152 COMPARATIVE ANATOMY. 



that in the ancestors of existing Vertebrates the dorsal and ventral 

 roots remained distinct. 



The common trunk formed by the junction of the two roots 

 divides up again into a dorsal, a ventral, and an intestinal 

 branch. The first of these goes to the muscles and skin of the 

 back, the second supplies the lateral and ventral portions of the 

 body-wall, while the intestinal branch comes into connection with 

 the sympathetic (see p. 160). 



1. SPINAL NERVES. 



As a general rule, each corresponding pair of dorsal and ventral 

 roots lies in the same transverse plane : an exception to this is seen 

 however, in Amphioxus, Cyclostomes, and Elasniobianchs. In 

 Amphioxus the mesoblastic somites of the right and left side 

 are arranged alternately, and thus the points of exit of the 

 nerves also alternate, while in the two last-named groups of Fishes 

 each ventral pair alternates with a dorsal pair. In Ganoids also 

 lateral displacements of the nerve-roots are to be met with. 



While in Fishes the greatest variations are seen as regards the 

 mode of exit of the nerves (which pass through the intercalary 

 pieces of the vertebral column, through the arches, or between them), 

 from the Amphibia onwards the nerves always make their exit on 

 each side between the arches, through the intervertebral foramina. 

 In their primitive undifferentiated condition the spinal nerves have 

 a strictly metameric arrangement, and are equally developed in all 

 regions of the body. As already pointed out in the chapter on the 

 spinal cord, this condition becomes modified by the development of 

 the appendages, so that a number of spinal nerves unite together to 

 form plexuses, 1 which according to their position are spoken of as 

 cervical, brachial, lumbar, sacral, &c. The number and size 

 of the nerves composing them is usually directly proportional to 

 the development of the appendages : a special description of them, 

 however, cannot be given here, and only the following points will 

 be touched upon. 



In contrast to Fishes, the great variation in the plexuses of which 

 renders it impossible to reduce them to a common plan, we find 

 from the Amphibia onwards a typical grouping of the branches 

 of the brachial plexus. The following brandies may be distin- 

 guished : (1) Anterior thoracic nerves (the dorsalis scapulas 

 and thoracicus posterior s. lateralis of human anatomy) ; (2) 

 anterior brachial nerves, the homologues of the subscapulares, 

 cutaneus brachii internus minor (with limitations), axillaris, and 

 radialis; (3) posterior brachial and thoracic nerves (tho- 

 racici s. pectorales aiitcncivs, cutaneus internus major s. medius 

 musculo-cutaneus, median, and ulnar nerves (with limitations). 



1 For ci description of their composition, see Wiedt'rslirim's Lchrlufb der vergl 

 Anatomic. 



