IV. VEBTEBRATA. 535 



ture. It is distinguished from the cord externally in that it 

 gradually increases in size in front, while its roof is reduced to a 

 thin epithelium, often torn away in dissection, leaving an opening, 

 the fossa rhomboidalis, into the ventricle. In front of this fossa is 

 the cerebellum, often a thin transverse nervous lamella, but usually 

 is a considerable part of the brain, composed of a median ' vermis ' 

 and two lateral cerebellar hemispheres. 



Although these five parts are present in all vertebrates, the 

 appearance of the brain in the various classes is very different, 

 because the relative size and form of the parts undergo great 

 variations. In the lower vertebrates optic lobes and medulla 

 oblongata are disproportionately large, while the cerebrum, and 

 often the cerebellum, are insignificant in size; in the cerebrum, 

 again, the hemispheres may be smaller than the corpora striata 

 and the olfactory lobes. In the higher vertebrates, on the other 

 hand, the cerebrum and cerebellum far surpass the other parts, 

 the increase in size of the cerebrum being proportional to the in- 

 crease in intelligence. The cerebral hemispheres grow backwards, 

 in man and the apes covering the other parts, while in front the 

 olfactory lobes are carried by a similar overgrowth to the lower 

 surface. Since the capacity of the skull is limited, the cortex of 

 the cerebrum, the seat of intelligence, is increased in amount by 

 the development of folds, gyri, separated by sulci. Somewhat 

 similar conditions exist in the cerebellum, which in mammals and 

 birds is, next to the cerebrum, the largest part of the brain. 



Connected with the 'twixb brain are two problematical organs, one, the 

 epiphysis (pinealis), being dorsal; the other, the hypophysis (pituitary 

 body), ventral. The hypophysis arises like a gland by an outgrowth from 

 the embryonic mouth. This hypophysial pocket cuts off from its source, 

 increases by budding, and fuses with parts derived from the end of the 

 infundibulum to a single two-lobed body. It has been compared with the 

 subneural gland of the Tunicata (p. 509). The epiphysis is an outgrowth 

 from the roof of the brain, from which develops in many vertebrates the 

 parietal organ. In many reptiles this has the structure of an eye (pineal 

 eye), and in these, separated from the brain, but connected with it by a 

 nerve, it lies in a special cavity in the parietal bone, which occurs not only 

 in recent but in fossil forms. Above the eye the skin may be transparent. 



The nerves which come from the brain mostly arise from the 

 region between the mid brain and the spinal cord, especially from 

 the medulla oblongata. The olfactory and optic nerves are an 

 exception, the one arising from the cerebrum, the other from the 

 'twixt brain, but both, and especially the optic, differ so much from 

 the peripheral nerves that they can hardly be classed with them. 



