114 HAND-BOOK OF PHYSIOLOGY. 



motor and the tegmentum sensory impressions. As nerve-centres they 

 are probably connected with the functions of the third cerebral nerve, 

 which arises from the locus niger, and through which are directed the 

 chief of the numerous and complicated movements of the eyeball. The 

 crura cerebri are also in all probability connected with the co-ordination 

 of other movements besides those of the eye, as either rotatory (p. 119, 

 Vol. II.) or disorderly movements result after section of either of them. 



COEPOKA QUADRIGEMIKA. 



The corpora quadrigemina (from which, in function, the corpora gen- 

 iculata are not distinguishable), are the homologues of the optic lobes in 

 Birds, Amphibia, and Fishes, and may be regarded as the principal 

 nerve-centres for the sense of sight. 



Functions. (1) The experiments of Flourens, Longet, and Hert- 

 wig, show that removal of the corpora quadrigemina wholly destroys the 

 power of seeing; and diseases in which they are disorganized are usually 

 accompanied by blindness. Atrophy of them is also often a consequence 

 of atrophy of the eyes. Destruction of one of the corpora quadrigemina 

 (or of one optic lobe in birds), produces blindness of the opposite eye. 

 This loss of sight is the only apparent injury of sensibility sustained by 

 the removal of the corpora quadrigemina. The (2) removal of one of them 

 affects the movements of the body, so that animals rotate, as after 

 division of the crus cerebri, only more slowly: but this may be due to 

 giddiness and partial loss of sight. (3) The more evident and direct in- 

 fluence is that produced on the iris. It contracts when the corpora quad- 

 rigemina are irritated: it is always dilated when they are removed: so 

 that they may be regarded, in some measure at least, as the nervous 

 centres governing its movements, and adapting them to the impressions 

 derived from the retina through the optic nerves and tracts. (4) The 

 centre for the co-ordination of the movements of the eyes is also contained 

 in them. This centre is closely associated with that for contraction of 

 the pupil, and so it follows that contraction or dilatation follows upon 

 certain definite ocular movements. 



CORPORA STRIATA A^D OPTIC THALAMI. 



Structure. (1.) The corpora striata are situated in front of the optic 

 thalami, partly within and partly without the lateral ventricle. Each 

 corpus striatum consists of two parts. 



(a.) Intraventricular portion (caudate nucleus) is conical in shape, 

 with the base of the cone forward; it consists of grey matter, with white 

 substance in its centre, which comes from the corresponding cerebral 

 peduncle, (b.) Extraventricular portion (lenticular nucleus) is separated 



