584 THE NERVOUS SYSTEM 



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 action 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 min- 

 ute, 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 imperfect and 

 disorderly movements of the muscles of the opposite side of the body. 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 movement," 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 also result on section of 

 one or other of the following parts: viz., medulla, pons, cerebellum, corpora 

 quadrigemina, corpora striata, optic thalami, and even, it is said, of the 

 cerebral hemispheres. But these structures are parts that involve tracts 

 used in the co-ordination of complex muscular movements. 



THE MID-BRAIN. 



The mid-brain includes the crura cerebri and the corpora quadrigemina. 



The Peduncles of the Cerebrum, or Crus Cerebri. The crura 

 diverge from the anterior edge of the pons Varolii and pass upward on either 

 side toward the cerebral hemispheres. At their anterior termination each 

 of them appears to have upon its dorsal surface, to the inner and outer sides, 

 respectively, two large masses of gray matter which have been already spoken 

 of, viz., the optic thalamus and the corpus striatum. The crus is made up 

 of two principal parts. The crusta or 'pes is in the ventral position, and the 

 tegmentum in the dorsal position. The two are separated by the substantia 

 nigra. 



The pes consists of longitudinal fibers which pass anteriorly between the 

 optic thalamus and the posterior part (lenticular nucleus) of the corpus stria- 



