38 ' VETERINARY PHYSIOLOGY 



down each side of the vertebral column of the embryo. Each 

 cell elongates. The nucleus divides across, but the cell, 

 instead of also dividing, lengthens and continues to elongate 

 as the two daughter nuclei again divide. The cytomitoma 

 becomes arranged longitudinally, and a series of transverse 

 markings appear across the cell. Lastly, a covering, the 

 sarcolemma, develops, and the fully-formed fibre is produced 

 (fig. 15, 6). 



This consists of three parts 



1. The Sareolemma is a delicate, tough, elastic membrane 

 closely investing the fibre, and attached to it at Dobie's lines. 



2. The Muscle corpuscles consist of little masses of proto- 

 plasm each with a nucleus, which lie just under the sarcolemma. 



3. The Sareous substance is made up of a series of longi- 

 tudinal fibrils consisting of alternate dim and clear bands the 



a b 



FIG. 15. (a) Fibres of Visceral Muscle ; (b) Fibres of Skeletal Muscle to show 

 sarcolemma, muscle corpuscles, and sarcous substance composed of fibrils 

 showing transverse markings. 



former staining deeply with eosin. In the middle of the 

 clear band is a narrow dim line, Dobie's line. The fibres 

 and fibrils tend to break across in the region of the clear 

 band, showing that they are weakest at that part. The clear 

 band differs from the dim band, not only in not taking up 

 eosin, but also in the fact that it entirely prevents the passage 

 of polarised light except in one position of the analysing 

 prism, while the dim band allows polarised light to pass, 

 whatever be the position of the prisms. 



A probable explanation of these facts is that the sarcous 

 substance is made up, like other protoplasm, of a mitoma and 

 plasma ; that the mitoma is arranged in a series of longitudinal 

 fibres, which are broader and stronger in the dim band, and 

 lie closely applied to one another, side by side, while in the 

 clear band they are thinner, and are separated from one 

 another by plasma ; and that at Dobie's line there is a swelling 

 on each fibril. 



