808 OP THE STRUCTURE AND FUNCTIONS OF MUSCULAR TISSUE. 



The percentage proportion of nitrogen in the flesh of different animals has 

 been shown by Petersen, 1 to vary : thus mutton contains 3.15; veal, 3.18; 

 pork, 3.25; beef, 3.29 ; and horseflesh, 3.48. Its amount in dried muscular 

 tissue varies from 11.88 to 15.07 per cent. According to Dauilewsky 2 tetan- 

 ized muscle yields a little more nitrogen to warm alcohol, which dissolves 

 the products of its disintegration, than muscle at rest. Amongst these is 

 lecithin. Muscular tissue, whether living or dead, absorbs oxygen, and elimi- 

 nates carbonic acid, 3 though more energetically in the former case than in 

 the latter. The specific heat of muscle exceeds that of water, and is higher 

 than that of any known solid or fluid body. 4 It is, on the other hand, an 

 exceedingly bad conductor of heat. 



659. Development of Muscular Tissue. In Man, according to Mr. Lock- 

 hart Clarke, 5 Muscular Fibre can first be distinguished about the 4th or 5th 

 week of utero-gestation. In a foetus of three-fourths of an inch in length, 

 it forms a gelatinous mass consisting of fibres and nuclei imbedded in a semi- 

 fluid granular blastema. In the formation of the fibres, granular processes 

 of condensed blastema extend from the sides or from around the nuclei, which 

 are subsequently bounded by an investing substance in the form of a baud 

 or fibre, sometimes plain, but sometimes longitudinally fibrillated. In these 

 bands the transverse striation first makes its appearance, and only subse- 

 quently extends through the central band or axis. The muscular fibres vary 

 in size at different parts, and they may sometimes be seen to increase in 

 diameter by the adhesion of fresh nuclei and fresh processes of blastema. A 

 very similar account has been given by Mr. Savory for the Mammalia gen- 

 erally, 6 except that in his observations the nuclei, at first irregularly dis- 

 tributed, were found to arrange themselves with great regularity in single 

 rows, almost in contact with one another, the fibres being formed by the de- 

 velopment of two nearly transparent bauds inclosing and bounding the rows 

 of nuclei ; subsequently the nuclei began to separate from one another, and 

 ultimately broke up into granules and disappeared. The striation began 

 within the border and gradually extended to the centre. Increase in the 

 size of the fibres was effected by the addition of fresh blastema to the outer 

 side of existing fibres. The observations of Braidwood 7 and of Eckhard 8 

 are, on the whole, confirmatory of the observations of Mr. Clarke and Mr. 

 Savory ; and it would hence appear that the essential features of the process 

 occur not in the nuclei, nor in cells, as was originally maintained by Schwann, 

 but in a material which may be regarded as intercellular substance ; and 

 Mr. Lockhart Clarke regards even involuntary muscular fibres as by no 

 means theproduet of a nucleated cell, but rather as a kind of cell-formation, 

 presenting at first a nucleus iucrusted with blastema, around which an 

 investing sarcous substance representing the cell-wall is subsequently de- 

 veloped. 9 



OliO. The frequently renewed exercise of muscles, by producing a determi- 

 nation of blood towards them, occasions an increase in the nutrition ; so that 



1 Zeits. f. Biologic, Bd. vii, Heft 2. 2 Centralblatt, 1872, p. 433. 



3 Du Bois-Reymond ; G. Liebig; Hermann. See the Grundriss dor Physiol. of 

 the latter author, 1867, p. 212; K. Gscheidlcn, Plliiger's Archiv., Bd. viii, p. 500. 



4 Adumkifwicz, Centralblatt, 1874, No. 22. 



6 Proceed ings of the Royal Society, vol. xi, No. 48. 



6 Sec Part ii of the Phil. Trans, for 1855. See also Petrowsky, Centralblatt, 1873, 



p. 7i;'.i. 



7 Mcd.-Ohir. Ilov., 1866, p. 447. 8 Zoitsch. f. rat. Mod., Bd. xxix, p. 55. 



9 Dr. Wilson Fox (Phil. Trans., 1865-60) has, however, resuscitated the oldm- view 

 of Lebert, Kolliker, and others, that the fibres of muscle proceed from the cells of 

 tin 1 embryo, the contents of which become fibrilluted, whilst the cell-wall becomes 

 the sarcolemrna. 



