THE NERVOUS SYSTEM. 649 



movements. A harmonious combination of muscular actions must depend as 

 much on the capability of appreciating the condition of the muscles with regard 

 to their tension, and to the force with which they are contracting, as on the 

 power which any special nerve-centre may possess of exciting them to contrac- 

 tion. And it is because the power of such harmonious movement would be 

 equally lost, whether the injury to the cerebellum involved injury to the seat 

 of muscular sense, or to the centre for combining muscular actions, that ex- 

 periments on the subject afford no proof in one direction more than the other. 



Forced Movements. The influence of each half of the cerebellum 

 is directed to muscles on the opposite side of the body ; and it would 

 appear that for the right ordering of movements, the actions of its two 

 halves must be always mutually balanced and adjusted. For if one of its 

 crura, or if the pons on either side of the middle line, be divided, so as 

 to cut off from the medulla oblongata and spinal cord the influence of one 

 of the hemispheres of the cerebellum, strangely disordered movements 

 ensue (forced movements). The animals fall down on the side opposite 

 to that on which the crus cerebelli has been divided, and then roll over 

 continuously and repeatedly; the rotation being always round the long 

 axis of their bodies, and generally from the side on which the injury has 

 been inflicted. The rotations sometimes take place with much rapidity; 

 as often, according to Magendie, as sixty times in a minute, and may last 

 for several days. Similar movements have been observed in men ; as by 

 Serres in a man in whom there was apoplectic effusion in the right crus 

 cerebelli ; and by Belhomme in a woman, in whom an exostosis pressed 

 on the left crus. They may, perhaps, be explained by assuming that 

 the division or injury of the crus cerebelli produces paralysis or imper- 

 fect and disorderly movements of the opposite side of the body; the 

 animal falls, and then, struggling 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 complete. 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 cerebri, medulla, 

 pons, cerebellum, corpora quadrigemina, corpora striata, optic thalami, 

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



FUNCTIONS OF THE CORPORA QUADRIGEMINA AND GENICULATA. 



The corpora quadrigemina are the homologues of the optic lobes in 

 birds, amphibia, and fishes. The anterior pair may be regarded as the 



