FUNCTIONS OP THE BRAIN. 



807 



The basal ganglia likewise depend for their degree of development 

 upon the position of the animal in the zoological series. Thus the 

 corpora quadrigemina are, as their name implies, divided into four emi- 

 nences in all mammals, the anterior pair being larger in herbivora and 

 the posterior in carnivora. 



In birds, reptiles, and fishes they consist only of a single pair of 

 ganglionic masses, and are termed the corpora bigemina, or optic lobes. 



FIG. 349.— Orbital Surface of the 

 Left Frontal Lobe and the 

 Island of Reil, the Tip of 

 the Temporo - Sphenoidal 

 Lobe Removed to Show the 

 Latter. (Landois.) 

 17, convolution of the margin of the longitu- 

 dinal fissure; 0. olfactory fissure, with tho olfac- 

 tory lobe removed ; TR, triradiate fissure : 111 and 

 i"i, convolutions of the orbital surface ; 1, 1, 1, 1, 

 under surface of the infero-frontal convolution: I, 

 nnder surface of the ascending frontal, and, 5, of 

 tne ascending parietal convolutions: C, central 

 lobe, or island. 



Fig. 350.— Brain of Dog, Superior Surface. 

 (Colin.) 



1, 2, 3, 4, the four primary convolutions ; a, centre for superior 

 cervical muscles : ft, for adductors and extensors of anterior ex- 

 tremity; c, for flexors and rotators of anterior extremity; d, 

 motor centre for posterior extremity ; /, centre for facial muscles. 



So, also, the corpora striata in all vertebrates are covered by the cere- 

 bral hemispheres, but in descending the animal series the reduction in 

 the size of the cerebral lobes gives a relatively greater importance to the 

 corpora striata. The size of the optic and olfactory lobes varies in dif- 

 ferent groups of animals, being largest in those in -which the special 

 senses associated with these parts of the brain are most highly developed. 



