THE SPINAL NE LIVES. 



also contains fibres which pass upwards from the communicating branches and run peripherally 

 to enter the nerves of the thyro-hyoid and genio-hyoid muscles. (See M. Holl. " Beobachtungen 

 iiber die Anastomosen des Nervus hypoglossus," Zeitschr. f. Anat. u. Entw., 1876.) 



The spinal origin of the nerves of the infrahyoid muscles has also been demonstrated ex- 

 perimentally in the monkey by Beevor and Horsley, who found that stimulation of the hypo- 

 glossal roots within the skull produced no effect on these muscles, while they were called into 

 action by stimulating the first and second cervical nerves. The sterno-hyoid and sterno -thyroid 

 were supplied mainly from the first nerve, and the omo-hyoid from the second (<>/>. fit. on 

 p. 270). 



The fibres of the hypoglossal nerve are of medium size, resembling in this respect those of 

 the facial nerve, and being smaller than those of most of the motor nerves passing to 

 voluntary muscles (Gaskell). 



Varieties. In one instance, recorded by Riidinger, the hypoglossal nerve was found 

 taking its superficial origin from the posterior surface of the medulla oblongata. The vertebral 

 artery is not unfrequently found passing forwards between, very rarely above, the roots of the 

 nerve. The right and left nerves are occasionally united by a cross branch or loop in the sub- 

 stance of the genio-hyoid, or between that and the genio-glossus muscle. In rare cases, the 

 twelfth nerve gives filaments to the mylo-hyoid, to the digastric, or to the stylo-hyoid muscle. 



The descending cervical nerve sometimes appears to be derived either altogether from the 

 pneumo-gastric or from both the pneumo-gastric and hypoglossal nerves, but it can always be 

 shown by dissection that these varieties of origin are only apparent, resulting from the tem- 

 porary adhesion of the filaments of this branch to those of the pneumo-gastric. A filament is 

 occasionally continued from the descending cervical nerve into the thorax, where it joins the 

 phrenic or the cardiac nerves ; in the latter case it is probably composed of fibres from the 

 vagus or the sympathetic, which have joined the hypoglossal or the descending nerve. This 

 nerve has also been seen sending a branch to the sterno-mastoid muscle. 



In some animals the twelfth nerve possesses a posterior root furnished with a ganglion, like 

 a spinal nerve. A similar condition has been met with in a few instances in man. 



SUMMARY. The hypoglossal nerve proper supplies only the muscles of the tongue, 

 with the exception of the palato-glossus and the pharyngeo-glossus. Fibres derived 

 from- the first three cervical nerves, which are associated with the hypoglossal for a 

 part of their course are distributed to the infrahyoid muscles and the genio-hyoid. 

 Others of uncertain origin pass to the skull and dura mater, and to the internal 

 jugular vein. The hypoglossal forms connections with the pneumo-gastric, lingual, 

 upper three cervical nerves, and sympathetic. 



SPINAL NERVES. 



The spinal nerves are characterised by their origin from the spinal cord, and 

 their direct transmission outwards from the spinal canal in the intervals between 

 the vertebrae. There are, in all, thirty-one pairs of these nerves, and, according to 

 the region in which they issue from the spinal canal, they are named cervical, 

 dorsal or thoracic, lumbar, sacral, and coccygeal. 



By universal usage each pair of nerves in the dorsal, lumbar and sacral regions is 

 named in correspondence with the vertebra below which it emerges. Of the eight 

 pairs of nerves between the cranium and the first dorsal nerve, the uppermost is 

 placed above the atlas, and the second and following nerves below the seven cervical 

 vertebrae in succession. These eight pairs are usually reckoned as eight cervical 

 nerves, but the first is also distinguished by the name of suboccipital nerve. The 

 nerves of the thirty-first pair emerge from the lower end of the sacral canal, pass 

 below the first vertebra of the coccyx, and are named coccygeal. 



Varieties. The spinal nerves necessarily vary in number with any deviation from the 

 usual number of the segments of the vertebral column. Sometimes an additional coccygeal 

 nerve exists. Among seven cases which were examined by Schlemm two coccygeal nerves 

 were found on each side in one instance, and on one side in another case. According to 

 Rauber vestiges of these, and also of a third pair of coccygeal nerves, are normally present in 

 the bundles of medullated fibres contained in the filum terminale (see Part I, p. 6). On the 

 other hand, the coccygeal nerve often appears to be wanting, being united to the filum termi- 

 nale (Rauber, Kadyi). 



