xm PHYLUM CHORDATA 153 



respectively, are supplied with venous blood. The caudal vein, 

 which brings back the blood from the tail, running, along with the 

 caudal artery through the inferior arches of the vertebrae, divides on 

 entering the abdominal cavity into right and left renal portal veins, 

 which end in a number of afferent renal veins supplying the kidneys. 



The hepatic portal vein (h. port, v.) is formed by the confluence of 

 veins derived from the intestine, stomach, pancreas, and spleen, 

 and runs forwards to enter the liver a little to the right of the 

 middle line. In Hemiscyllium a large branch connects the genital 

 sinus with the intestinal tributaries of the hepatic portal system : 

 the blood from the liver enters the sinus venosus by two hepatic 

 sinuses placed close together. 



Nervous System. The fore-brain consists of a rounded 

 smooth prosencephalon (Fig. 831, V.H.) divided into two lateral 

 parts by a very shallow median longitudinal groove. From its 

 antero-lateral region each half gives off a thick cord, which dilates 

 into a large mass of nerve-matter, the olfactory bulb (L. ol), closely 

 applied to the posterior surface of the corresponding olfactory 

 capsule. The diencephalon (ZH) is comparatively small ; its roof 

 is very thin, while the lateral walls are composed of two thickish 

 masses the optic thalami. Attached to the roof is a slender tube, 

 the epiphysis cerebri or pineal organ (Gp.), which runs forwards and 

 terminates in a slightly dilated extremity fixed to the membranous 

 part of the roof of the skull. Projecting downwards from its 

 floor are two rounded bodies, the lobi inferior es (UL), which are 

 dilated portions of the 'infundibulum. Behind these give off a 

 thin- walled vascular outgrowth the saccus vasculosus (Sv.). 

 Attached to the infundibulum and extending backwards from it 

 is a thin- walled sac the pituitary body or hypophysis cerebri (HS), 

 having on its ventral surface a median tubular body attached at 

 its posterior end to the floor of the skull. In front of the infundi- 

 bulum, and also on the lower surface of the diencephalon, is the 

 optic chiasma, formed by the decussation of the fibres of the two 

 optic nerves. The mid-brain (M H) consists of a pair of oval optic 

 lobes dorsally, and ventrally of a band of longitudinal nerve-fibres 

 corresponding to the crura cerebri of the higher vertebrate brain. 

 The cerebellum (HH) is elongated in the antero-posterior direction, 

 its anterior portion overlapping the optic lobes, and its posterior 

 the medulla oblongata. Its surface is marked with a few fine 

 grooves. The medulla oblongata (NH), broad in front, narrows 

 posteriorly to pass into the spinal cord. The fourth ventricle or 

 fossa rhomboidalis (F. rho.) is a shallow space on the dorsal aspect 

 of the medulla oblongata covered over only by a thin vascular 

 membrane, the choroid plexus : it is wide in front and gradually 

 narrows posteriorly. At the sides of the anterior part of the 

 fourth ventricle are a pair of folded ear-shaped lobes, the corpora 

 restiformia. 



