236 



ZOOLOGY 



SECT. XIII 



A large post-caved vein (i. v. c.) brings back the greater portion 

 of the blood from the posterior parts of the body ; it is situated 

 somewhat to the right of the middle line, and opens into the sinus 

 venosus between the two hepatic veins. The post caval is present 

 in no other Fishes, but is universal in all the higher classes. Pos- 

 teriorly the posterior cardinal and the post-caval are formed by the 

 bifurcation of a median caudal vein ; close to its origin each re- 

 ceives the efferent renal veins bringing back the blood from the kid- 

 ney. The blood from the pielvic fin is brought back by an iliac vein 

 which divides into two branches. One of these, running forwards and 

 inwards, unites mesially with the corresponding vessel of the op- 

 posite side to form a median abdominal veina, vessel not present in 

 the Fishes, but universal in the Amphibia ; it opens into the sinus 

 venosus. The other branch is the renal portal vein ; after receiving 



tributaries from the posterior region of 

 the body it passes to th^ corresponding 

 kidney. 1 



Brain. The whole brain (Fig. 860) 

 is enclosed in a tough and thick mem- 

 brane, which becomes glandular in two 

 positions on the roof of the diacoele 

 and on that of the metacoele. In the 

 former position this glandular deve- 

 lopment of the enclosing membrane or 

 choroid plexus passes downwards into the 

 diaccele and is developed into a spongy 

 mass which is prolonged forwards to the 

 anterior end of the prosencephalon. 

 The prosencephalon (pros.} presents two 

 elongated hemispheres, which are com- 

 pletely separated except posteriorly, 

 where they are united by a narrow com- 

 missure. The contained cavity is divided 

 into two by the prolongation of the choroid 

 plexus already referred to. The nervous 

 wall of the hemisphere is very thin and 

 is incomplete dorsally and internally. 

 There is a pair of large olfactory lobes 

 (rh.), each with its cavity or rhinocoele. 



The pineal body is situated on the 

 summit of a conical membranous cap 

 on the roof of the third ventricle. The 

 infundibulum developes a pair of lobi 



inferiores. The mesencephalon (meso.) is bilobed, but the division 

 is not strongly pronounced. The cerebellum (cbl.) is very small, 



1 How far this arrangement combines fish-like and amphibian characters will 

 'be best understood at a later stage. 



FIG. 860. Brain of Ceratodus 

 Forsteri, dorsal view. and. 

 auditory nerve ; cbl. cerebellum ; 

 fac. facial nerve ; gl. glosso- 

 pharyngeal; med. medulla ob- 

 longata ; mes. mesencephalon ; 

 oc. oculo-motor nerve ; opt. optic 

 nerve ; pros, prosencephalon ; 

 rh. rhinencephalon ; vg. vagus 

 nerve. (Chiefly after Sanders.) 



