650 HANDBOOK OF PHYSIOLOGY. 



principal nerve-centres for visual sensations, the posterior possibly with 

 auditory sensation. 



Functions. (1) The experiments show that removal of the anterior 

 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 removal of the eyes. 

 Destruction of one of the anterior corpora quadrigemina (or of one optic 

 lobe in birds) produces hemiopia of opposite field of vision. 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 influence is that produced on the 

 iris. It contracts when the anterior corpora quadrigemina 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 move- 

 ments, 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. 



As we have peen, the lateral corpus geniculatum is associated on 

 either side with the anterior corpus quadrigeminum, and the median 

 corpus geniculatum with the posterior corpus quadrigeminum. 



The Sympathetic System. Having in the preceding chapters com- 

 pleted the description of the Cerebro-spinal nervous system proper, there 

 remains to be considered the structure and functions of the so-called 

 Sympathetic nervous system, and to this it is now necessary to direct 

 attention. 



It should, however, be distinctly borne in mind that the cerebro- 

 spinal and sympathetic systems are not distinct from one another. The 

 separation of the one from the other may be considered to be purely for 

 the sake of convenience. 



Distribution. It consists of: (1) A double chain of ganglia and 

 fibres, which extends from the cranium to the pelvis, along each side of 

 the vertebral column, and from which branches are distributed both to 

 the cerebro-spinal system, and to other parts of the sympathetic system. 

 With these may be included the small ganglia in connection with those 

 branches of the fifth cerebral nerve which are distributed in the neigh- 

 borhood of the organs of special sense : namely, the Ophthalmic, Otic, 



