204 HISTORY OF THE HUMAN BODY 



followed into the thoracic region, although here a new char- 

 acter is introduced by the presence of the ribs, which occupy 

 the place of the myocommata, of which they are direct deriva- 

 tives, and thus impart to the layers a segmental character. 

 The obliquus externus (profundtis) and internus are thus con- 

 tinued forward as the external and internal intercostals respec- 

 tively, and the transversalis is represented by the transversus 

 thoracis (triangularis sterni), in man very variable in extent. 

 The levatores costarum appear as a new element, peculiar to 

 the thorax, but they undoubtedly belong with the external in- 

 tercostals and the external oblique. 



In the cervical region the reduction of both ribs and body 

 cavity necessitates another series of modifications of the origi- 

 nal elements. There belong here, as ventral muscles of the 

 axial system, the scaleni, and the muscles of the prevertebral 

 group. Of these the scalenus posterior is a continuation of 

 the levatores costarum, and the other two scaleni, anterior 

 and medius, belong to the system of the intercostals and are 

 attached to those portions of the transverse processes of the 

 cervical vertebrae which are in reality anchylosed ribs. In the 

 same category belong the short lateral muscle, rectus capitis 

 later alls } and the "anterior" (ventral) series of intertrans- 

 versarii, which are innerved from ventral branches and are 

 also attached to the rib elements of the cervical vertebrae. 

 The prevertebral muscles, lying on the latero-ventral side of 

 the vertebrae, consist of the longus colli, and its anterior ex- 

 tension, longus capitis (rectus capitis anticus major), also of 

 the little rectus capitis anterior (rectus capitis anticus minor), 

 which extends between the skull and the atlas, near the rectus 

 lateralis. The history and serial homology of these latter 

 muscles, other than the fact that they belong with the ventral 

 axial muscles, is not known. 



Both ventral and dorsal axial muscles are continued beyond 

 sacrum and cloaca along the caudal vertebrae, where their de- 

 velopment varies as much as does the tail itself. As it is more 

 convenient to treat of the caudal musculature as a single sub- 

 ject, the discussion of the ventral muscles of this region will 



