EriSKELETAL AND IIYFOSEELETAL MUSCLES. 45 



In the lowest Yertehrata — as, for example, in ordinary 

 fishes — the chief muscular system of the trunk consists of the 

 episkeletal muscles, which form thick lateral masses of longitu- 

 dinal fibres, divided by transverse intermuscular septa into 

 segments (or Myotomes) corresponding with the vertebrte. 

 The lateral muscles meet in the middle line below, and divide, 

 in front, into a dorso-lateral mass connected with the skull, 

 and a ventro-lateral attached, in part, to the pectoral arch, and, 

 in part, continued forward to the skull, to the hjoidean appa- 

 ratus, and to the mandible. Posteriorly, the lateral muscles 

 are continued to the extremity of the tail. The hyposkeletal 

 muscular system appears to be undeveloped. 



In the higher Yertehrata^ both the episkeletal and hypo- 

 skeletal muscular systems are represented by considerable 

 numbers of more or less distinct muscles. The dorso-lateral 

 division of the lateral muscle of the fish is represented b}' the 

 superior caudal muscles, and by the erector spinoe • which, as 

 it splits up, anteriorly, and becomes attached to the vertebrie, 

 and to the ribs, and to the skull, acquires the names of spi- 

 nalis^ semispi7iaUs, longissiniKS dorsi, sacrolumhalis, i7iter- 

 tr ail sver sails, levatores costarmn, complexiis, sptle^iius, recti 

 p)ostici, and 7'ecti later ales. 



The ventro-lateral division of the fish's lateral mnscle is 

 represented, in the middle line of the trunk and head, by a 

 series of longitudinal muscles ; and, at the sides, by obliquely- 

 directed muscles. The former are the recti ahdomi^iis, extend- 

 ing from the pelvis to the stern vim — the sterno-Jiyoidei, be- 

 tween the sternum and the hyoidean apparatus — the genio- 

 hyoidei, which pass from the hyoid to the symphysis of the 

 mandible. The latter are the ohliqui exteymi of the abdomen 

 — the external intercostales of the thorax — the sifbclaviifs 

 stretching from the first rib to the clavicle ; the scaleoii from 

 the anterior dorsal ribs to the cervical ribs and transverse 

 processes, and the ster^io- and cleido-niastoidei from the ster- 

 num and clavicle to the skull. 



The fibres of all these obhque muscles take a direction, 

 from parts which are dorsal and anterior, to others which are 

 ventral and posterior. 



The trunk muscles of the lower Amphibia exhibit arrange- 

 ments which are transitional between those observed in Fishes 

 and that which has been described in Man, and which substan- 

 tially obtains in all abranchiate Yertehrata. 



The muscles of the jaws and of the hyoidean apparatus 

 appear to be, in part, episkeletal, and, in part, hj^poskeletal. 



