VIIl] THE XERVOITS SYSTEM 101 



the cerebellum is followed by laek of coordination in nuisciilar 

 movement. 



The pons Varolii is a stout band of nerve fibres passing trans- 

 versely across and in front of the medulla oblongata. It connects 

 together the two sides of the cerebellum. It contains the centre 

 for closing the eyelids in the presence of strong illumination. 



The medulla oblongata or hind bram is a continuation of the 

 spinal cord within the cranial cavity. It differs from the cord 

 in having white matter penetrating the internal grey matter. 

 In the medulla large strands of white fibres in passing back from 

 the cerebral hemispheres cross over one another to the opposite 

 side, and it is for this reason that injury (as in an apoplectic fit) 

 affecting one side of the fore bram produces a condition of 

 paralj'sis, not on the same side, but on the opposite side of the 

 body. The medulla contams nvimerous centres presiding over 

 particular functions, some of which have been referred to in 

 previous chapters. Thus the centres regulatmg the respiratory 

 movements, the beating of the heart, the secretion of saliva and 

 gastric juice, and the movements of the oesophagus, stomach 

 and infest hies are situated in the medulla. 



The cavities of the cerebral hemispheres are termed the 

 lateral ventricles, that of the thalamencephalon is the third 

 ventricle, and that of the medulla is the fourth ventricle. 



The cranial nerves AWth their points of origin may here be 

 enumerated, but for detailed accounts of their respective courses 

 the reader is referred to text-books on anatomy. The nerves 

 arise in pairs and are as follows : 



1. Olfactory, or nerves of .smell, arising from the front of each cerebral 

 hemisphere. 



2. Optic, or nerves of sight, arising from the thalamencephalon. 



3. Ocular-motor innervating some of the muscles of the eyeball and arising 

 from the region of the corpora quadrigemina. 



4. Trochlear, motor nerves supplj'ing the superior oblique muscle of the 

 eyeball, and arising from the same region as the 3rd nerves. 



5. Trigeminal, so-called because on each side they divide into three; they 

 contain sensory fibres for the mouth and tongue, and motor fibres for the 

 muscles used in mastication ; they arise from the pons. 



0. Abducens, motor nerves supplying the external rectus of the eyeball, 

 and arising just behind the pons from the medulla. 



7. Facial, motor nerves supplying the muscles of expression (in face, 

 mouth and lips) and arising immediately behind the trigeminals. 



8. Auditorv, or nerves of hearing, arising from the sides of the medulla. 



