MUSCULAR SYSTEM OF VERTEBRATA. 195 



observe a symmetrical arrangement of the upper and lower halves ; 

 the persistent dorsal portion is, however, still separated by inter- 

 muscular ligaments into separate pieces, and is quite fish-like in 

 character. 



§ 373. 



In the Amniota, other muscles are derived from the ventral 

 portion of the lateral musculature of the trunk, but it is retained, with 

 modifications, in the tail of the Reptilia and Mammalia, where it 

 undergoes the same changes as the dorsal portion, which is still 

 continued with great regularity on to the tail. 



In the Saurii, it is still possible to make out a separation of the 

 dorsal lateral muscles by the intermuscular ligaments, but in all the 

 rest of the Amniota they are still further differentiated, and give 

 rise to a series of separate dorsal muscles. In the Mammalia we 

 find them divided into a superficial and a deep portion. The former 

 consists of the splenius, which is limited to the cervical region, and 

 is partly inserted on to the skull, and partly on to the transverse 

 processes of the anterior cervical vertebras. The sacrospinalis is also 

 one of the superficial muscles ; it is broken up into a median and a 

 lateral portion (iliocostalis and longissimus). They both contain 

 masses of muscle which have their origin in the sacrum and ilium. 

 As the muscle passes up to the skull, accessory masses are added on 

 to it, which take their origin partly from the ribs, and partly from the 

 transverse processes. The insertions of the iliocostalis and longis- 

 simus extend to the ribs, and those of the latter muscle to the 

 transverse processes also. The deep layer is formed by the 

 trans verso- spinalis, which is formed of a system of muscles which 

 arises from the transverse processes, and is inserted into the spinous 

 processes; it is more or less broken up into various layers 

 (semispinalis, multifidus). 



Those parts of these muscles which reach the neck are often 

 developed in size in proportion to the freedom of movement 

 possessed by this region, and they may therefore be described as 

 special muscles. The same remark applies to those ends of these 

 muscles, which become developed into independent muscles, and 

 extend to the skull. The trachelomastoid is the cranial portion of 

 the longissimus, and the biventer and complexus of the semispinalis. 

 The musculi spinales and interspinales belong to this group. The 

 rectus capitis posticus major forms the most anterior spinalis muscle ; 

 and the rectus capitis posticus minor is the first of the interspinales. 



The small muscles which move the vertical fins of Fishes must 

 be regarded as being derived from the primitive lateral muscles of 

 the trunk. 



§ 374. 



The intercostal muscles must be regarded as a group derived 

 from the lateral trunk muscles. In Fishes these muscles are not 



