIGHT
___
_ _
grams
0.60
0.71
0.73
0.72
0.81
0.81
0.67
0.71
0.79
0.60
0.95
0.87
0.94
1.06
0.87
0.97
1.06
WATER IN
BRAIN
~~
per cent
82
82
82
82
81
81
84
82
84
80
83
79
79
81
80
80
82
62
GEORGE G. SCOTT
Moreover, eight females and two males from the standpoint of
length would be regarded as recently born, accmding to the age
criterion given above. But when we look at the weights we find
these to be much greater than what is called for, namely, 75
grams. And yet it must be remembered that we are now discussing a species other than that for which Kellicott constructed
his age table.
The average percentage of water in the brain of these seventeen Squalus acanthias is 81.4 per cent. On the whole this group
is smaller in length and weight and so younger than the smooth
dog-fishes, Mustelus canis. There is some slighl; indication then
of a small decrease in the percentage of water in t:he brain between
birth and the first year. This should not be emphasized, however, since we are dealing with two different species of fishes.
Kellicott has shown that during the period of which we have
data, the brain of Musttlus has increased from about 1.5 grams
in weight to about 4.0 grams. During this time also it has decreased from about 0.6 per cent to 0.2 per cent of the total body
weight. And yet we have seen that the percentage of water in
the brain has remained quite constant. How can we account
for this?
Mammals are characterized by determinate growth. As soon
as maturity is reached the organs have reached their size limit.
For example, the bones increase in length no further. On the
other hand, fishes have indeterminate growth, t h a t is, they grow
as long as they live. As far as the brain is concerned, in the
case of mammals growth is very rapid during the first few months.
On the other hand, in fishes the brain grows steadily as long as
the animal lives. As Kellicott says, “After birth (smooth dogfish) the brain weight increases rapidly but at a slightly diminishing rate. Among the large individuals the diminution is much
slower but is continued during life. Donaldson shows that the
diminution in percentage of water is most rapid during the first
thirty days of the albino rat’s life, that is, when the central
nervous system is growing most actively. Amphibians also possess indeterminate growth. Tigerstedt,3 reviewing the work of
Tigerstedt, Textbook of human physiology, 1906, p. 574.
WATER I N BRAIN OF DOG-FISH
63
Birge, says that he “counted the motor cells in the spinal cord
and nerve fibres in the anterior spinal roots in frogs of different
sizes” and convinced himself “that both either multiply from
preexisting nerve elements or from other elements throughout
life.” He found “unmistakable relation between the weight of
the animal and the number of cells and fibres. On the average
for each 1 gram increase in weight, 52 motor fibres had been
added.”
The most significant difference between the rat and the dogfish, as far as our present discussion is concerned, is the postbirth condition of the two. The rat is born helpless, blind and
cannot move about for some time. On the other hand, the dogfish is born, free-swimming, active and apparently mature with
the exception of the reproductive system. Donaldson shows a
correlation between the period of rapidly forming nerve cells and
the percentage of water in the brain. Very possibly the dog-fish
has a greater percentage of mature nerve cells at birth than the
rat. We should expect a smaller percentage of water than in the
case of the rat. This is borne out by the conditions in the young
spiny dog-fishes discussed above. If the discoveries of Birge are
correct and apply equally well to the dog-fishes, as we have considerable reason to believe, then the continued constancy in the
percentage of water in the elasmobranch brain is due to the multiplication of new nerve cells and fibres keeping pace with the
growth of the brain in other respects.
According t o Donaldson’s table, about seven-tenths of the percentage decreasi: in water takes place in the first one-eighth of
the rat’s life, between birth and maturity. There is a decrease of
only three-tenths during the remaining seven-eighths of this maturing period, that is, it occurs during the first thirty out of the
total two hundred and forty days. The period of greatest loss
in water is that during which profound neurological changes take
place. May not these changes take place in the dog-fish in utero?
The two cases make a strong argument for considering the change
in water content of the central nervous system to be correlated
with the growth intensity of this system. And that in the dog-
64
GEORGE G . SCOTT
fish the greatest change takes place in-utero, while in the rat and
man it is extra-utero. The collection of data from the brains of
embryonic stages is necessary to decide this hypothesis.
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