THE NERVOUS SYSTEM i6i 



juxtaposition of these two nerves, but it does not appear to be quite 

 constant. 



The first spinal nerve now turns anteriorly and in doing so passes 

 between the subclavian vein and the small cutaneous vein. 



(iv) Immediately ventral to the subclavian vein a strong anasto- 

 mosis from the second spinal nerve is received, so that the vein passes 

 through the angle between the anastomosis and the proximal portion 

 of the main nerve. The nerve thus formed, containing fibres from 

 the ventral branches of both first and second spinal nerves, is the 

 N. hypoglossus proper. 



(iv a) From the middle of the anastomosis a fine twig is given off to 

 the M. pectori-scapularis (p.s.). 



(v) N. hypoglossus (n.hyp.) (N. hypobranchialis, Driiner). Imme- 

 diately after its formation, as the result of the fusion of branches 

 from the first two spinal nerves, the hypoglossal nerve turns 

 directly forwards along the lateral edge of the M. rectus cervicis. 

 It supplies several twigs to both the superficial and deep portions 

 of this muscle, then passes ventral to the thyroid gland, and at the 

 anterior end of the gland turns somewhat mesially to pass through 

 the substance of the M. genio-hyoideus. It supplies several fine 

 twigs to this muscle (g.hy.) and emerges from it at the anterior end 

 of the cerato-hyal (see Figs. 3 7 and 38) to enter the tongue and inner- 

 vate the MM. genio-glossus and hyo-glossus. 



With regard to the homology of the hypoglossus of Urodeles with 

 that of the higher vertebrates the statement of Goodrich (P.Z.S. 

 191 1, p. 118) may be quoted here. He says '. . . the hypoglossus of 

 Amphibia and Amniota may certainly be considered as homologous, 

 although not necessarily composed of the same segmental nerves. 

 It is owing to the shortness of the skull in Amphibia that the hypo- 

 glossal roots come out behind it.' In the same paper he also shows 

 that the urodele hypoglossus innervates muscles derived from the 

 ventral outgrowths of the second, third, and fourth myotomes. 



4. The Second Spinal Nerve. 



N. spinalis 2 (PL XI, fig. ^z^^ n.sp.2) possesses both dorsal and ven- 

 tral roots and also a ganglion, so that its dorsal branches contain both 

 cutaneous and motor-fibres, and hence conform to the normal plan. 



The R. ventralis, after emerging from the subvertebral muscula- 

 ture, gives off the R. communicans ad sympatheticus and turns postero- 

 laterally to the anterior border of the shoulder. Here, according to 

 Fiirbringer, it divides into four main branches: (i) N. thoracicus 

 superior 2; (ii) N. thoracicus inferior 2 anterior; (iii) N. thoracicus 



4038 M 



