xii. 21 BLOOD OF AMPHIBIA 339 



produced. Clearly, however, it is essential for this mechanism that 

 the pressure of the ventricular heart-beat shall exceed the colloid 

 osmotic pressure of the blood. This it does by about three times in the 

 frog. 



The lymphatic system consists of a set of spaces which, in the frog 

 at least, communicate with the tissue spaces around the capillaries. 

 Injection of gum into the lymphatic system, by increasing the colloid 

 osmotic pressure in the tissue spaces, prevents the back suck of fluid 

 into the venules and hence leads to swelling of the part injected. The 

 lymph spaces in the tissues join to form larger channels and great 

 sinuses, such as that below the loose skin of the back of the frog. The 

 lymph is kept circulating by the action of lymph hearts. In the frog 

 there are anterior and posterior pairs of these, opening into veins. The 

 more posterior pair lies on either side of the coccyx and can be seen if 

 the skin is removed. The lymphatic vessels also assist in the process 

 of repair. If, after injury, red cells come to lie in the tissues the lym- 

 phatics send out sprouts for as far as -^ mm to pick them up and 

 return them to the blood-stream. 



21. The blood of Amphibia 



The red corpuscles of amphibia are much larger than those of 

 mammals, reaching in the urodele Amphiuma the immense size of 

 yofx; they nearly always exceed 20 fi. The red cells are formed mainly 

 in the kidney, and are destroyed, after a life of about 100 days, by the 

 spleen and liver. The bone-marrow is a source of red cell formation 

 in Ra?ia temporaria but not, except during the breeding-season, in 

 R. pipiens. A process of breaking up of the red cells occurs after they 

 have entered the blood-stream, giving a number of enucleated frag- 

 ments, and this, when the part remaining with the nucleus is small, 

 produces a result like the extrusion of the nucleus during the develop- 

 ment of the red cell of mammals. In Rana only small portions of the 

 cytoplasm are broken off in this way, but in Batrachoseps a large 

 proportion of enucleated corpuscles is produced. 



The haemoglobin of the frog has a lower affinity for oxygen than 

 that of mammals, even when both are considered at the same tempera- 

 ture, and in this respect is notably less efficient. Also, although the 

 power of the blood to combine with carbon dioxide is great, there is a 

 less delicate regulation of the reaction of the blood than in mammals. 



The white cells of amphibia are of three types, lymphocytes, with a 

 large nucleus and small cytoplasm, monocytes, which are larger 

 phagocytic macrophages, and polymorphonuclear granulocytes. 



