THE EFFECT O F CONTRACTION ON THE VOLUME O F THE SMOOTH MUSCLE NUCLEUS. UY CAROLINE McGILL. Zii8trtictor in Aiiatoiny, Uniccrsity of Uissouri. a recent paper (American Journal of Snatomy, Vol. IX, KO. 4, 1909) I have described the histological changes which occur in the smooth muscle nucleus during the contraction of the tissue. Briefly these changes are as follows : The nucleus decreases markedly in length and increases in thickness. The chromatin, which is distributcd uniformly through thc resting nucleus in a fine reticulum, as the nucleus shortens streams towards the poles, and there forms into coarse, deeply-staining strands. The nucleus thus appears to take an active part in the process of contraction. RJ most authors, contraction in smooth n~usclehas been assumed to be due to an increase in the roliime of the myofibrilb brought about by the absorptioii of water from the interfibrillar substance. The observations reported in the present paper were made to determine whether there is any change in the ~ o l u m eof the smooth mnscle nucleus during contraction, accompanying the marked changes in structure described in the preceding paragraph. The material chosen was the intestinal muscle of Nccturus. IIere the nuclei are very large, in extreme extension reaching a length of eight? micra. They are large enough for fairly accurate reconstruction. Pieccs of the miiscle mere fixed in Zenker's fluid, embedded in paraffi and cut serially. Some of the series were cut at four micra, others at five micra in thickness. The sections were stained either by Delafield's haematoxylin or i l l Heidenhain's iron haematoxylin. Measurements of the 1-olumes of a large number of nuclei, both from contracted and expanded muscle, were made. Two methods were used in determining the volume. The first method is as fol111 (633) Caroline McGill. 634 lows: I n a given area of a longitudinal section of smooth muscle, the nuclei showing the greatest size were taken as nuclei cut in median longitudinal section. These were projected and drawn by means of n camera lucida at a magnification of 1200 diameters. Assuming that the nuclei were round in cross section, the volume was calculated. This was done by breaking each niicleus u p into several frustra of cones, finding the volume of each and adding the several volumes. The following table is from a series of nuclei taken as representatives from a large number of measurements. I n this table the magnified size has been reduced to thc actual size. The length and width of the nucleus give a fair index of the amount of contraction. The first of the series therefore represents a niicleus fully contracted, the last is fully extended. No, Length in Mirra. Width i n Micra. Volume in Cubic Micra. 1 28 18 236.74 2 30 17 2447.65 3 35 15 2938.52 4 43 9 1763.30 5 55 9 1940.82 6 62 9 2537.64 7 70 ” f 2013.68 8 72 7 2024.72 9 76 7 2031.12 10 82 5 1766.24 In the expanded muscle the nuclei are almost round in cross section, but in contracted muscle they are often considerably flattened. This probably accounts for the apparently slightly smaller volume in the expanded muscle nucleus shown in the above table. Therefore, as a check on this first series of measurements by (probably) a more accurate method, the volumes of another series of nuclei were found by reconstruction. The size of the nucleus made it easy to do this. Thin serial sections in cross section of the muscle fibres were made from both contracted and expanded muscle. The Effect of Contraction. 635 Definite areas in series from these sections were projected and drawn by aid of the camera lucida a t a magnification of 1500 diameters. I n these series the nuclei were easily traced throughout their entire length. From the serial drawings the volumes of the nuclei were found either by means of a planimeter or by reconstruction by Born’s wax plate method (the volume in the latter case being measured by water displacement). The following table gives a fern representative volumcs taken from a large number of measurements by the planimeter method. As in the preceding table, the magnified size has been reduced to the actual size. Volume in Cubic Micra. Number. Length in Micru. 1 27 2370.00 2 32 2400.00 3 37 241 1.90 4 43 2240.00 5 47 2370.00 6 47 2134.00 7 53 2340.00 8 64 2472.00 9 70 21 12.00 10 77 2408.00 The wax models of the nuclei showed very similar volumes, so a table is not given. The above table shows little difference betxeen the roliimes of the resting and of the contracted smooth muscle nucleus. From thc data it seems probable that during the contraction of the smooth muscle nucleus of Necturus there is no change in rolnme. Reccil-ed for publication September 9, InOD.