Chap, ii.] THE BRAIK 815 



the whole body is thus concerned ; and there is some foundation 

 for the view that of this part, one portion belongs, so to speak, 

 to the optic tract and another portion to the cortical fibres of the 

 optic radiation. Possibly still another part is concerned in 

 bringing, as we have (§ 500) suggested, visual impulses to bear 

 on the coordination of movements. 



Stimulation of the surface of the posterior pair, besides giv- 

 ing rise to movements of various parts of the body, has in monkeys 

 and some other animals, the singular effect of producing a vocal 

 utterance in the form of a cry or bark. But we cannot make 

 much use of these results for the purpose of drawing conclusions 

 as to what share these bodies take in the whole work of the 

 brain. In the frog, the optic lobes correspond to the two pair 

 of corpora quadrigemina together ; and the cry just mentioned 

 may perhaps be put side by side with the fact that in the frog 

 the optic lobes seem to furnish a mechanism for croaking ; when 

 the optic lobes are destroyed, the characteristic reflex croaking 

 is done away with. The probable connection of the posterior 

 corpora quadrigemina with hearing is also interesting in this 

 connection ; but we have no satisfactory evidence of any special 

 ties between the bodies in question and either the cortical area 

 for phonation or the vocal mechanism in general ; the occurrence 

 of the cry remains so far an isolated fact. 



In frogs, in which the cerebellum is very small, the optic 

 lobes seem to be particularly concerned in the coordination of 

 movements. When the brain is removed by means of a section 

 behind the optic lobes the animal loses the power of balancing 

 itself (§ 474), which it possesses when the section passes in 

 front of the optic lobes ; and injury to the optic lobes produces 

 incoordination of movement and often ' forced movements.' It 

 has been maintained that the loss of coordination is in these 

 cases due to removal of or injury to the central grey matter in 

 the walls of the third ventricle, and not to mere removal of or 

 injury to the optic lobes ; but the whole evidence goes to shew 

 that in the frog and in the bird the optic lobes do play a part 

 in the coordination of movement, though lesions of the central 

 grey matter around the third ventricle, or indeed of the thala- 

 mus or other parts of the tegmentum, may give rise to loss of 

 coordination or to ' forced movements. ' 



In the mammal removal of or injury to the posterior cor- 

 pora quadrigemina does not cause blindness, but may, like a 

 lesion of the anterior pair, give rise to loss of coordination or 

 to forced movements ; the effect, however, is in most instances 

 very temporary. The connection of the anterior pair with 

 vision suggests a clue as to how this -pair takes part in coordi- 

 nation ; but as to how the posterior pair could intervene in the 

 matter we have hardly so much as a hint ; for, even if we admit 

 a connection between them and the sense of hearing, and, 



