198 HUMAN PHYSIOLOGY. 



Function. The crura are conductors of motor and sensor impulses ; 

 the gray matter assists in the coordination of the complicated move- 

 ments of the eyeball and iris, through the motor oculi communis 

 nerve. It also assists in the harmonization of the general muscular 

 movements, as section of one crus gives rise to peculiar movements 

 of rotation and somersaults forward and backward. 



THE CORPORA QUADRIGEMINA. 



The corpora quadigemina are four small, rounded eminences, 

 two on each side of the median line, situated immediately behind 

 the third ventricle, and beneath the posterior border of the corpus 

 callosum. 



The anterior tubercles are oblong from before backward, and 

 larger than the posterior, which are hemispheric in shape ; they are 

 grayish in color, but consist of white matter externally and gray 

 matter internally. 



Both the anterior and posterior tubercles are connected with the 

 optic thalami by commissural bands, named the anterior and pos- 

 terior brachia, respectively. They receive fibers from the olivary 

 fasciculus and fibers from the cerebellum, which press upward to 

 enter the optic thalami. 



The corpora geniculata are situated, one on the inner side and 

 one on the outer side of each optic tract, behind and beneath the 

 optic thalamus, and from their position are named the corpora 

 geniculata internet and externa ; they give origin to fibers of the 

 optic nerve. 



Functions. The corpora quadrigemina are centers associated with 

 the visual centers. Destruction of these tubercles is immediately 

 followed by a loss of the sense of sight ; moreover, their action in 

 vision is crossed, owing to the decussation of the optic tracts, so that 

 if the tubercle of the right side be destroyed by disease or extir- 

 pated, the sight is lost in the eye of the opposite side, and the iris 

 loses its mobility. 



The tubercula quadrigemina as nerve centers preside over the 

 reflex movements which cause a dilatation or contraction of the 

 iris, irritation of the tubercles causing contraction, destruction 

 causing dilatation. Removal of the tubercles on one side pro- 



