Muscular Tissues and Muscles. 



19 



restricted to a single plane. The gliding joint or arthrodia is one in 

 which a slight degree of motion is made possible by one surface slipping 

 over the other ; it is exemplified in the accessory articulations of the 

 vertebral arches. 



3. Muscular TisspES. 



Muscular tissues are the active portions of the individual muscles 

 of the skeleton and of the muscle coats of visceral organs. Their chief 

 feature consists in the elongation of the cells to form fibres. These 

 fibres may be considered to possess the contractile properties of 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 is the shortening of the 

 longitudinal axis and the increase of the transverse axis. 



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. 14 B) are elongated, spindle-like cells, the substance 

 of which is longitudinally striated, but possesses no transverse 

 markings. The single nucleus of the cell occupies 'a central position. 

 The. muscles which they form are dis- 

 tinguished as involuntary because 

 their operation is not under the con- 

 trol of the will, their connections 

 being with the sympathetic nervous 

 system. The striated fibres (Fig. 14 A) 

 are very much larger, cylindrical 

 structures, the substance of which 

 possesses characteristic transverse 

 striations. 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 muscular substance of the heart differs both from striated and 

 smooth muscle in being composed of branched anastomosing fibres, 



I ! I 



I 



Fig. 14. A, Striated (skeletal) muscle of the 

 rabbit. B , Unstriated mubcle from the 

 muscular tunic of the intestine. 



*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 bandies of the interior of the fibre. 



