34 Anatomy of the Rabbit. 



fibres. These fibres may be considered to possess the contractile 

 properties of protoplasm, 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 con- 

 traction in both is the shortening of the longitudinal axis and the 

 increase of the transverse axis. Muscles are important structures 

 in the production of heat. The latter is liberated not only in 

 action but also when the muscle is 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 com- 

 pose the individual or voluntary muscles of the 

 skeleton. Smooth fibres (Fig. 14) are elongated, 

 spindle-like cells, the substance of which is longitud- 

 inally striated, 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 sympathetic nervous 

 system. The striated fibres (Fig. 15) are very 

 Fig. is. Striated much larger, cylindrical structures, the substance 

 the^rabbit""^'^'^ °' of which possesses characteristic transverse stria- 

 tions. Each fibre is enclosed by a loosely attached 

 membrane, the sarcolemma, on the inner surface of which many 

 nuclei occur.* The presence of these nuclei indicates that the 

 fibre is not a single cell but a syncytium, i.e., an association of 

 cells unseparated by cell boundaries. The muscles formed by such 

 fibres are under the control of the will, their connections being 

 directly with the central nervous system. They comprise not 

 only the typical muscles of the skeleton, but also the special muscles 

 connecting the skeleton with the skin. 



*The position of the nuclei is characteristic of the so-called white muscles. 

 In the semitendinosus of the rabbit, which is a red muscle, the nuclei occur 

 between the fibril bundles of the interior of the fibre. Red fibres contract more 

 slowly but are more resistant to fatigue. The proportion of red and white 

 fibres varies in different muscles. 



