120 HAND-BOOK OF PHYSIOLOGY. 



ments of the opposite side of the body; the animal falls, and then, strug- 

 gling with the disordered side on the ground, and striving to rise with the 

 other, pushes itself over; and so again and again, with the same act, rotates 

 itself. Such movements cease when the other crus cerebelli is divided; but 

 probably only because the paralysis of the body is thus made almost com- 

 plete. Other varieties of forced movements have been observed, especially 

 those named "circus movements," when the animal operated upon moves 

 round and round in a circle; and again those in which the animal turns 

 over and over in a series of somersaults. Nearly all these movements may 

 result on section of one or other of the following parts; viz., crura cere- 

 bri, medulla, pons, cerebellum, corpora quadrigemina, corpora striata, optic 

 thalami, and even, it is said, of the cerebral hemispheres. 



THE CEREBRUM. 



The Cerebrum (composed of two so-called Cerebral hemispheres) is 

 placed in connection with the Pons and Medulla oblongata by its two 

 crura or peduncles (III., Fig. 326): it is connected with the cerebellum by 

 the processes called superior crura of the cerebellum, or processus a cere- 

 bello adtestes, and by a layer of grey matter, called the valve of Vieussens, 

 which lies between these processes, and extends from the inferior vermiform 

 process of the .cerebellum to the corpora quadrigemina of the cerebrum. 

 These parts, which thus connect the cerebrum with the other principal 

 divisions of the cerebro-spinal system, may, therefore, be regarded as the 

 continuation of the cerebro-spinal axis or column; on which, as a kind 

 of offset from the main nerve-path, the cerebellum is placed; and on 

 the further continuation of which in the direct line, is placed the cerebrum 

 (Fig. 331). 



The Cerebrum is constructed, like the other chief divisions of the 

 cerebro- spinal system, of grey (vesicular and fibrous) and white (fibrous) 

 matter; and,. as in the case of the Cerebellum (and unlike the spinal cord 

 and medulla oblongata), the grey matter (cortex) is external, and forms a 

 capsule or covering for the white substance. For the evident purpose of 

 increasing its amount without undue occupation of space, the grey matter 

 is variously infolded so as to form the cerebral convolutions. 



Convolutions of the Cerebrum. For convenience of description, the 

 surface of the brain has been divided into five lobes (Gratiolet). 



1. Frontal (F. , Figs. 332, 333), limited behind by the fissure of Rolando 

 (central fissure), and beneath by the fissure of Sylvius. Its surface con- 

 sists of three main convolutions, which are approximately horizontal in 

 direction and are broken up into numerous secondary gyri. They are 

 teAied the superior, middle, and inferior frontal convolutions. -In addi- 

 tion, the frontal lobe contains, at its posterior part, a convolution which 

 runs upward almost vertically ("ascending frontal"), and is bounded in 

 front by a fissure termed the praecentral, behind by that of Rolando. 



