THE MUSCULAR SYSTEM 



211 



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. Among the other ontogenetic changes in 

 trunk myotomes is tangential splitting of muscles into sheets. One of 

 the most characteristic ontogenetic processes affecting the trunk muscles 



SOMITES lO-l-f 



'MCXJTH I I MliOTOMIC BUDS 



YPOPHYSIS HYPOGLOSSUS' VISCERAL ARCH 



A CYCLOSTOKC 



NERVE '■" 'OLR«CTORY PIT VISCERAL ARCH 4 



B ELASMOBRANCH 



C. REPTILE D. MAMMAL 



Fig. 200. — Diagrams illustrating the mode of origin of hypoglossal (hypobranchial) 

 muscles in A. Cyclostome, £. Elasmobranch, C. Reptile, and/?. Mammal. \t\A,B, and 

 C cervical myotomes send myotomic buds into the hypobranchial region. In mammals 

 such buds are not formed but a migration of mesenchyme cells from cervical myotomes 

 provides material for these muscles. The number of myotomes which participate is 

 usually four or five. 



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, intercostalis, 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 are innervated 

 by dorsal rami of the spinal nerves, the latter by ventral rami. The muscles 

 of the diaphragm, which are peculiar to man and mammals, migrate into 

 the chest from the neck, as is evidenced by the fact that they are innervated 

 by branches of the third, fourth, and fifth cervical nerves. 



