THE SENSES. 145 



the sterno-mastoid and trapezius muscles contract to fix the head 

 and hold the spine steady. This action seems to be prevented by 

 the section of this muscular branch of the spinal accessory. 



What is the origin of the hypoglossal or twelfth cranial nerve ? 



It arises in the gray matter at the inferior extremity of the 

 floor of the fourth ventricle, near the origin of the spinal accessory 

 and pneumogastric nerves. The fibres pass through the substance 

 of the medulla oblongata, skirting (and perhaps gaining fibres from) 

 the olivary body, and, emerging in a number of small bundles, 

 collect into a nerve-trunk which emerges from the skull by the 

 anterior condyloid foramen. 



How is the hypoglossal nerve distributed? 



It passes down the neck to about the level of the hyoid bone, 

 where it curves forward and into the tongue, giving off branches to 

 the muscles which move that organ. 



What is the function of the hypoglossal nerve ? 



It is a motor nerve, but possesses some sensory fibres derived 

 from the cervical spinal nerves and from the trigeminus, with 

 whose lingual branch it inosculates on the side of the tongue. 

 Filaments from it are distributed to all the muscles which move 

 the tongue, and to the depressors of the hyoid bone -through its 

 descending branch. 



What influence has this nerve upon digestion? 



It is important in mastication, for its muscles move the food 

 about for the better action of the teeth. In animals, after division 

 drinking is impossible, because they are unable to lap up fluids, and 

 the food is swallowed with difficulty because it is not carried back 

 into the pharynx by the tongue after mastication. 



What is its connection with speech? 



Articulation of most sounds involves movements of the tongue. 

 Impaired articulation is an early symptom in bulbar or glosso- 

 labio-laryngeal paralysis. 



THE SENSES. 



What organs are necessary for sensation? 



A peripheral organ for the reception of an impression, a nerve 

 for its conduction, and a centre in the brain for the perception. 

 10 Phy. 



