THE BLOOD. 87 



4. Temperament. But little more is known concerning the connection 

 of this with the condition of the blood, than that there appears to be a 

 relatively larger quantity of solid matter, and particularly of red corpuscles, 

 in those of a plethoric or sanguineous temperament. 



5. Diet. Such differences in the composition of the blood as are due to 

 the temporary presence of various matters absorbed with the food and 

 drink, as well as the more lasting changes which must result from gener- 

 ous or poor diet respectively, need be here only referred to. 



Effects of Bleeding. The result of bleeding is to diminish the specific 

 gravity of the blood; and so quickly, that in a single venesection, the portion 

 of blood last drawn has often a less specific gravity than that of the blood 

 that flowed first. This is, of course, due to absorption of fluid from the 

 tissues of the body. The physiological import of this fact, namely, the 

 instant absorption of liquid from the tissues, is the same as that of the 

 intense thirst w r hich is so common after either loss of blood, or the ab- 

 straction from it of watery fluid, as in cholera, diabetes, and the like. 



For some little time after bleeding, the want of red corpuscles is well 

 marked; but with this exception, no considerable alteration seems to be 

 produce^ hi the composition of the blood for more than a very short time: 

 the loss of the other constituents, including the pale corpuscles, being 

 very quickly repaired. 



VARIATIONS IN THE COMPOSITION OF THE BLOOD, IK DIFFERENT PAKTS 



OF THE BODY. 



The composition of the blood, as might be expected, is found to vary 

 in different parts of the body. Thus arterial blood differs from venous; 

 and although its composition and general characters are uniform through- 

 out the whole course of the systemic arteries, they are not so throughout 

 the venous system, the blood contained in some veins differing remarka- 

 bly from that in others. 



Differences between Arterial and Venous Blood. The differ- 

 ences between arterial and venous blood are these: 



(a.) Arterial blood is bright red, from the fact that almost all its 

 haemoglobin is combined with oxygen (Oxy haemoglobin, or scarlet haemo- 

 globin), while the purple tint of venous blood is due to the deoxida- 

 tion of a certain quantity of its oxyhaemoglobin, and its consequent reduc- 

 tion to the purple variety (Deoxidized, or purple haemoglobin). 



(#.) Arterial blood coagulates somewhat more quickly. 



(c. } Arterial blood contains more oxygen than venous, and less carbonic 

 acid. 



Some of the veins contain blood which differs from the ordinary stand- 

 ard considerably. These are the Portal, the Hepatic, and the Splenic 

 veins. 



Portal vein. The blood which the portal vein conveys to the liver is 

 supplied from two chief sources; namely, that in the gastric and mesen- 

 teric veins, which contains the soluble elements of food absorbed from the 



