3 86 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



and this fusion has extended to their blood-vessels, their venous channels 

 having come to communicate with one another. As a result of this 

 arrangement, blood flowing forwards in the right posterior cardinal vein 

 when it reaches the level of the front end of the kidney has before it 

 two alternative routes by which it can reach the heart the original 

 somewhat circuitous route by way of posterior cardinal vein and duct 

 of Cuvier, and a new route direct to the sinus venosus through the venous 

 channels of the liver and the hepatic vein. Very naturally, as it seems, 

 the latter direct route has tended in the course of evolution to supplant 



the former. The anterior portion of 

 the right posterior cardinal vein has 

 dwindled away while a wide direct 

 channel has developed through the liver 

 substance, continuous behind with the 

 renal portion of the posterior cardinal 

 and in front with the enlarged hepatic 

 vein. It is this vein, draining the 

 blood from the right kidney and the 

 liver substance, which we now call by 

 the name posterior vena cava. 



It has been mentioned that the renal 

 portions of the right and left posterior 

 cardinal veins communicate here and 

 there. These anastomoses are of much 

 importance for they pave the way for 

 a further step in evolution whereby 

 the blood from the left kidney as well 

 as that from the right drains into the 

 posterior vena cava, and the anterior 

 portion of the left posterior cardinal 

 dwindles away just as the right had 

 done in the Lung-fish. In the higher vertebrates this step foreshadowed 

 in the Lung-fish has actually taken place. 



The chief peculiarities of the brain of the Lung-fishes are these. In 

 correlation with the simplicity of the creature's movements the cere- 

 U'Hum remains very slightly developed, in the form of a slight transverse 

 thickening of the brain roof (Fig. 167, c). In correlation with the small 

 size of the eyes and comparatively feeble vision the roof of the mesen- 

 < < phalon shows hardly any thickening to form optic lobes (t.o). And 

 finally, perhaps in correlation with the high development of the sense 



s.c. 



FIG. 167. 



Dorsal view of brain of Lepidosiren. 



c Cerebellum ; h, hemisphere ; p.b, pineal 



body s.c, spinal cord ; t o, roof of mesen- 



n; l\'y, roof of fourth ventricle. 



Human figures indicate cranial nerves. 



