xii PHYLUM CHORDATA 351 



In all the three examples the veins which carry the venous 

 blood towards the heart from the stomach, intestine, and 

 pancreas, unite together to form a large vein, the hepatic 

 portal (Fig. 204, h.p.v.), which ramifies in the substance of 

 the liver and forms the main source of the blood supply of 

 that organ. In the Dog-fish and Lizard, but not in the 

 Rabbit, veins convey blood from the posterior region to the 

 kidneys, forming what is termed a renal portal system (r.p.v.). 



The nervous system is highly developed. The central 

 nervous system consists of the brain and spinal cord. The 

 brain is, as already stated, contained in the cavity of the 

 cranium : the spinal cord, continuous with the posterior end 

 of the brain, extends through the neural canal roofed over 

 by the series of neural arches of the vertebrae. 



The spinal cord is similar in essential respects in all. 

 three examples. It is a cylindrical cord of nerve-matter, 

 having running along the middle of its dorsal surface a 

 fissure, the dorsal longitudinal fissure, and along the middle 

 of its ventral surface, a second fissure, the ventral longi- 

 tudinal fissure. Through its substance from end to end 

 runs a narrow canal the central canal. 



In the brain of the Dog-fish (Fig. 205) the most 

 anterior portion is a thick mass of nerve matter indis- 

 tinctly divided into two lateral portions by a shallow de- 

 pression. This ( VH), is the prosencephalon. A pair of 

 lobes given off from this in front are the olfactory lobes. 

 (L. <?/.). The prosencephalon with a narrow region, dience- 

 phalon or thalamencepKalon (ZH\ behind it, constitute the 

 fore-brain. Behind the fore-brain a pair of oval lobes, the 

 optic lobes, (MH\ constitute the dorsal portion of the 

 mid-brain, which comprises, in addition, a thick mass of 

 longitudinal nerve-fibres, lying below, and connecting the 

 hind brain with the fore-brain. An elongated median mass, 



