284 Elementary Biology. 



the plasma is expressed. We might therefore classify the 

 various constituents of blood thus : 



Blood 



Corpuscles Plasma 



Colourless Red Fibrin Serum 



Coagulum 



The two most important functions of blood have been 

 mentioned above ; it will be sufficient to add here that the 

 plasma performs the double duty of carrying nutritive 

 material to the various tissues, and of removing from them 

 many of the waste products always being formed there 

 during life, while on the colouring matter of the red cor- 

 puscles, haemoglobin (p. 21), devolves the duty of carrying 

 oxygen gas from the exterior to the tissues. 



Naturally, therefore, a considerable difference exists 

 between blood passing to the tissues from the heart (ar- 

 terial blood) and blood passing from the tissues to the 

 heart (venous blooi). Arterial blood contains about 17 % 

 of oxygen gas (by volume), while venous blood contains 

 only 6 %. Similarly arterial blood contains about 30 % of 

 carbonic acid gas (by volume), while venous blood may have 

 as much as 40 % to 45 %. There are also other points of 

 difference between arterial and venous blood, the most 

 noticeable of which is the bright scarlet colour of arterial, 

 contrasting in this respect with the dull red of venous blood. 



The red corpuscles are derived from the white in a manner 

 not yet accurately determined. The white in their turn are 

 developed in a variety of situations in the body, chief amongst 

 which are the so called blood-glands. 



The heart, or pump which drives the blood through the 

 vessels, is a hollow, muscular, pyramidal sac, lying in the 

 anterior part of the body-cavity known in the higher animals 

 as the thorax, where indeed it forms a distinct subdivision 



