46 THE ANATOMY OF VERTEBEATED ANIMALS. 



ments as there are vertebrae, the interspaces between them 

 appeai-ing as intermuscular septa. The development of the 

 hyposkeletal muscles has not been worked out, but it appears 

 to take place much later than that of the episkeletal set. 



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

 fishes — the chief musciilar system of the trunk consists of 

 the episkeletal muscles, which form thick lateral masses of 

 longitudinal fibres, divided by transverse intermuscular 

 septa into segments (or Myotomes) corresponding with the 

 vertebrae. The lateral muscles meet in the middle line 

 below, and divide, in front, into a dorso-lateral mass con- 

 nected with the skull, and a ventro-lateral attached, in part, 

 to the pectoral arch, and, in part, continued forwards to the 

 skull, to the hyoidean apparatus, and to the mandible. Pos- 

 teriorly, the lateral muscles are continued to the extremity 

 of the tail. The hyposkeletal muscular system appears to 

 be undeveloped. 



In the higher Vertelrata, 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 repre- 

 sented by the superior caudal muscles, and by the erector 

 spincB ; which, as it splits up, anteriorly, and becomes 

 attached to the vertebrae, and to the ribs, and to the skull, 

 acquires the names of spinalis, semispinalis, longissimus 

 dorsi, sacrolumhalis, inter-transversalis, levatores costarwm, 

 complexus, splenius, recti postici, and recti laterales. 



The ventro-lateral division of the fish's lateral muscle 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 abdominis, ex- 

 tending from the pelvis to the sternum — the sterno-hyoidei, 

 between the sternum and the hyoidean apparatus — the 

 genio-hyoidei, which pass from the hyoid to the symphysis 

 of the mandible. The latter are the obliqui externi of the 

 abdomen — the external intercostales of the thorax — the sub- 

 clavius stretching from the first lib to the clavicle; the 

 scaleni from the anterior dorsal ribs to the cervical ribs and 



