494 



PHYSIOLOGY 



CHAP. 



part of these fibres terminate in the external geniculate body ; 

 others which pass ventrally and laterally to the geniculate body 

 enter the pulvinar ; other less numerous fibres take a more 

 medial direction and reach the anterior corpus quadrigendnum. 

 The finer internal branch of the optic tract is the continuation 

 of Gudden's commissure, and therefore contains no optic fibres 

 properly so-called. It enters the internal geniculate body, and 

 through it reaches the posterior quadrigeminal body. 



III. The mid - brain and inter - brain are the parts of the 

 central nervous system which from their situation in the higher 

 animals have been least satisfactorily studied by ordinary physio- 

 logical methods. The results of researches on the lower animals, 



in which the methods of ablation 

 succeed fairly easily, are not 

 directly applicable to mammals 

 and man, in which these segments 

 of the brain probably have a less 

 important or different physio- 

 logical value, owing to the pre- 

 ponderating importance and 

 influence of the other centres, 

 especially the cerebrum and the 

 cerebellum. 



Owing to the incomplete and 

 not infrequently incoherent and 

 contradictory results of experi- 

 ment, we must, therefore, confine 

 ourselves to a critical discussion 

 of the most definite fundamental 

 facts. 



The observations on the effects 

 of total extirpation, either of the 



fore -brain alone, or of the inter- and hind -brain in different 

 classes of vertebrates, are most important to the physiology of 

 these three parts of the brain. Certainly they do not exactly 

 define the functions of the individual centres contained in the 

 parts that are destroyed or preserved; but they undoubtedly 

 place us in a position to form a general conception of their physio- 

 logical significance. The functions lost depend on the segments 

 destroyed ; the functions that remain, on the surviving segments. 

 Arnpliioxus, the lowest type of vertebrate, has no true brain, 

 but the anterior end of the cord is slightly enlarged owing to the 

 presence of a sinus ovalis which is continued into the vertebral 

 canal. This represents a rudimentary brain, which does not 

 differ essentially in structure from the rest of the cord, since it 

 consists of internal white matter and an outer layer of nerve- 

 fibres that run longitudinally. 



FIG. 249. Comparative extent of the retinal 

 areas connected with the direct and crossed 

 bundles of the optic nerve, in fundus of 

 left eye. (Testut.) n, nasal portion 

 connected with the crossed ; t, temporal, 

 with the direct bundle ; re, x, separating 

 line between the two portions. 1, sclerotic ; 

 2, choroid ; 3, retina ; 4, pupil ; 5, fovea 

 centralis and yellow spot. 



