470 CHORDATA 



striata; the root of this \'esiclc de\'clops no iicr\'ous suhstanco, Init remains 

 a ihin la\'cr of epilhclium closing in tiic ihirj \'cnlriclc aliox'c {III). 

 The lloor is also ihin-walletl between the tlialanii and is pushed down- 

 wards, forming a funneldike pocket, the 'nifuiidibuluin [:). ']"hc mesen- 

 cephalon is divided on ils dorsal side 1>\' a longitudinal groove into a pair 

 of oplii lobes, divided in die mammals 1>\' a transNcrse groo\"e into four 

 corpora qiuuhigcui iiii. In the sanie group the \eiiIricleof the Tuid lirain is 

 reduced to a narrow canal, the dqncJiicI, so that the fourth \"enlricle 

 is the ca\"ity of the hind liraln. 



This hist region is called the itiodiiUa ol'loiigoSa {i)!vcliiioopl:oloii); 

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

 similar structure. It is distinguished from the cord externallv in tliat it 

 graduaUy increases in si.:e in front, while its roof is reduced to a thin 

 epithelium {cliorioid pic.yiis). often torn a\\a\' in dissection, leaving an 

 opening, the fossa rhoiiihoidalis, into the ventricle. In front of (his fossa 

 is the cerebellum, often a tliin transverse nerwnis lamella, but usualh' is 

 a considerable part of the brain, composed of a median 'oorj)!:s and two 

 lateral ocrcbcllar liODiisplicres. 



Although these li\e parts are present in all vertebrates, the appearance 

 of the brain in the various classes is \'erv dilTerent. liecause their relali\'e 

 size and form van" greath'. In the Knver \ertebrates optic lobes and 

 medulla oblongata are disproportionately large, while the cerebrum, and 

 often the cerebellum, are insigniticant in si/e. In the higher vertebrates, 

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

 piarts, the increase in si..;e of the cerelirum lieing proportional to the in- 

 crease in intelligence. The cereliral 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 seal of intelligence, 

 is increased in amount by the development of foKls (gyri). separated by 

 grooves (sulri). Somewhat similar conditions exist in the cerebellum, 

 ^\hich in mammals and birds is, next to the cerebrum, the largest part of 

 the lirain. 



Connected with the 'Iwixt hrain are two problematical organs, one, the 

 epiphysis (Itint'dlis), being dorsab the other, the liypopliysis [piliiilarY boa'v), 

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

 onic mouth. This pocket cuts otT from its source, increases bv buddini,', aiid 

 fuses with parts derived from the end of the infundibuluni to a sin';le twod'obed 

 body. It has been compared with the subneural gland of the Tunicala (p. 4451. 

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

 in many vertebrates the parietal or;;aii. Tn cyclostomes and many reptiles this 

 has the structure of an eye (J>ari,ial eye), and in these, separated from the brain, 



