STRUCTURE OF THE VERTEBRATES 



187 



1. Pectoral limb. The extrinsic muscles of the pectoral limb 

 are most primitive in the fish. They are fan-shaped groups of 

 fibers, the apex inserting on the basal cartilages. In the urodeles 

 the dorsal group becomes divided into several distinct muscles 

 which can be homologized with those of the mammal: (a) an 

 anterior sheet, the trapezius, arising on the spines of the verte- 

 brae; (b) a larger, more posterior, muscle, the latissimus dorsi, 

 which draws the forelimb posteriorly and slightly dorsally; (c) 

 the deltoid, which lies between the two former muscles, and is 

 an abductor of the arm; and (d) one or two other dorsal muscles 



Myotome. 



Fig. 106. Head and Shoul- 

 der of Necturus (Order 

 Urodela). Note the meta- 

 meric structure of the axial 

 muscles, the simple arrange- 

 ment of the extrinsic mus- 

 cles of the arm, and the 

 modification of the bran- 

 chial musculature. 



which are not clearly homologized. The ventral, pectoralis, 

 muscles are almost undivided in the urodeles. During the course 

 of evolution other muscles are added to the anterior group, and 

 the pectoralis muscle is divided into two or three parts. All the 

 muscles change their shape and relative size, but their position 

 and function are very constant throughout the different classes. 

 The intrinsic muscles of the anterior limb undergo much 

 greater specialization. In the dogfish these muscles arise on the 

 basal cartilages and pass out to the distal cartilages as a group 

 of separate slips or bands. With the development of tetrapod 

 limbs in the urodeles the muscles take a more definite shape 

 with a division of function. The ventral biceps, which flexes the 

 forearm, is well developed and remains in the higher classes as 



