188 COMPARATIVE ANATOMY 



leaves the skull through the same foramen as the vagus, to which 

 it gives off motor elements. It supplies certain of the muscles 

 related to the pectoral arch, e.g. the sternocleidomastoid and the 

 trapezius. 



Hypoglossal. — The hypoglossal corresponds to one or several 

 of the anterior spinal nerves, and its transformation into a cerebral 

 nerve can be traced in passing through the Vertebrate series. 

 In some Fishes and all Amphibia it does not pass through the 

 cranial wall and is a true spinal nerve ; and in most Fishes and in the 

 Dipnoi, its inclusion within the skull can be seen to be due to a 

 gradual assimilation of the anterior part of the vertebral column with 

 the skull (comp. p. 45). In addition to its numerous ventral-roots 

 one or more dorsal, ganglion ated roots have been observed in the 

 embryos of various Vertebrates (Figs. 148 and 149). Two dorsal 

 roots, each with a ganglion, persist in Protopterus, and the same is 

 apparently true as regards Polypterus and certain Elasmobranchs : 

 even amongst Mammals, these roots can exceptionally be recog- 

 nised subsequently to the embryonic period.^ 



In Fishes (Fig. 148) the hypoglossal, like the next following 

 spinal nerves, sends branches to the muscles of the body, the- 

 floor of the mouth, and skin of the back, as well as being connected 

 with the brachial plexus. In higher Vertebrates (Fig. 149) it 

 supplies the intrinsic and extrinsic muscles of the tongue. 

 These lingtial branches are most marked in Mammals, in which 

 the tongue reaches its highest development. Elements of the 

 cervical spinal nerves also run along with the hypoglossal, and 

 give rise to the so-called ramus descendens with which further 

 cervical nerves are associated ; and from the " ansa hypoglossi " 

 thus formed, branches pass to the sterno-hyoid, sternothyroid, 

 omohyoid, and thyrohyoid muscles. 



Sympathetic. 



The sympathetic system arises in close connection with the 

 spinal system, with which it remains throughout life in close 

 connection by means of rami communicantes. It is distributed 

 mainly to the intestinal tract (in the widest sense), the vascular 

 system, and the glandular organs of the body. The sympathetic 

 ganglia, like those of the spinal nerves, show originally a segmental 

 arrangement. They usually become united together later by 

 longitudinal commissures and thus give rise to a chain-like 

 paired sympathetic cord lying on either side of the vertebral 

 .column. From its ganglia nerves pass off to the above-mentioned 



^ The dorsal root of the first spinal nerve may lie reduced or wanting in 

 Mammals — even in Man, so that here the modification of the primary 

 character of the nerves is not limited to those within the skull. 



