THE MUSCULAR SYSTEM 



287 



even in the adults of the lower vertebrates, becomes largely lost in adult 

 man and mammals as the result of a number of processes among which 

 fusion is the most important. As the myotomes grow in size and thick- 

 ness through cell multiplication, the connective tissue septa between them 

 disappear. In this way are formed such elongated muscles as the spinalis 

 and iliocostalis. Among the processes tending to obscure the original 

 metamerism is the degeneration of myotomes into connective tissue fasciae 

 and aponeuroses which may be very extensive. Migration of muscles 

 may accompany their fusion, as in the case of the tongue and throat 

 muscles which develop from four or five occipital myotomes. In man, 



SOMATIC MOTOR NERVE 

 SPINAL CORD 

 SPINAL GANGLION 

 RAMUS DORSALIS,/^ 

 NOTOCHORO 

 CENTRUM 

 NEPHROTOME 

 CORIUM 

 VISCERAL MUSCLE 

 RAMUS VENTRAL IS 

 EVTREMITY 



SOMATIC MUSCLE 



INTESTINE 

 ENDODERM 

 COELOM 



NEURAL CREST 



/ DERMATOME 

 MYOTOME 

 SCLEROTOME 

 PARIETAL MESODERM 



VISCERAL MESODERM 



Fig. 239. — A stereogram of the trunk region of a vertebrate embryo, based upon 

 elasmobranch embryos. The figure shows an earlier stage of development on the right 

 side — a later stage on the left. The extension of the myotome to form the lateral trunk 

 musculature is shown. The lateral trunk musculature of the ventral half of the body- 

 wall thus arises as a secondary invasion. (Redrawn after Braus.) 



the number is reduced to three. Among the other ontogenetic changes in 

 trunk myotomes, is tangential spHtting of muscles into sheets. One of 

 the most characteristic ontogenetic processes affecting the trunk muscles 

 is the subdivision of a muscle mass into a number of bellies each of which 

 acquires an independent origin or insertion, or both. The original seg- 

 mentation of the trunk myotomes is, however, retained in such muscles as 

 the transversospinalis, intercostal, and rectus abdominis. By the growth 

 of a horizontal connective tissue septum which extends laterally from 

 the transverse processes of the vertebrae, the lateral trunk muscles 

 become divided into epaxial and hypaxial portions, of which the former 

 is innervated by dorsal rami of the spinal nerves, the later by ventral rami. 

 Hypaxial muscles are further divided into a prevertebral and a lateral 



