Anatomy of the Cat

by Jacob Reighard; H. S. Jennings

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).

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.

In a general view of the cat’s brain from the dorsal side ([Fig. 137]) four subdivisions are discernible. At the caudal end is a small stalk-like portion (IV) which is clearly a slightly modified continuation of the spinal cord; this is the myelencephalon or medulla oblongata. Just craniad of this, rising high above it, is an irregularly lobulated rounded portion, the cerebellum (III). This is produced as a thickening of the wall of the original tube; it forms the dorsal part of the metencephalon. These two portions of the brain have a common cavity, lying within the medulla oblongata and ventrad of the cerebellum, and formed by a widening of the cavity of the spinal cord; it is known as the fourth ventricle. Craniad of the cerebellum, separated from it by a deep transverse fissure, are two very large portions, the hemispheres of the cerebrum (II), separated from each other by a deep longitudinal fissure; they constitute together the telencephalon. The hemispheres are produced by a sort of forking of the original central tube;—by outgrowths on the sides of the tube, the central cavity extending into the outgrowths. The two outgrowths extend dorsad, as well as craniad and caudad from the original place of origin, forming the larger part of the brain; the extensions of the central cavity which they contain are known as the lateral ventricles. Finally, in front of the two hemispheres are seen the two small olfactory bulbs (I) which are mere extensions of the two hemispheres and contains cavities which are extensions of the lateral ventricles.

In a ventral view of the entire brain ([Fig. 138]) certain parts are visible which in the dorsal view are hidden by the large cerebral hemispheres. At the caudal end is seen, as before, the medulla oblongata, or myelencephalon, and craniad of this, on the ventral side of the cerebellum, a broad transverse tract, the pons (i); this, like the cerebellum, forms part of the metencephalon. All this part of the brain formed by the myelencephalon and metencephalon lies caudad of (behind) the rest of the brain and is marked off from it by a great fissure; it therefore receives as a whole the name hindbrain or rhombencephalon.