506 COMPARATIVE PHYSIOLOGY. 



and the corpus striatum as motor ; but the clinical and patho- 

 logical evidence is conflicting — all lesions of these parts not 

 being followed by loss of sensation and motion respectively ; 

 though an injury to the internal capsule generally results in 

 paralysis. All are agreed that the symptoms are manifested on 

 the side of the body opposite to the side of the lesion, so that a 

 decussation must take place somewhere between the ganglion 

 and the periphery of the body. 



There, is no doubt that the optic thalamus, especially its 

 posterior part, is concerned with vision, for injury to it is fol- 

 lowed by a greater or less degree of disturbance of this func- 

 tion. As has been already pointed out, unilateral injury of 

 either of these ganglia leads to inco-ordination or to forced 

 movements. That these regions act some intermediate part in 

 the transmission of impulses to and from the brain cortex, and 

 that the anterior one is concerned with motor, and the pos- 

 terior possibly with sensory (tactile, etc.), and certainly with 

 visual impulses, may be stated with some confidence, though 

 further details are not yet a subject of general agreement. 



Corpora Quadrigemina. — The function of these parts in vis- 

 ion, as in the co-ordination of the movements of the ocular 

 muscles, and their relations to the movements of the pupil, will 

 be considered later. However, the actual centers for these func- 

 tions seem to lie in the anterior portion of the floor of the 

 aqueduct of Sylvius, and are indirectly affected by stimulation 

 of the corpora quadrigemina. Extirpation of these parts on 

 one side produces blindness of the opposite eye, and in birds, 

 etc., the same result follows when their homologues — the optic 

 lobes — are similarily treated. There can be no doubt, therefore, 

 that they are a part of the central nervous machinery of vision, 

 and it seems to be probable that the anterior parts of the cor- 

 pora quadrigemina alone have this visual function. But, since 

 it is the opposite eye that is affected, and in some animals 

 (rabbits) that alone, we are led to infer a decussation of the 

 optic fibers, or at least of impulses. In dogs, on the other hand, 

 the crossing seems to be but partial. 



It begins to appear that there are several parts of the brain 

 concerned with vision. After removal of almost any part of 

 the cerebral cortex, if of suflBcient extent, vision is impaired. 

 We may say, then, that before an object is " seen " in the high- 

 est sense, processes beginning in the retina undergo further 

 elaboration in the corpora quadrigemina, optic thalami, and. 



