THE BULB. 589 



folded in a remarkable manner and often spoken of as cortex. The cere- 

 bellum, though the lateral portions, called the hemispheres, project above 

 the median portion, called the vermis, is, unlike the cerebrum, a single 

 mass ; each lateral half, however, sends down ventrally a mass of fibres 

 which, running transversely, partly end in thepons and partly are continued 

 across the pons into the other lateral half; this mass of fibres, thus con- 

 stituting, as we have said, a considerable part of the pons, forms on each 

 side, just as it leaves the cerebellum to enter the pons, a thick strand, 

 called the middle peduncle of the cerebellum. From the cerebellum there 

 also proceeds backward into the bulb on each side a thick strand of fibres, 

 the inferior peduncle of the cerebellum or restiform body ; and a third 

 strand, the superior peduncle of the cerebellum, passes forward on each 

 side into the region of the corpora quadrigemina. As the cerebellar 

 peduncles converge behind the corpora quadrigemina the angle between 

 them is filled up by a thin sheet of nervous matter, the valve of Vieussens, 

 which thus for a little distance backward forms a roof for the front part of 

 the fourth ventricle, just where the lozenge-shaped cavity is narrowing to 

 become the aqueduct. Behind the cerebellum and pons comes the bulb, 

 which, as we have said, is the thickened floor of the hind part of the hind- 

 brain, the roof of the cavity being here practically wanting. 



Of these several divisions the first division, that of the cerebral hemi- 

 spheres, including the corpora striata, stands apart from the rest by reason 

 both of its origin and the character of its development. As we shall see, 

 this anatomical distinction corresponds to a physiological difference. 



Of the other parts of the brain the crura cerebri deserve special atten- 

 tion. We may regard these as starting in the cord, but largely augmented 

 in the bulb; they traverse the pons, where they are still further increased, 

 and passing beneath the corpora quadrigemina, with which as well as with 

 the cerebellum they make connections, end partly in the region of the optic 

 thalami arid walls of the third ventricle, but to a great extent in the cere- 

 bral hemispheres. We may, in a certain sense, consider the rest of the 

 brain as built upon and attached to these fundamental basal or ventral 

 strands. 



517. Connected with the brain are a series of paired nerves, the 

 cranial nerves. The first and second pair, the olfactory nerves and the 

 optic nerves, differ in their origin and mode of development from all the 

 rest so fundamentally as to cause regret that they are included in the same 

 category. We shall consider these by themselves in due course. The re- 

 maining pairs, from the third pair to the twelfth, forming a much more 

 homogeneous category, we shall also consider in their proper place. We 

 must now turn to study in greater detail some of the structural features of 

 the brain, and we may with advantage begin with the bulb. 



THE BULB. 



518. The spinal cord, as it ascends to the brain, becomes changed 

 into the more complex bulb, partly by a shifting of the course of the tracts 

 of white fibres, partly by an opening up of the narrow central canal into 

 the wide and superficial fourth ventricle, but chiefly by the development of 

 new gray matter. 



When the anterior ventral aspect of the bulb is examined (Fig. 131, C.), 

 it will be seen that the anterior columns of the cord are interrupted for 

 some distance in the median line by bundles of fibres (Py. dec.) which, 

 appearing to rise up from deeper parts, cross over from side to side and so 

 confuse the line of the anterior fissure. This is the decussation of the pyra- 



