MEMBRANES AND VESSELS. 81 



backward around the crura cerebri to the posterior lobes 

 of the cerebrum, which they supply. The arteries are 

 joined near their commencement by the posterior com- 

 mit ni eating arteries from the internal carotid. 



The internal carotid artery (p. 52) supplies the anterior 

 and middle cerebral lobes, and at the base of the brain lies 

 just behind the optic nerves. 



The anterior cerebral arteries are given off from the 

 internal carotids ; they penetrate the fissure between the 

 cerebral hemispheres and supply the inner surfaces of the 

 anterior lobes ; as the two arteries are about to enter 

 this fissure, they are united by a short transverse branch 

 called the anterior communicating artery. 



The middle cerebral arteries, larger than the preceding, 

 pass outward from the internal carotid to enter the fissure 

 of Sylvius and supply the anterior and middle cerebral 

 lobes. Close to their origin, these arteries give off the 

 posterior communicating branch which unites them with 

 the posterior cerebral arteries of the basilar, thus com- 

 pleting the anastomoses of the cerebral arteries, to which 

 the term circle of Willis has been applied. 



The arachnoid and pia mater, together with the vessels, may now 

 be removed by carefully tearing them away with the forceps ; it must 

 be done in such a manner as to respect the origins of the nerves, many 

 of which are small, and so easily confounded with the membranes, that 

 they may be torn away unawares. 



The removal of the membranes gives an opportunity 

 to define better than has yet been done, the different parts 

 of the encephalon, their limits, and the subdivisions into 

 which they are separated. 



The ENCEPHALON consists of the medulla oblongata, pons 

 Yarolii, cerebellum, and cerebrum. The medulla oblongata 

 is the upper part of the spinal cord. The pons Yarolii is 

 a broad band of transverse fibres stretching across the 

 upper part of the medulla oblongata. The cerebellum, or 

 little brain, lies beneath the posterior part of the cerebrum, 

 and is composed of two lateral halves or hemispheres. 

 The cerebrum, or large brain, constituting the principal 

 part of the encephalon, is composed likewise of two hemi- 

 spheres, but is also divided into anterior, middle, and pos- 

 terior lobes ; the anterior and middle lobes are separated 

 by a sulcus called the fissure of Sylvius; the limit between 

 the middle and posterior lobes is indicated by a line cor- 



