STRUCTURE AND PHYSIOLOGY OF ANIMALS 



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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. Mus- 

 cular tissue varies greatly in 

 minute structure in different 

 groups of animals, and even in 

 different parts of the same animal. 

 It consists of microscopic fibres 

 aggregated together into large 

 bundles or layers. These fibres 

 are composed of a substance the 

 muscle- substance which when 

 living has the special property of 

 contractility, contracting or be- 

 coming shorter and thicker on the 

 application of a stimulus. There 

 are two principal varieties of 

 muscular tissue to be distin- 

 guished, termed respectively 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 un- 

 altered 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 represented 

 by their nuclei (n). Sometimes 

 there is a core of protoplasm ; but 



more usually the entire fibre is composed of muscle-substance, 

 with perhaps a remnant of protoplasm in the neighbourhood 

 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 enclosed 

 in a delicate sheath the sarcolemma. Striated muscular tissue 



FIG. 18. Transverse section of compact 

 bone, a, lamelte concentric with the 

 outer surface ; b, lamellae concentric 

 with the surface of the marrow cavity ; 

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

 tion of a Haversian canal just dividing 

 into two ; d, interstitial lamella?. (From 

 Huxley's Lessons in Physiology.) 



