60 MANUAL OF ELEMENTARY ZOOLOGY 



p. 42). This vessel receives a small vesical vein from the 

 bladder, several pairs of vessels from the recti muscles of the 

 abdomen, and a little backward vessel from the heart-wall. 

 It ends in front by passing into the liver and there breaking 

 up into capillaries again. The blood from the stomach, 

 bowel, pancreas, and spleen is gathered up into a great 

 hepatic portal vein, which also breaks up in the liver. Thus 

 the liver has a portal system, which is fed with blood (a) 

 from the dorsal aorta, (p) from the anterior abdominal vein, 

 (c) from the hepatic portal vein, and discharges by the 

 hepatic veins into the inferior vena cava. 



The general course of the circulation in the frog is 



summed up in the table on the opposite page. 

 oh-ou S iatioii he The thick lines indicate venous blood, the 



narrow lines arterial blood. 

 The elaborate arrangements whereby the blood circulates 



through all parts of the body point to the fact 

 the mood. * tri at it is a universal means of transport between 



them. It conveys nourishment from the gut 

 to the rest of the body, bears oxygen from the organs of 

 respiration to the tissues and the waste products of 

 metabolism from the tissues to the organs of excretion, 

 and carries various substances which are secreted into 

 it by the liver and other organs to the regions of the body 

 where they are made use of. It also conveys heat — which 

 is set free by most chemical changes inside, as outside, the 

 body — from the organs where there is much chemical 

 activity, such as the muscles and glands, to those where 

 there is little, such as the skeleton and nervous system, and 

 to the surface of the body, where what is excessive is lost. 

 In some animals, as in man, heat is produced at such a rate 

 that the body is kept at a temperature which is a good deal 

 higher than that of its normal surroundings, and constant. 

 This is not the case with the frog, whose temperature is only 

 a few tenths of a degree above that of the air or water, and 

 varies with it. The frog is therefore said to be "cold- 

 blooded." 



The preceding paragraph must not be taken to indicate 



that the blood comes itself into contact with 



ymp ' the tissues. The blood vessels are completely 



closed, and the tissues are actually bathed by another 



