CORPORA QUADRIGEMINA. 179 



cerebellum, constitutes the sensor tract (the tegmentum), which terminates in 

 the optic thalamus and cerebrum. 



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

 gray matter assists in the coordination of the complicated movements 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 superior 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 superior and posterior tubercles are connected with the optic 

 thalami by commissural bands, named the superior and posterior 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 interna and externa; the 

 latter is a terminal for some of the true visual fibers. 



Functions. The corpora quadrigemina are centers associated with the vis- 

 ual centers. Destruction of these tubercules 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 de- 

 stroyed by disease or extirpated, 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 produces a temporary loss of power of the opposite side 

 of the body, and a tendency to move around an axis is manifested, as after a 

 section of one crus cerebri, which, however, may be due to giddiness and loss 

 of sight. 



They also assist in the coordination of the complex movements of the eye, 

 and regulate the changes of the iris during the movements of accommodation. 



