628 THE CRANIAL NERVES. 



pneumo-gastric. It is probable, moreover, that the descendens noni has little if any 

 real origin from the hypoglossal nerve : Luschka states it as the result of numerous 

 researches on the human subject that the descendens noni usually contains no fila- 

 ments from the hypoglossal, but is a branch from the first and second cervical, tem- 

 porarily associated with the ninth nerve ; and this quite agrees with the circumstance 

 that in the domestic animals the branches supplied to those muscles to which the 

 descendens noni of the human subject is distributed come from the cervical plexus. 



Branches to muscles and the tongue. The branch to the thyro-hyoid 

 muscle is a separate twig given off from the hypoglossal nerve as it 

 approaches the hyoid bone. The nerve supplies branches to the stylo -hyoid, 

 hyo-glossus, genio-byoid, and genio-hyo-gloasus muscles as it becomes 

 contiguous to each, and, when arrived close to the middle of the tongue 

 with the ranine artery, gives off several long slender branches, which pass 

 upwards into the substance of the organ. Some filaments join with others 

 proceeding from the gustatory nerve. 



A branch is described as uniting with its fellow of the opposite side, in the sub- 

 stance of the genio-hyoid muscle, or between it and the genio-glossus. This loop, as 

 also the ansa hypoglossi, is recommended by Hyrtl as a particularly favourable 

 example for the observation of nerve-fibres returning to the nervous centres without 

 distribution, to which he gives the name of " nerves without ends." (" Nat. Hist. 

 Review," Jan. 1862.) That in the ansa hypoglossi an interchange of fibres takes 

 place, so that a filament of the spinal nerve is directed upwards along the branch of 

 the hypoglossal, and vice versa, was noticed by Cruveilhier. 



Summary. The hypoglossal nerve supplies, either alone or in union 

 with branches of the spinal nerves, all the muscles connected with the 

 os hyoides, including those of the tongue, with the exception of the digastric 

 and stylo-hyoid, the mylo-hyoid, and the middle constrictor of the pharynx. 

 It also supplies the sterno -thyroid muscle. 



It is connected with the following nerves, viz., pneumo-gastrie, gustatory, 

 three upper cervical nerves, and the sympathetic. 



B. 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. Taken together, these nerves consist of thirty-one 

 pairs ; and, according to the region in which they issue from the spinal 

 canal, they are named cervical, dorsal, 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 beneath which it 

 emerges. There are thus left eight pairs of nerves between the cranium 

 and the first dorsal nerve, the first being placed above the atlas and the 

 eighth below the seventh cervical vertebra, which are reckoned by the 

 majority of writers as eight cervical nerves. The nerves of the thirty-first 

 pair emerge from the lower end of the sacral canal, below the first vertebra 

 of the coccyx, and are named coccygeal. 



Although the plan of counting eight cervical nerves is continued in this work for 

 the sake of convenience, it being that which is most frequently followed, it is by no 

 means intended to represent this method as scientifically correct. The plan of 

 \Villis, who reckoned the suboccipital as a cranial nerve, had at least the advantage 

 that it made the numbers of the remaining seven cervical nerves correspond each 

 with the vertebra beneath which it emerged, as do the dorsal, lumbar and sacral 

 nerves; and if the suboccipital nerve, while recognised as the first spinal nerve, be 



