MUSCULAR SYSTEM 175 



FIG. 137. A C. (After P. Buffa.) 



A, Diagram showing the various phases in the movement of the scutes in Snakes. 



a and b, two consecutive scutes ;'c, the intervening integument ; d, fixed 

 point at free margin of scute ; e, distance along which the scute a is moved ; 

 A , resting stage ; B, stage in which a is raised and in which there is the 

 greatest forward extension of the skin (c), while the free margin of the scute 

 catches against the ground ; O, stage in which the scute a again takes on a 

 horizontal position, the skin (c) shows the greatest backward extension, and 

 the scute b is moved forwards along the distance e. 



B, Semi-diagrammatic figure of a longitudinal section through the ventral and 



lateral parts of the skin of Tropidonotus natrix, and of the costo-cutaneous 

 muscles in connection with the rib. c, rib ; c.c.i, c.c.s, inferior and superior 

 costo-cutaneous muscle ; m.c.i, intrinsic musculature of the skin ; s.g, longi- 

 tudinal sections of ventral scutes ; s.l, transverse sections of lateral scutes ; 

 r, vertebra. 



0, Inner side of part of the ventral integument of Tropidonotus natrix. The 

 intrinsic muscles of the skin are not indicated, c, a pair of ribs with the 

 corresponding inferior (c.c.l) and superior (c.c.s) costo-cutaneous muscles ; 

 /?, free raised border of the ventral (-s-.gr) and r of the lateral (s.l) scutes. 



rule flat, while those of the extremities have usually an elongated, 

 cylindrical, or prismatic form. In some cases, however, they assume 

 the most varied shapes; for instance, there may be more than 

 one origin (bicipital, tricipital, or quadricipital forms), the belly 

 may be double (biventral or digastric form), or the muscle may 

 be saw-shaped, or have its fibres arranged in a single or double 

 series like the barbs of a feather. 



Most of the muscles are separated by fibrous sheaths, or fasciae, 

 and may be continuous with tendons for connecting the muscles to 

 the skeleton, or with flattened membranous expansions (aponeuroscs). 

 Wherever marked friction occurs, ossifications (sesamoids') may be 

 developed in the course of a muscle or tendon. 



The differentiation of independent muscles may take place 



(1) by the separation of the originally single muscle into proximal 

 and distal parts by the formation of an intermediate tendon; 



(2) by the splitting of a muscular mass into layers ; (3) by a 

 longitudinal splitting: or (4) by a fusion of primarily distinct 

 muscles. A muscle may undergo very considerable modification 

 both in form and position by a change of origin and insertion ; 

 and when the action of a muscle becomes unnecessary, it either 

 disappears, or what remains of it contributes to the strengthening 



C T--U 1 B ' 6 



oi a neighbouring muscle. 



The most important point in determining the morphological 

 value of a muscle is its nerve-supply ; but other factors must also 

 be taken into consideration e.g. the homologies of the parts of 

 the skeleton, and the relative positions of the neighbouring parts. 



Integumentary Muscles. 



While most muscles have intimate relations to the skeleton, 

 which usually forms their points of origin and insertion and on 

 which they act directly, certain others are found in the derm or 



