64 



THE ESSENTIALS OF HISTOLOGY 



Besides the sarcolenmia and striated substance, a muscular fibre also 

 exhibits a number of oval nuclei which have the usual reticular structure 

 of cell-nuclei. Sometimes there is a little granular substance (proto- 

 plasm) at each pole of the nucleus. In mammalian muscle the nuclei 

 lie immediately under the sarcolemma (figs. 69, 70), except in certain 

 fibres, e.g. those which compose the red muscles of some animals, such 

 as the hare and rabbit, and which occur scattered amongst the ordinary 

 fibres in mammalia generally. In these the nuclei are distributed 

 through the thickness of the fibre, and this is also the case in all the 

 muscular fibres of the frog. 



The transverse section of a muscle shows the fibres to be nearly 

 cylindrical in figure. Between the fibres there is a certain amount of 

 areolar tissue, which serves to support the blood-vessels and also unites 

 them into fasciculi ; the fasciculi are again united together by a larger 

 amount of this intramuscular connective tissue. 



On examining the transverse section of a fibre with a high power, 

 it is seen to be subdivided everywhere into small angular fields, the 

 areas of Cohnheim. These probably represent sections of the longi- 

 tudinal fibrils into which a muscular fibre splits after death, or after 

 being hardened in certain reagents, e.g. alcohol, chromic acid, or osmic 

 acid. 



FIG. 73. SECTION OF A MUSCULAR FIBRE 

 SHOWING AREAS OF CoHNHEIM. 



FIG. 74. LIVING MUSCLE OF WATER- 

 BEETLE (DYTISCUS MAKGINALIS). 

 ( Highly magnified.) 



s, sarcolemma ; a, dim stripe ; b, bright 

 stripe ; c, row of dots in bright stripe, 

 which seem to be the enlarged ends of 

 rod-shaped particles, d. 



In the muscles of insects the stripes are relatively broad, and their 

 structure can be more readily seen than in mammals. In the living 

 fibres from the muscles of the legs, the appearance of fine longitudinal 

 lines traversing the dark stripes, and terminating within the light 

 stripes in rows of dots, is very obvious. When the fibres contract, the 

 light stripes are seen, as the fibre shortens and thickens, to become 



