CEREBRAL NERVES 247 



of the mouth, and the skin of the back, and also sends twigs to the 

 brachial plexus. In higher Vertebrates the hypoglossal becomes 

 gradually more differentiated from the other cervical nerves, and 

 innervates the intrinsic muscles of the tongue, takes up cervical 

 elements, and with them gives rise to the so-called n.inius descendcns 

 and the ansa hypoglossi, from which arise branches to the sterno- 

 hyoid and other muscles. 



In the Gymnophiona, Urodela, and Aglossa amongst the Anura, 

 the first spinal nerve perforates the first vertebra : in other Anurans 

 this nerve has disappeared, though occasionally recognisable in the 

 embryo, and the nerve which arises behind the vagus and emerges 

 between the first and second vertebrae in reality corresponds to the 

 second spinal nerve (hypoglossal, cf. Figs. 145 and 164). 



From the Sauropsida onwards, the hypoglossal, which arises 

 postero-ventrally to the vago-accessory group, leaves' the skull 

 through one or more apertures : it has three roots, 1 which corre- 

 spond to three spino-occipital nerves of the Anamnia. 



Dorsal roots may be present temporarily or permanently in con- 

 nection with the hypoglossal of Sauropsida and Mammalia, and may 

 be provided with ganglia, as in the case of the accessory and of the 

 spino-occipital nerves of many Fishes. A reduction of dorsal roots 

 may also take place further backwards : in many Mammals, including 

 Man, that of the first cervical nerve (and even of the second in e.g. 

 the Orang) may be reduced or entirely wanting. 



Sympathetic. 



The sympathetic system is a derivative of the spinal system, with 

 which it remains throughout life in close connection by means of 

 raini communicantes (Fig. 145). It is distributed mainly to the 

 alimentary tract, the vascular system, and the glandular organs 

 of the body. 



The sympathetic ganglia are derived from the developing spinal 

 ganglia, and, like these, show originally a segmental arrangement. 

 They contain typical ganglion-cells, 2 and usually become united 

 together secondarily by longitudinal commissures, thus giving rise 

 to a chain-like paired sympathetic cord lying on either side of the 

 vertebral column and aorta. From its ganglia nerves pass off 

 to the above-mentioned organs, and form plexuses. Numerous 

 peripheral ganglia, derived from the others, are also present in 

 the plexuses. 



1 Other, more anterior elements occur in the embryo in Sauropsida. 



- A special small form of cell occurs in the embryonic sympathetic ganglia, 

 and may extend beyond them to a greater or less degree into other parts. Thus 

 these chromaffin edit are found, e.ff., in the pancreas (islets of Langerhans), 

 coccygeal gland, hypophysis (Fig. 151), and suprarenals (medullary substance, q.v. ) : 

 in fact, in all " glands with internal secretion." 



