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 ina 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 bobdy—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 
Fic. 15. Striated much larger, cylindrical structures, the substance 
(skeleta)) muscle of 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, 7.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. 
