CHAPTER XX. 



BLOOD ELABORATING GLANDS. 



In the preceding chapters we have seen that the blood under- 

 goes important changes as it courses through the different parts 

 of its circuit. Where it comes in contact with the tissues it 

 yields to them nutrient material for assimilation, and oxygen for 

 their metabolism, and carries away from them some waste pro- 

 ducts. In the lungs it receives oxygen and gives off carbonic 

 acid. While it flows through the minute vessels of the alimentary 

 tract some of the materials elaborated by the digestion of food 

 are absorbed and directly added to the blood ; at the confluence 

 of the great veins in the neck the stream composed of lymph and 

 chyle is poured into the blood before it enters the heart, so as to 

 be thoroughly mingled with it on its return from the general cir- 

 culation. Moreover, in various glands, different substances are 

 used in the manufacture of their secretions. 



Thus it is obvious that there is a kind of material circulation, 

 a constant income and output going on in the blood itself as it 

 passes through the different parts of the body. The investigation 

 of the exact changes which take place in the blood in each organ 

 or part is surrounded with difficulty, and in many cases it is quite 

 impossible to ascertain what changes occur. In some parts it may 

 be made out by noting the results produced, or the substances 

 given off or taken up by the blood, as seen in the changes found 

 in the air after its exposure to the blood in the lungs, where we 

 can definitely state that the blood has lost or gained certain mate- 

 rials, and is so far altered. In other parts, such as the muscles 

 or the ductless glands, where, no doubt, profound changes in the 

 blood occur, we have no separate outcome which we can analyze, 

 and we must therefore trust altogether for the elucidation of the 

 change going on in them to the differences which may be found 

 to exist in the blood flowing to, and that flowing from, such an 

 organ. For this purpose one can either examine samples of the 



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