136 PROCEEDINGS OF THE ACADEMY OF [1881. 



having shown that the external carotid through the sphenoidal 

 branch of its internal maxillary communicates with the carotid 

 rete mirabile, this sphenoidal branch in the Hippopotamus is as 

 large as the internal carotid and plays the part of an " anterior 

 internal carotid." In this connection I may say that it appeared 

 to me that the elevation of the hyoid bone would compress the 

 internal carotid arterj' as well as the external, the common carotid 

 bifurcating between the digastric muscle and the hyoid bone in my 

 specimens. The return of the venous blood to the heart from 

 the head, however, was not impeded in any way, the jugular veins 

 lying to the outside of the muscles which compressed the carotid 

 arteries. The superior mesenteric artery came off the aorta in 

 common with the coeliac, the inferior mesenteric separately. The 

 common trunk of the external and internal iliacs was short. I did 

 not notice any rete mirabile in the arteries of the body or extremi- 

 ties. In this respect the venous system, however, differed very 

 considerably from the arterial. I was struck with the large size 

 of the cutaneous and subcutaneous veins and of the many anasto- 

 moses between them, especially in the extremities, where numei'ous 

 rete exist. Another peculiarity about the venous system in the 

 Hippopotamus is the difference between the superior and inferior 

 vena cavae. The superior being very lai'ge and readily transmit- 

 ting the blood to the heart, whereas the inferior cava, at least that 

 part of it above the diaphragm, is rather small. According to 

 Macalester,^ in Choeropsis a left superior vena cava is partly 

 represented by a small vein. As Gratiolet first showed, there 

 is found in the walls of the vena cava above the diaphragm a cir- 

 cular band of muscular fibres which in contracting will entirely 

 or partially constrict the vessel. The effect of such action is that 

 the blood in the inferior cava is prevented returning to the heart. 

 The circular muscular band in the Hippopotami examined by me 

 was ^ an inch broad. Such a disposition of the vena cava is also 

 seen in the SeaP and in some other mammals which habitually 

 remain under water for a certain length of time.^ Below the 

 diaphragm the vena cava was very much dilated, while the open- 

 ings into it of the hepatic veins were enormous. It will be seen 

 from the above that while the venous blood readily returns from 



1 Op. cit., p. 495. 



2 Burrow, MuUer's Archiv, 1838. 



^ Mihie Edwards' Physiologie, Tome lit, p. 594. 



