FUNCTIONS OP THE ENCEPHALON. 207 



expiration, by which the air iorces its way out at tlie con- 

 tracted opening. In sneezing, an irritation of the fifth nerve 

 leads to a convulsive expiration with the glottis open, but 

 the tongue raised so as to divert the expelled air from 

 escaping by the mouth, and send it through the nostrils. In 

 hiccough, the glottis is shut, and a momentary spasmodic 

 contraction of the diaphragm and abdominal walls takes 

 place ; while in vomiting, a similar action is more prolonged. 

 The walls of the stomach appear to take no part, or only a 

 secondary part in vomiting; for a dog, in which the stomach 

 was replaced by a bladder, was made to vomit perfectly, by 

 injection of tartar emetic into its veins (Magendie). The 

 excitation of the reflex action in vomiting is not always 

 the same, for it may be the result of irritation of the fauces, 

 or may be induced by nausea, an ill-understood sensation 

 depending on disturbance of the cerebral circulation. 



It may be furthur mentioned that irritation of the medulla 

 oblongata in the floor of the fourth ventricle, produces 

 artificial diabetes, that is to say, sugar in the urine. This it 

 does by paralysing the blood-vessels of the liver, and so lead- 

 ing to an abnormal amount of sugar being thrown into the 

 blood. 



152. When one of the crura cerebri or optic thalami is divided 

 or destroyed, total paralysis of both sensation and voluntary 

 movement is the result. When the cerebellum is removed, 

 the power of standing, and of all steady and definite move- 

 ment is lost, although the animal continues to move its limbs 

 in its attempts to stand and walk. It seems as if the impulse 

 to voluntary movement descended from the optic thalami, 

 while the power of co-ordination of movements resided in 

 the cerebellum. 



When the corpora quadrigemina are destroyed, total blind- 

 ness results; and when only one side is destroyed, there is 

 blindness in the opposite eye, a result to be accounted for by 

 the crossing of fibres in the optic commissure. It is curious 

 that injury to the optic thalami appears to have no effect on 

 vision, although the optic tracts arise in part from those 

 bodies, as well as from the corpora quadrigemina. 



With regard to the corpora striata, experiment gives none 

 but negative evidence, while the study of development and 



