TRANSPORT OF MATERIALS 135 



continuous work done by the heart requires a copious supply of 

 oxygen and, in fact, a large part of the blood sent out by the beats 

 of the heart is used to feed itself. The next channel in the figure 

 is that through the stomach, intestines, and their attached glands. 

 We notice the fact, to which attention has been directed above 

 (p. 75), that the venous blood from these organs does not pass at 

 once to the great veins, but traverses the liver on its way. Thus 

 products of digestion are subjected to the action of this organ. 

 The liver would thus receive only venous blood, except that pro- 

 vision is made for its oxygen supply by a special artery which 

 proceeds directly to it. The third of the parallel paths represents 

 that of the viscera, whose venous blood does not pass through the 

 liver, and must be imagined to be itself composed of a number of 

 separate channels. Similarly, the lowest one represents a large 

 number of separate parallel paths. 



The Blood 



The blood is a liquid consisting of an immense number of tiny 

 corpuscles suspended in a clear liquid, the plasma. These corpuscles 

 are of two kinds, the red ones containing the haemoglobin, which 

 we have seen to be responsible for the carriage of oxygen and 

 carbon dioxide ; and colourless ones, like small amoebae, which are 

 in much smaller number than the red ones, are called leucocytes 

 (E., p. 223). The chief function of the latter cells is to take up, kill, 

 and digest disintegrating tissues, such as the tail of the tadpole 

 when the frog stage is reached, and also the micro-organisms which 

 invade the body and cause disease. The process is known as 

 phagocytosis, although it is just like the ordinary feeding of the 

 amoeba. It seems probable that these corpuscles may have other 

 functions, but very little is known of these. 



The red corpuscles, as already mentioned, consist almost entirely 

 of haemoglobin, together with water. In vertebrates other than 

 mammals they possess nuclei, and have the general properties of 

 living cells. In mammals they lose the nuclei which are present 

 when the corpuscles are young. It was pointed out in our first 

 chapter that living cells when deprived of their nuclei degenerate 

 and die, so that it is a matter of some difficulty to know whether 

 the red corpuscles are actually to be regarded as living or not. 

 At any rate, they disintegrate after a certain number of clays. 

 The destruction takes place almost entirely in the liver, and the 

 bile pigments are formed from their haemoglobin. These pigments 

 contain no iron, that present in the haemoglobin being taken up 

 by the liver cells. 



New corpuscles are formed in the red marrow found inside a 



