250 MANUAL OF AGPJCULTURE. 



of the atmospheric air which they inhale, and having undergone 

 the consequent important change of constitution it passes next 

 into veins communicating directly with the left auricle, having 

 thus completed the puhuonary circulation. The vessels leading 

 from the ventricles are arteries ; into the auricles, veins. Both 

 orders of vessels are connected by means of the delicate capil- 

 laries so as to form a continuous channel. The blood is kept 

 circulating in one direction by means of the valvular arrange- 

 ment, which prevents the backward impulse when the auricles 

 and ventricles contract. Blood suitable for the demands of tissue 

 supply is the arterial blood only; when that function has been 

 discharged it becomes venous blood. Thus the blood in the 

 veins leading from the pulmonary circulation to the heart, and 

 thence to the systemic capillaries, is arterial ; the blood flowing 

 into the ri^jht auricle, and thence through the ri^ht ventricle to 

 the lungs, is venous. Arterial blood is of a bright scarlet hue ; 

 venous, dark purple. For which difference the reason would 

 appear to be that, whereas the colouring matter, which contains 

 a large proportion of iron, is fully ox'idised in the case of arterial 

 blood, in venous blood, on the other hand, it has parted with 

 much of its oxygen during its passage through the capillaries 

 into the veins. 



Blood absorption consists in the taking up and conveyance of 

 new material to supply the continual wants of tissue waste, and, 

 in addition, the removal of this waste, which otherwise would 

 effect the destruction of the parts throwing it off. The other 

 absorbents besides the blood-vessels are the lacteals and lym- 

 phatics. The lacteals are confined to the intestinal canal ; the 

 lymphatics are distributed through all parts of the body where 

 the presence of blood-vessels occurs. Both alike are connected 

 with and pass through the lymphatic glands, which are supposed 

 to have the power of further organising the compounds absorbed 

 by the vessels. The absorption of substances received into the 

 alimentary canal is effected by the blood-vessels and lacteals. 

 Blood-vessels absorb all soluble matter, whether albuminous or 

 saccharine, &c., directlv throuoii the enclosing!- membranes of the 

 vessels. Hence, fluids and matter in solution can be absorbed 

 by the blood-vessels before the passage of food from the stomach. 

 The other two sets appear to have an affinity for particular sub- 

 stances. The lacteals chiefly absorb oleaginous substances, and 

 their contents are then called chyle. In the intestines are 

 numerous processes called villi, consisting of a network of 

 minute blood vessels surrounding one or more lacteals, and all 

 covered with mucous membrane like the other portions of the 

 intestinal canal ; and here it is that absorption principally takes 

 place, although it does so more or less at all parts, even to the 

 canal's extremity. It is still matter of doubt and conjectm^e as 



