936 GENERAL STRUCTURE. [BOOK in. 



the middle peduncle of the cerebellum. From the cerebellum 

 there also proceeds backwards 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 forwards on each side into the region of the corpora 

 quadrigemina. As the latter converge towards each other 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 backwards 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 

 hemispheres, including the corpora striata, stands apart from the 

 rest by reason both of its origin and the character of its develop- 

 ment. As we shall see, this anatomical distinction corresponds to 

 a physiological difference. 



Of the other parts of the brain the crura cerebri deserve 

 special attention. 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 

 and walls of the third ventricle, but to a great extent in the 

 cerebral 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. 



604. 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 develop- 

 ment 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 remaining 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. 



