26 



ANATOMY OF THE RABBIT 



Fig. 13. Involun- 

 tary muscle, from 

 a section of the 

 muscular tunic of 

 the intestine. 



organs. Their chief feature consists in the elon- 

 gation of the cells to form fibres. These fibres 

 possess the contractile properties of simple proto- 

 plasm, but with the contraction limited to one 

 direction. Except in a few cases, the fibres are 

 arranged in a parallel fashion, so that the line of 

 contraction of the muscle or muscle layer is the 

 same as that of each of its fibres. The result of 

 contraction in both the muscle and its individual 

 fibres is the shortening of the longitudinal axis and 

 the increase of the transverse axis. Muscles are 

 important in the production of heat, which is 

 liberated not only when the muscle is in action 

 but also, though less rapidly, in repose. 



Two chief types of muscle fibres occur in the 

 body — the smooth or unstriated fibres, which are characteristic of 

 the involuntary muscles or muscle coats of the visceral organs or 

 of the skin, and the striated fibres which compose the individual or 

 voluntary muscles of the skeleton. Smooth fibres 

 (Fig. 13) are elongated, spindle-like cells, the 

 substance of which is longitudinally striated as a 

 result of the presence within each of numerous 

 fine myofibrils, but possesses no transverse mark- 

 ings. The single nucleus of the cell occupies a 

 central position. The muscles which they form 

 are distinguished as involuntary because their 

 operation is not under the control of the will, 

 their connections being with the autonomic ner- 

 vous system. They respond slowly to stimula- 

 tion but are capable of prolonged contraction. 

 The striated fibres (Fig. 14) are very much lar- 

 ger, cylindrical structures, the substance of which 

 possesses characteristic transverse striations in 

 addition to the longitudinal ones. These are 

 due to the myofibrils being composed of alter- 

 FiG 14 Parts of two mating light and dark portions, those of adjacent 

 preparation of a'piece ^brils being placed side by side so as to give an 

 musciTo?! rSbit^^'^ appearance of transverse bands. Each fibre is 



