398 NEKVOUS SYSTEM. 



which the size of the pupil is modified is effected through 

 the optic lobes as nerve-centres. The mechanism of the 

 movements of the iris and their regulation through nervous 

 action are questions of great interest, and are somewhat com- 

 plex. We have already treated of them to some extent, in 

 connection with the physiology of the third pair of nerves, 

 and they will be considered still more fully in the section 

 on the special sense of sight. 



Ganglion of the Tuber Annulare. 



The tuber annulare, called the pons Yarolii, or the 

 mesocephalon, is situated at the base of the brain, just above 

 the medulla oblongata. It is white externally, and contains 

 in its interior a large admixture of gray matter. It presents 

 both transverse and longitudinal white fibres. Its transverse 

 fibres connect the two halves of the cerebellum. Its longi- 

 tudinal fibres are connected below, with the anterior pyrami- 

 dal bodies and the olivary bodies of the medulla oblongata, 

 the lateral columns of the cord, and a certain portion of the 

 posterior columns. Above, the fibres are connected with 

 the crura cerebri, and pass to the brain. The superficial 

 transverse fibres are wanting in animals in which the cere- 

 bellum has no lateral lobes. 



The general properties of the tuber annulare have been 

 Demonstrated in the most satisfactory manner by Longet. 

 In his experiments, direct excitation of the superficial trans- 

 verse fibres did not produce well-marked convulsive move- 

 ments, and there were no convulsions when the posterior 

 fibres were stimulated. When galvanization was applied to 

 the deeper anterior fibres, convulsive movements were dis- 

 tinct at each excitation. Stimulation of the posterior portion 

 always produced pain. This was not constantly observed to 

 follow irritation of the anterior portion, and, when pain oc- 

 curred, it was thought to be due to irritation of the root of 

 the fifth nerve. 1 



1 LONGET, Traiti de physiologic, Paris, 1869, tome iii., p. 394. 



