MUSCULAR SYSTEM. 



133 



from muscles which originally ran in a transverse (circular) or in a 

 longitudinal direction. 



To the first category belong the epibranchial muscles, the sub- 

 spinales and interbasales, which lie in the dorsal part of the branchial 

 region, while the coraco-arcuales are in the ventral or hypobranchial 

 half. The most anterior of the circular group are those which open 

 (digastric or depressor mandibulae) or close (adductors) the mouth, 

 and the mylohyoid which extends between the two rami of the lower 



FIG. 141. Dorsal and ventral head muscles of the skate (Raia), after Marion; the dorsal 

 muscles more deeply dissected on the left side, the ventral on the right, ami, lateral man- 

 dibular adductors; amm, medial mandibular adductors; csd, csv, dorsal and ventral con- 

 strictors; cm, coraco-mandibularis; chy, coraco hyoideus; chm, coraco-hyomandibularis; 

 cbr, cora co-bra chialis; cac, common coraco-arcual; intbr, interbranchials; Us, superior 

 labial levators; Imi, levator of lower jaw; Ihm, hyomandibular levator; Ir, levator of rostrum; 

 tr, trapezius; VII, seventh nerve; dm, depressor mandibulae (digastric). 



jaw. Usually there are several adductors, known as masseter, tem- 

 poralis, pterygoideus, accordingly as they have their origin from 

 different parts of the skull. The longitudinal muscles are largely con- 

 fined to small slips which pass from one arch to the next. In the 

 amphibians these various muscles undergo considerable differentia- 

 tion, while in the amniotes this is in part carried farther, in part is re- 

 duced on account of the loss of branchial respiration and the degenera- 

 tion of the parts connected with it. Hence the most noticeable of the 

 visceral muscles are those connected with opening and closing the 

 mouth. 



