142 THE HUMAN BODY. 



fascicles, on the contrary, do not cross each other, and their 

 action is direct. 



Pons Frtn?///. -The movements of locomotion are origi- 

 nated specially, according to M. Longet, in the pons Varolii. 

 This portion of the encephalon has a cross action on motion. 

 It is a centre of perception for tactile sensations, but nothing 

 authorizes us to believe that it can appreciate sensation by 

 itself alone, and without the aid of the cerebral lobes. 



Peduncles of the brain. These organs unite the greater 

 and lesser brain to the isthmus of the encephalon, and to the 

 spinal cord, and seem to be solely devoted to the transmis- 

 sion of motion and sensation. An injury to one of the 

 middle peduncles of the cerebellum causes the body to turn 

 on its axis; a phenomenon which has been variously explained 

 by different writers. 



Corpora quadrigemina, or quadrigeminal bodies. These 

 bodies take an essential part in vision, either by inducing 

 the contractions of the iris, or in contributing to visual per- 

 ceptions. 



Pineal gland. The hypothesis of Descartes has popular- 

 ized, so to speak, this organ, whose functions are entirely 

 unknown. The illustrious philosopher believes the pineal 

 gland to be "the source from whence the most subtle parts of 

 the blood, the spirits, flow to all parts in the brain, and are 

 directed to a particular point, according as the gland is in- 

 clined one way or the other." This idea of Descartes has 

 been parodied, by making the pineal gland the seat of the 

 soul, from whence it directs the impulses of the brain by two 

 nervous prolongations, called the " reins of the mind " 

 (habena animi). 



The optic beds. In spite of the name which has been given 

 them, these portions of the encephalon do not seem to have 

 any appreciable action on the sense of vision; but they act 

 upon the voluntary movements in such a manner that the 

 influence of the right half is felt on the left, and vice versa; 

 this is called cross action, which is caused, as already stated, 

 by the crossing of the cerebral fibres. The optic beds seem 

 to have no influence over the movements of the upper 

 extremities, as has been thought by several physiologists. 



