COMPOSITION OP MUSCLE. 



441 



an elongated band. A granular material then occupies the interior of 

 the tube. Viewing the sarcolemma as the sum of the coalesced cell walls, 

 the fibrils are to be regarded as a development from the granular cell con- 

 tents. They form, by a sort of endogenous process, from without to 

 within. The nuclei, as has already been remarked, are on the inner sur- 

 face of the sarcolemma, and not within the cells. The structure is not 

 evident until after the end of the second month of foetal life, but by the 

 fourth month it has so much advanced that the muscle assumes a pale 

 red aspect; the tendons, which have already begun to be distinctly dif- 

 ferentiated, are gray. At birth the structure has become so far com- 

 pleted that the fibres can be isolated. The condition which the non- 

 striated fibre presents is, therefore, that beyond which the striated fibre 

 has passed, and in this respect the former may be regarded as an embry- 

 onic state of the latter. In some insect muscles an instructive interme- 

 diate condition is seen ; fibres may be found striated toward the middle, 

 and non-striated at the ends, as though imperfectly developed. The 

 thoracic muscles of insects, which offer a beautiful example of muscular 

 structure, are not, however, to be regarded as presenting primitive fibrils, 

 but rather non-fibrillated primitive bundles. This I consider to be the 

 case with the specimens from which the photographs, Figs. 221, 222, 

 were taken. Though not so apparent, nuclei exist in the striated fibre 

 even of adult life, and discharge an active function. At this period, the 

 increase of thickness of the muscles is to be attributed to an increase in 

 the number of the contained fibrils, which individually have about the 

 same dimensions as before birth. 



Composition of Ox Muscle. 



Composition of Human Muscle. 



The result of the chemical change which muscular fibre undergoes dur- 

 ing the periods of its activity is eventually manifested by the Chemical 

 appearance of carbonic acid and urea, and also salts of sulphuric ^Sct^ity 

 acid, the two latter escaping from the system through the uri- of muscle. 



