THE CRANIAL NERVES 285 



shows that the special sensibility distributed to the tongue by 

 the lingual branch of the fifth is furnished by the chorda tympani. 

 The fifth nerve sends filaments to give sensibility to the velum 

 palati. The reflex act of deglutition is due to impressions carried 

 from the velum and neighboring parts to the centers; when the 

 fitth nerve is cut no such impressions are conveyed and the reflex 

 act cannot be excited. 



Regarding nutrition it is noticed that, besides the sloughing of 

 the cornea, there is also, about the same time, the appearance of 

 ulcers in the mouth and on the tongue, and animals thus experi- 

 mented upon soon die. These lesions are much less marked 

 when the section is behind the semilunar ganglion. Explanations 

 of this difference are not altogether satisfactory, but it is rational 

 to suppose that section of sympathetic fibers when the nerve 

 is cut in front of Gasser's ganglion is responsible for the dis- 

 turbances of nutrition; for this is the system of nutrition, and 

 changes following its section in other parts of the body are not 

 unlike those under discussion. Why, however, the changes 

 should be inflammatory in character is not explained by this hy- 

 pothesis, unless it be an explanation to say that the inflammation 

 is set up by the impairment of nutrition in these structures the 

 impairment resulting in part from the impoverished condition of 

 the blood as a consequence of the inability of the animal to chew. 



Sixth Nerve (Abducens). 



Origin. This is a motor nerve entirely. Its apparent origin 

 is from the lower border of the pons in the groove separating it 

 from the anterior pyramid of the medulla. Its deep origin is 

 close to the median line beneath the floor of the fourth ventricle 

 a little below the motor root of the fifth. 



Course and Distribution. The nerve enters the cavernous 

 sinus, runs forward to enter the orbit by the sphenoidal fissure, 

 passes between the two heads of the external rectus, and is dis- 

 tributed to the ocular surface of that muscle. In the cavernous 



