CORPORA STRIATA AND OPTIC THALAMI. 119 



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 geniculata 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- 

 fested, 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 movements of the iris during the movements of 

 accommodation for distance. 



CORPORA STRIATA AND OPTIC THALAMI. 



The Corpora Striata are two large ovoid collections of gray matter, 

 situated at the base of the cerebrum, the larger portions of which are 

 imbedded in the white matter, the smaller portions projecting into the 

 anterior part of the lateral ventricle. Each striated body is divided, by a 

 narrowband of white matter, into two portions, viz.: 



1. The Caudate micleus, the intra ventricular portion, which is conical 

 in shape, having its apex directed backward, as a narrow, tail-like process. 



2. The Lenticular nucleus, imbedded in the white matter, and for the 

 most part external to the ventricle ; on the outer side of the lenticular 

 nucleus is found a narrow band of white matter, the external capsule ; 



