138 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



THE VISCERAL MUSCLES 



In the gill-bearing vertebrates a special system of muscles is devel- 

 oped in connexion with the visceral arches; these are to open and 

 close the visceral clefts, including the mouth. With the loss of the 

 gills some of these muscles disappear while others become changed in 

 function, several retaining their connection with the hyoid. These 

 visceral muscles may be divided into two sets according as they are 

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

 or in a longitudinal direction. 



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



FiG. 148. — Dorsal and ventral head muscles of the skate (Rata), after Marion; the 

 dorsal muscles more deeply dissected on the left side, the ventral on the right, amd, 

 lateral mandibular adductors; amm, medial mandibular adductors; csd, csv, dorsal and 

 ventral constrictors; cm, coraco-mandibularis; chy, coraco-hyoideus; chm, coraco- 

 hyomandibularis; cbr, coraco-brachialis; cac, common coraco-arcual; intbr, interbran- 

 chials; Us, superior labial levators; Itni, levator of lower jaw; Ihm, hyomandibular levator; 

 Ir, levator of rostrum; tr, trapezius; VII, seventh nerve; dm, depressor mandibulae 

 (digastric). 



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 

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

 temporalis, pterygoideus, accordingly as they have their origin from 

 different parts of the skull. The longitudinal muscles are largely 

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



