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 mandibule) or close (adductors) the mouth, 
and the mylohyoid which extends between the two rami of the lower 
Fic. 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. aml, lateral man- 
dibular adductors; emm, medial mandibular adductors; csd, csv, dorsal and ventral con- 
strictors; cm, coraco-mandibularis; chy, coraco hyoideus; chm, coraco-hyomandibularis; 
cbr, coraco-brachialis; cac, common coraco-arcual; intbr, interbranchials; l/s, superior 
labial levators; /mz, levator of lower jaw; /4m, hyomandibular levator; /r, levator of rostrum; 
tr, trapezius; VII, seventh nerve; dm, depressor mandibule (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. 
