THE CRANIAL NERVES 295 



mater in the posterior fossa of the skull; (2) descendens hypo- 

 glossi, which, running downward across the sheath of the great 

 vessels, meets branches of the second and third cervical nerves 

 to form a loop from which are supplied the sterno-hyoid, the 

 omo-hyoid and the sterno-thyroid muscles; (3) thyro-hyoid to 

 the muscle of that name; (4) muscular to the muscular sub- 

 stances of the tongue and to the styloglossus, hyoglossus, genio- 

 hyoid and genio-hyoglossus muscles. 



Functions. This nerve possesses no sensibility at its root, 

 but receives sensory fibers from anastomoses with other nerves. 

 Its stimulation, therefore, causes movements of the tongue and 

 some pain. Section of both nerves causes difficult deglutition, 

 loss of power over the tongue and consequent disturbances in 

 mastication and articulation. When the twelfth is affected in 

 hemiplegia the tongue, on protrusion, deviates to the affected 

 side because it is pushed out by the genio-hyoglossus. 



It will be seen from the foregoing that, classified according to 

 their properties at their roots, the I., II. and VIII. are nerves of 

 special sense; the III., IV., VI., XI. and XII. are motor; the X. 

 is sensory; and the V., VII. and IX. are mixed. It is to be remem- 

 bered, however, that most of these (excepting the nerves of 

 special sense) are mixed in their distribution by reason of the 

 reception of fibers from other nerves. The term " mixed" in the 

 above classification is used as meaning the association of special 

 sensory fibers with motor or common sensory fibers as well as the 

 association of these latter with each other. The VII. is classed 

 as a mixed nerve only by allowing that the intermediary nerve of 

 Wrisbery is to be considered a part of it. Its own proper root is 

 purely motor. 



THE SPINAL NERVES. 



The spinal nerves, thirty-one on each side, are so called from 

 the fact that they originate in the spinal cord and escape from the 

 spinal canal by the intervertebral foramina. Eight pairs come 



