i8o VETERINARY PHYSIOLOGY 



The crusta, or anterior parts of each peduncle of the cerebrum, 

 is composed, in its central part, of the pyramidal fibres passing 

 down from the cerebrum to the spinal cord (P.), and, on each 

 side, of the cerebello-cerebral fibres passing upwards from the 

 pons (CO.). The posterior part, or tegmentum, contains 1st. 

 The fillet fibres going partly to the corpora quadrigernina, 

 partly onwards to the thalamus opticus (F. ) ; 2nd. The nuclei 

 of the 3rd and 4th cranial nerves; 3rd. The fibres of the 

 superior peduncles of the cerebellum which cross the middle 

 line (S.C.P.); and 4th. The red nuclei in which most of 

 these fibres end. 



The functions of this segment of the brain are chiefly con- 

 ducting, but the anterior corpora quadrigemina form the 

 shunting station between the incoming fibres of the optic tract 

 and the oculo-motor mechanism (see p. 141). 



F. THE CEREBRUM 

 Structure 



Each crus or peduncle terminates in its half of the cerebrum 

 (fig. 76, p. 164). As the fibres pass from peduncle to cerebrum 

 and vice versa they come into relationship with three masses of 

 grey matter lying in the midst of the cerebrum. These are the 

 thalamus opticus, into which the ingoing fibres enter; the 

 lenticular nucleus, between which and the thalamus the outgoing 

 fibres run ; and the caudate nucleus, the main part of which lies 

 in front of the other two (fig. 91). 



The fibres, above these nuclei, spread out to form the corona 

 radiata and enter a crust of grey matter, the cortex cerebri, 

 which covers over the cerebrum, and which in the higher 

 animals is raised into a number of folds or convolutions 

 marked off from one another by fissures and sulci. 



In the lower vertebrata the differentiation of the cortex from 

 the basal ganglia is incomplete, and it is only in the higher 

 mammals, monkeys and man, that the cortex reaches its full 

 physiological importance. 



The structure of the cortex cerebri as regards the arrangement 

 of cells and fibres is somewhat complex, and varies greatly at 

 different parts, but the general type is as follows (see fig. 95) : 



