132 HUMAN PHYSIOLOGY. 



striatum, but to some extent, also, in the cerebrum; the deep portion, 

 made up of the fasciculi teretes and posterior pyramids and accessory fibres 

 from the cerebellum, constitute the sensory tract (the tegmentum\ which 

 terminates in the optic thalamus and cerebrum. 



Function. The crura are conductors of motor impulses and sensory 

 impressions ; the gray matter, the locus niger, assists in the coordination of 

 the complicated movements of the eyeball and iris, through the motor oculi 

 communis nerve. They also assist in the harmonization of general muscular 

 movements; section of one crus giving rise to peculiar movements of rotation 

 and somersaults forward and backward. 



CORPORA QUADRIGEMINA. 



The Corpora Quadrigemina 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 hemispherical 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 posterior brachia, 

 respectively. They receive fibres from the olivary fasciculus and fibres 

 from the cerebellum, which pass 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 genictilata interna and 

 externa ; they give origin to fibres of the optic nerve. 



Functions. The Tubercula quadrigemina are the physical centres of 

 sight, translating the luminous impressions into visual sensations. Destruc- 

 tion 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 extirpated, in a pigeon, the sight is lost in the eye of the oppo- 

 site side, and the iris loses its mobility. 



The tubercula quadrigemina as nerve centres preside over the reflex 

 movements which cause a dilation 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 mani- 



