THE FROG 65 



from the skin, but also, as its name implies, from the muscles of the 

 side of the head and body. The common vein is known as the sub- 

 clavian, as it is situated just inside the clavicle and it goes inwards 

 and slightly forwards in the direction of the heart. 



The external jugular, the innominate and the sub-clavian veins 

 all unite, often at one point, but sometimes the first two join a very 

 short distance before flowing into the third, and give rise to the 

 pre-caval vein, or the vena cava anterior. The pre-caval vein on 

 each side enters the sinus venosus at its anterior corner. 



From each kidney arise four or five renal veins (venae 

 renales revehentes), and the two sets unite in the middle line to form 

 the inter-renal portion of the post-caval vein, or vena cava posterior. 

 Into the anterior of these renals flows a genital vein (spermatic in 

 the male and ovarian in the female), coming from the gonad and fat 

 body. The post-caval vein then runs forward, parallel with and 

 immediately ventral to the dorsal aorta to the liver. It goes on 

 partly embedded in the substance of this gland to which it gives 

 no branches, but from which it receives the wide short hepatic 

 vein on each side, and then enters the posterior corner of the 

 sinus venosus. 



It will be seen from the foregoing account that a number of 

 parts of the body have not been included in those drained by the 

 vessels entering the sinus venosus. The blood from these portions, 

 instead of being conveyed straight to the heart, passes first by special 

 veins to another organ, the kidney or liver, in which these veins 

 break up into capillaries, so that both these glands have a double 

 blood supply, venous and arterial. These capillaries are greatly 

 distended, for which reason they are sometimes called sinusoids, 

 and form comparatively large vessels with extremely thin walls, 

 so that the blood they contain is brought into close proximity to 

 the active secretory cells of these two organs. Such an arrangement, 

 whereby the blood is collected up by a vein which then takes it to 

 the capillaries of one of the organs of the body, is termed a portal 

 system, so that in the frog we have renal portal and hepatic portal 

 systems.* 



The blood conveyed by these systems to the organs is, of course, 

 taken from them by the renal and hepatic veins respectively. 



The blood from the hind limb is collected by two veins, the femoral 

 and the sciatic. The femoral, returning blood from the major part 

 of the leg, is situated on the anterior dorsal side of the thigh and runs 



* The portal arrangement is not to be confused with that met with, for 

 example, in the carotid " gland," where a vessel, usually an artery, breaks up 

 into a network of small branches or even capillaries, but then reunites and 

 continues as a single trunk. This is often designated a rete mirabile. 



