THE MUSCULAR SYSTEM 



67 



of the eye show practically no indications thereof, the connective 

 tissue septa between segments having disappeared during develop- 

 ment. The fusion indicated in the last sentence may, moreover, 

 be accompanied by transformation of parts of the resulting sheets 

 into connective tissue, forming "aponeuroses," and by splitting in 

 new planes, for example, parallel with the surface. 



A transverse septum of connective tissue extends laterally from 

 the transverse processes and divides the trunk musculature into 

 dorsal, or epaxial, and ventral, or hypaxial, portions supplied 

 respectively by dorsal and ventral branches of the spinal nerves. 

 This division is indicated in Fig. 20. The epaxial portions then 

 produce the dorsal musculature, the hypaxial forming the pre- 

 vertebral and lateral musculature of the trunk. In the neck region, 

 the hypaxial portion is divided in early stages by the gill pouches, 

 so that epibranchial and hypobranchial groups of muscles are 

 produced. The muscles of each limb are developed from cells that 

 have migrated from the ventral ends of several adjacent myotomes 

 and those of the diaphragm have migrated from myotomes in the 

 neck (corresponding with which fact, the phrenic nerve has its 

 origin in the neck) . 



Distribution 



While the bulk of voluntary muscle is 

 skeletal, part at least is related to the 

 skin. This forms a cutaneous sheet, 

 divisible into the cutaneus maximus and 

 platysma, and the facial, palpebral, and 

 auricular muscles of the head. The first 

 two and the last of these are commonly 

 developed to a considerable extent in 

 mammals, though reduced in man. The 

 trunk musculature comprises a special 

 group of cervical and occipital muscles in 

 relation to the neck and head, and the 

 general series which are more nearly 

 vertebral. The appendicular muscles are 

 distributed in special groups connecting 

 the limb as a whole with the trunk and 



Fig. Z7. Arm muscles of 

 rabbit from the medial sur- 

 face, illustrating muscle _ ac- 

 tion, flexion, and extension: 

 b, biceps (flexor) ; tr, long 

 head of triceps (extensor) ; i, 

 insertion; o, origin; sc, scap- 

 ula; h, humerus; r, radius; 

 u, ulna. 



