28 



ZOOLOGY 



membrane — the periosteum — which takes an active part in its 

 growth and nutrition. 



The connective tissues are all more or less passive in the 

 functions which they perform, serving mainly for support and for 

 binding together the various organs. Muscular tissue, on the 



other hand, has an active part to 

 play — this being the tissue by 

 means of which, in general, all 

 the movements of the body of 

 an animal are brought about. 

 Muscular tissue varies greatly in 

 minute structure in different 

 groups of animals, and even in 

 different parts of the same ani- 

 mal. It consists of microscopic 

 fibres aggregated together into 

 large bundles or layers. These 

 fibres are composed of a sub- 

 stance — the muscle-siibstanee — 

 which when living has the special 

 property of contractility, contract- 

 ing or becoming shorter and 

 thicker on the application of a 

 stimulus. There are two princi- 

 pal varieties of muscular tissue 

 to be distinguished, termed re- 

 spectively non-striated and striated 

 muscle. Each fibre of non-striated 

 muscle (Fig. 19) is usually a 

 single, greatly elongated cell, 

 sometimes branched, with a single 

 nucleus ; it may contain a core 

 of unaltered protoplasm, or all 

 except the nucleus may be altered 

 into muscle-substance ; cross- 

 striation is absent. A fibre of 

 striated muscular tissue (Fig. 20) 

 is formed by the close union 

 of several cells which are repre- 

 sented by their nuclei (n). Some- 

 times there is a core of proto- 

 plasm ; but more usually the entire fibre is composed of muscle- 

 substance, with perhaps a remnant of protoplasm in the neigh- 

 bourhood of each nucleus. The substance of the fibre is crossed 

 by numerous transverse bands and striae, the precise significance 

 of which is a matter of controversy. The fibre is usually en- 

 closed in a delicate sheath — the sarcolemma. Striated muscular 



i. 18. — Transverse section of compact 

 bone, a, lamellse concentric with the 

 outer surface ; b, lamellse concentric 

 with the surface of the marrow cavity ; 

 c, section of Haversian canals ; c', sec- 

 tion of a Haversian canal just dividing 

 into two ; d, interstitial lamellae. (From 

 Huxley's lessons in Physiology.) 



