THE ENCEPHALON. 513 



to be accounted for by the curving of the nerve-roots both toward and away from the 

 brain. By similar experiments it is made probable that motor impressions likewise 

 travel chiefly in the grey matter of the cord. 



Section of one lateral half of the cord is followed by loss of sensation in the opposite 

 hind limb, and of motion in the limb of the side operated on : and a prolonged 

 mesial incision produces loss of sensation in both hind limbs, without paralysis 

 of motion. But in the medulla oblongata, above the decussation of the anterior 

 pyramids, section of one side produces loss of both sensation and motion on the 

 opposite side. From these circumstances it appears probable that the sensory fibres, 

 viz., those of the posterior roots, decussate in the commissure ot the spinal cord, 

 while the motor fibres, those derived from the anterior roots, cross chiefly at the 

 decussation of the anterior pyramids of the medulla oblongata. (For further details, 

 see Brown-Sequard, " Central Nervous System," 1860; also for a succinct account of 

 the subject and for bibliography, J. Be"clard, " Physiologic Humaine," 4th ed., 1862 ; 

 " Carpenter's Human Physiology/' 6th edit., 1865.) 



B. THE ENCEPHALON. 



The encephalon admits of being conveniently divided into the medulla 

 oblougata, the cerebellum with the pons Varolii, and the cerebrum. 



Fig. 348. 



Fig. 348. PLAN IN OUTLINE OF THE ENCEPHA.LON, AS SEEN FROM THE RIGHT SIDE. J 



The parts are represented as separated from one another somewhat more than natural 

 so as to show their connections. A, cerebrum ; /, g, h, its anterior middle and posterior 

 lobes; e, fissure of Sylvius; B, cerebellum; C, pons Varolii; D, medulla oblongata; 

 a, peduncles of the cerebrum ; b, c, d, superior middle, and inferior peduncles of the 

 cerebellum ; the parts marked a, b, c, C, form the isthmus encephali. 



The medulla ollongata is the part continuous with the spinal cord : it 

 rests on the basilar process of the occipital bone, and on its superior or 

 dorsal surface presents a groove continuous with the central canal of the 

 spinal cord. 



The cerebellum occupies the posterior fossa of the cranium. By the 

 mesial part of its anterior and inferior surface, it forms the roof of a space, 



