4i 8 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



ad quartum ventriciilum, or more shortly as 

 the Her. 



The walls of these cavities undergo further 

 changes ; the hind-brain becomes divided into two 

 parts, one of which lies behind, and at a little 

 lower level than the other ; this is the medulla 

 oblongata, and it is directly continuous with 

 the spinal cord. The anterior half, which in the 

 frog is a narrow band, but in man forms a very 

 conspicuous part of the whole mass, is the so-called 

 little brain or cerebellum. The mid-brain does 

 not undergo transverse division ; its upper and late- 

 ral portions form the optic lobes, and the inferior 

 portion the so-called crura cerebri. The most re- 

 markable changes are undergone by the fore-brain 

 vesicle, which buds out a vesicle on either side, the 

 cavities in which are known as the lateral ventricles 

 (cv) ; these lateral outgrowths always become of con- 

 siderable size, and in the higher vertebrates form the 

 chief mass of the brain. They are the cerebral hemi- 

 spheres, and are the seat of the most important of 

 the functions performed by the brain ; they not only 

 increase in size, but by the development of grooves, 

 the presence of which permits an addition to the 

 quantity of grey or ganglionic material altogether 

 out of proportion to the increase in the area occupied, 

 they come to have not only a more complicated sur- 

 face, but also a much higher functional value. 



The cerebral hemispheres are continued anteriorly 

 into the olfactory lobes (Fig. 179 ; o/), and these 

 into the so-called olfactory nerves. More pos- 

 teriorly, the fore-brain gives off another vesicle on 

 either side, and this vesicle travels away from the 

 brain, with which it only remains connected by its 

 stalk ; the vesicle forms the hinder part of the eye, 

 and the stalk becomes the so-called optic nerve. 

 The remainder of the fore-brain forms the thalamen- 



