46 THE ANATOMY OF VERTEBRATED ANIMALS. 



The mandible is depressed by a muscle, the digastric^ arising 

 from the skull, and supplied by a branch of the seventh nerve : 

 it is raised by a muscular mass, which is separable into mas- 

 seter, temporal, and pterygoid muscles, according to its con- 

 nection with the maxillo-jugal bones, the sides of the skull, 

 or the palato-pterygoid bones, and is supplied by the fifth 

 nerve. 



The proper facial muscles belong to the system of cutane- 

 ous muscles, and receive branches from the seventh nerve. 



The hyposkeletal system is formed, partly, of longitudinal 

 muscles which underlie the vertebral column ; and partly, of 

 more or less oblique, or even transverse fibres, w4iich form the 

 innermost muscular walls of the thorax and of the abdomen. 



The former are the subcaudal intrinsic flexors of the tail ; 

 the pyriformis, psoas, and other muscles proceeding from the 

 inferior faces of the vertebrae to the hind-limb; the longus 

 colli, or intrinsic flexor of the anterior part of the vertebral 

 column ; and the recti capitis antici, or flexors of the head 

 upon the vertebral column. The latter are the ohliquus in- 

 ter nus of the abdomen, the fibres of which take a direction 

 crossing that of the external oblique muscle ; and the trans- 

 versalis, which lies innermost of the abdominal muscles, and 

 has its fibres transverse. In the thorax, the intercostales interni 

 continue the direction of the internal oblique, and the triangu- 

 laris sterni that of the transversalis. The diaphragm and the 

 levator ani must also be enumerated among the hyposkeletal 

 muscles. The hyposkeletal muscles of the posterior moiety 

 of the body attain a great development in those Vertebrata 

 which have no hind-limbs, such as Ophidia and Getacea. 



The Muscular System of the Limbs. — The muscles of the 

 limbs of Fishes are very simple, consisting, on each face of 

 the limb, of bundles of fibres, w4iich proceed (usuall}'- in two 

 layers) oblivquely, from the clavicle and supraclavicle to the 

 fin-rays. The pectoral and pelvic arches themselves are im- 

 bedded in the lateral muscles. 



In the Amp>hibia and all the higher Vertebrata, the muscles 

 of the limbs are divisible into — intrinsic, or those wdiich take 

 their origin within the anatomical limits of the limb (including 

 the pectoral or pelvic arch) ; and extrinsic, or those which 

 arise outside the limb. 



Supposing the limb to be extended at right angles to the 

 spine (its primitive position), it will present a dorsal aspect 

 and a vetitral aspect, with an anterior, or pre-axlal^ and a pos- 

 terior, or post-axial^ side. 



