470 CHORDATA 



striata; the roof of this vesicle develops no nervous substance, but remains 

 a thin layer of epithelium closing in the third ventricle above (///). 

 The floor is also thin-walled between the thalami and is pushed down- 

 wards, forming a funnel-like pocket, the infundibulum (/). The mesen- 

 cephalon is divided on its dorsal side by a longitudinal groove into a pair 

 of optic lobes, divided in the mammals by a transverse groove into four 

 corpora quadrigemini. In the same group the ventricle of the mid brain is 

 reduced to a narrow canal, the aqueduct, so that the fourth ventricle 

 is the cavity of the hind brain. 



This last region is called the medulla obhngata (myelencephalon); 

 it is a prolongation of the spinal cord, and in many respects shows a 

 similar structure. 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 (chorioid plexus), often torn away in dissection, leaving an 

 opening, the fossa rhomboidalh, 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 their relative 

 size and form vary greatly. 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 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 a similar growth for- 

 wards carries the olfactory lobes 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 

 grooves (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 'twixt brain are two problematical organs, one, the 

 epipliyxis (pinealis), being dorsal; the other, the hypophysis (pituitary body), 

 ventral. The hypophysis arises like a gland by an outgrowth from the embry- 

 onic mouth. This 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 

 burly. It has been compared with the subneural gland of the Tunicata (p. 445)- 

 The epiphysis is an outgrowth from the roof of the brain, from which develops 

 in many vertebrates the parietal or^an. In cyclostomes and many reptiles this 

 has the structure of an eye (parietal eye), and in these, separated from the brain, 



