CHAPTER XX. 

 BLOOD ELABORATING GLANDS. 



IN the preceding chapters we have seen that the blood un- 

 dergoes 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 

 products. 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 



