376 THE NERVOUS SYSTEM. 



somewhat larger and of triangular shape, are the ''hemispherical gan- 

 glia," corresponding to the hemispheres, or cerebrum in the higher 

 classes. Immediately following them are two quadrangular masses 

 ( 33 ) which give origin to the optic nerves, and are therefore called 

 the optic ganglia; but in some of the higher animals, where they are 

 imperfectly divided into four nearly equal parts, they are known as the 

 "tubercula quadrigemina." Behind them is a triangular collection of 

 nervous matter ( 4 ), the cerebellum. Finally, the upper portion of the 

 cord, just behind and beneath the cerebellum, is enlarged into a double 

 oblong mass ( 6i5 ), the medulla oblongata. 



In birds the cerebrum is comparatively larger, and nearly or quite 

 conceals the optic tubercles in a view taken from above. The cerebellum 

 is well developed, and presents on its surface a num- 

 FIG. 99. k er O f transverse foldings or convolutions by which 



its gray substance is increased in quantity; and 

 it extends so far backward as to completely cover the 

 medulla oblongata. 



In quadrupeds, the cerebrum attains a still greater 

 size, as compared with remaining parts of the brain, 

 and in the more highly developed orders it is so much 

 BRAIN OF PIGEON. increased as partly to cover the olfactory ganglia 

 Profile view. i. j n front and the cerebellum behind ; its surface at 



Cerebrum. 2. Optic ,. _,. _,. , ,. 



Tubercle. 3. Cere- tne same time presenting numerous convolutions. 



beiium. 4. Optic it also contains near its base, on each side the 

 oblongata. l median line, two additional collections of gray sub- 



stance; namely, the "corpora striata " and "optic 

 thalami." These bodies are often designated as the " cerebral ganglia," 

 since they occupy the inferior parts of the cerebrum, and receive the 

 tracts of white substance entering it from below. The cerebellum in 

 quadrupeds is enlarged by the development of its lateral lobes, and is 

 marked by abundant transverse convolutions. 



In man the cerebrum reaches its highest development, and prepon- 

 derates completely over all the remaining nervous centres. In the 

 human brain, accordingly, when viewed from above, there is nothing 

 to be seen but the convoluted surface of the hemispheres ; and even in 

 a posterior view they cover everything but a portion of the cerebellum. 

 The remaining parts, however, although concealed by the cerebrum 

 and cerebellum, participate in the structure of the encephalon, forming, 

 as in the lower animals, a series of associated nervous centres, with 

 connecting tracts of nerve fibres. 



As the spinal cord enters the cranial cavity, it enlarges to form the 

 medulla oblongata. This portion of the cerebro-spirial axis is dis- 

 tinguished from the cord below, not only by its form, but also by the 

 arrangement of its gray and white substance. The gray substance, 

 which in the cord presents on each side the projections of the anterior 

 and posterior horns, recedes, in the medulla oblongata, in a backward 

 direction, expanding into a continuous layer at its posterior surface. 



