104 HUMAN PHYSIOLOGY. 



1 2th Pair. Hypoglossal or Sublingual. 



Apparent Origin. By two groups of filaments from the medulla ob- 

 longata, in the grooves between the olivary body and the anterior pyramid. 



Deep Origin. From the hypoglossal nucleus situated deeply in the 

 substance of the medulla, on a level with the lowest portion of the floor of 

 the 4th ventricle ; some decussating filaments have been traced to a higher 

 encephalic centre. 



Distribution. The trunk formed by a union of the root filaments 

 passes out of the cranial cavity through the anterior condyloid foramen, 

 occasionally receiving a filament from the lateral and posterior portion of 

 the medulla oblongata. After emerging from the cranium, it sends filaments 

 to the sympathetic and pneumogastric ; it anastomoses with the lingual 

 branch of the 5th pair, and receives and sends filaments to the upper cer- 

 vical nerves. The nerve is finally distributed to the sterno-hyoid, sterno- 

 thyroid, omo-hyoid, thyro-hyoid, stylo-glossi, hyo-glossi, genio-hyoid, genio- 

 hyo-glossi, and the intrinsic muscles of the tongue. 



Properties. A purely motor nerve at its origin, but derives sensibility 

 outside the cranial cavity, from anastomosis with the cervical, pneumo- 

 gastric and 5th nerves. 



Irritation of the nerve gives rise to convulsive movements of the tongue 

 and slight evidences of sensibility. 



Division of the nerve abolishes all movements of the tongue, and inter- 

 feres considerably with the act of deglutition. 



When the hypoglossal nerve is involved in hemiplegia, the tip of the 

 tongue is directed to the paralyzed side when the tongue is protruded ; 

 due to the unopposed action of the genio-hyo-glossus on the sound side. 



Articulation is considerably impaired in paralysis of this nerve ; great 

 difficulty being experienced in the pronunciation of the consonantal sounds. 



Mastication is performed with difficulty, from inability to retain the food 

 between the teeth until it is completely triturated. 



Function. Governs all the movements of the tongue and influences the 

 functions of mastication, deglutition and articulate language. 



CEREBRO-SPINAL AXIS. 



The Cerebro-Spinal Axis consists of the spinal cord, medulla oblon- 

 gata, pons Varolii, cerebellum and cerebrum, exclusive of the spinal and 

 cranial nerves. It is contained within the cavities of the cranium and 

 spinal column, and surrounded by three membranes, the dura mater, 



