32 ZOOLOGY. 



all the veins except those of the abdominal region of the 

 alimentary canal the blood is gathered into the great caval 

 veins, and so to the right auricle and right ventricle. The 

 overflow of plasma, robbed of much of its nutritive material 

 and loaded with katastases (carbon dioxide, urea, etc.), 

 returns by the lymphatics to the precaval veins near the 

 heart, receiving on its way new white blood-corpuscles from 

 the lymph-glands, and abundance of fatty material from 

 the lacteals. 



From the capillaries of the stomach, spleen, pancreas and 

 intestines, the blood with newly absorbed sugar and proteids 

 is gathered into veins which unite into the great portal 

 vein ; this branches out into the capillaries of the liver, 

 where it mingles with the blood from the hepatic artery, 

 and finally, deficient in oxygen, but with its sugar-supply 

 " standardized " it passes by the hepatic vein into the 

 postcaval and thence to the right side of the heart. 



In all the capillaries the blood loses nutritive materials 

 (proteids, sugar, etc.), and, except in those of the lung, 

 oxygen also. In the capillaries of lungs, kidneys, and skin, 

 it most particularly loses waste-materials. It gains new 

 nutritive material most particularly in the portal capillaries 

 and by the inflow of lymph and chyle near the heart re- 

 ceiving waste -products also in the latter case. But it is 

 well to bear in mind the great rapidity of the circulation 

 and the small percentage of the blood that is really affected 

 in each capillary-journey : the variation in composition of 

 the blood in different parts of the body is far smaller than 

 this general statement of gains and losses suggests. 



16. The " Ductless Glands." Finally, we have to note 

 a series of organs of very varied character, in whose 

 capillaries the blood undergoes changes of an important but 

 somewhat mysterious character. These are often collec- 

 tively spoken of as the ductless glands, and several of 

 them belong to the category of structures whose position is 

 meaningless from the point of view of the rabbit alone, but 

 receives surprising elucidation when a wide range of animals 

 is studied, particularly as regards the embryonic develop- 

 ment of these structures. Such are the thyroid gland 



