The peripheral distribution of the spinal nerves is described later. 2. =The Brain. Encephalon.=--The brain is that portion of the central nervous system that is included within the cranial cavity. It is a direct continuation of the spinal cord, and presents many of the same essential characters as the latter, with great modifications in details. The structure of the brain can best be understood if it be considered as a modified continuation of the spinal cord, and the relation of the parts to the essential parts of the cord noted. The spinal cord is a nearly straight tube, with a central cavity and thick walls. The brain is likewise tubular, with the cavities enlarged or subdivided in places, with the walls greatly thickened, and with a number of bends and constrictions in the tube. The relation of the structure of the brain to that of the spinal cord is most easily perceived by an examination of the brain of some lower vertebrate, as the frog or shark, and such an examination should be made before proceeding to the study of the more complicated brain of the cat. In the following account of the brain of the cat all parts will be described as far as possible in relation with the tubular structure of the brain. The brain will be considered as a hollow structure, having central cavities, and the solid portions will be brought into relation as parts of the roof, sides, or floor of the cavities. The cavities of the brain, forming a direct continuation of the central canal of the spinal cord, are known as =ventricles= (=ventriculi=). [Illustration: FIG. 137.--DORSAL SURFACE OF THE BRAIN. _I_, olfactory bulbs; _II_, cerebral hemispheres; _III_, cerebellum; _IV_, medulla. _a_, præsylvian sulcus; _b_, cruciate sulcus; _c_, sulcus ansatus; _d_, coronal sulcus; _e_, anterior sulcus; _f_, posterior sulcus; _g_, suprasylvian sulcus; _h_, separate parts of the lateral sulcus; _i_, end of splenial sulcus; _j_, vermis of cerebellum; _k_, hemispheres of cerebellum; _l_, roof of fourth ventricle; _m_, first cervical nerve. 1, orbital lobe (or gyrus); 2, sigmoid gyrus; 3, anterior sylvian gyrus; 4, ectosylvian gyrus; 5, suprasylvian gyrus; 6, posterior sylvian gyrus; 7, parts of marginal gyrus; 8, gyrus compositus posterior.]