CHAPTER VIII. 



THE CIRCULATION. 



WE have now seen where the general combined body-liquid, the blood 

 and lymph, comes from, and in general terms its composition. We must 

 now try to get an idea of how and for what purpose it goes about the body. 

 Few if any mechanisms with which we are familiar are more complicated 

 than this, or do their work better. The precise mechanism of the human 

 circulation is described by visceral and microscopic anatomy. The 

 chief matters to be fully understood from these other text-books are the 

 heart, the arteries, the capillaries, the lymphatics, and the veins, and the 

 relations of the two sorts of capillaries to the tissue-cells. The heart in 

 man is a double force- and suction-pump of four chambers and a complex 

 system of valves which are marvels of hydraulic perfection. These valves 

 are so constructed as to compel the circulation continually in one direction. 

 There are distinct tubes for the blood and others for the lymph, and both 

 of these sorts of conducting vessels ramify almost endlessly in every 

 minute active portion of the body. It is now thought that the lymph- 

 system is a set of tubes closed in the same sense that the blood capillaries 

 are closed. It is only in a technical, anatomical sense, however, that 

 either the blood-capillaries or the lymph-capillaries are said to be closed 

 tubes. Their walls are so thin and so permeable to solutions or crystal- 

 loids (saline solutions) that the circulating fluid in them is in functional 

 continuity with the liquid in the tissue-spaces. This almost complete 

 unification of the circulating fluid with the more or less similar fluid 

 everywhere between the tissue-cells is an important matter which must 

 be well comprehended. For descriptive purposes these liquids are 

 defined as different, but practically the blood soaks with the utmost 

 freedom into the tissues and the lymph collects just as freely into the 

 lymphatics. The exceedingly thin layer of protoplasm which forms 

 the blood -capillaries and the lymph-capillaries is so permeable (through 

 the processes called filtration and osmosis) that functionally under 

 normal conditions it is almost as if it were not there. 



Besides the blood- and lymph-vessels it is necessary to become thoroughly 

 familiar with the elaborate system of nerves which control the heart and 

 the caliber of the arteries (and possibly the permeability of the capil- 

 laries); but these cannot be described here. There are many things 

 also in the histology of the arteries, veins, lymphatics, and capillaries 

 which it is essential to have in mind as one studies their functions. Again, 

 there are many principles and facts of physics connected with this elabo- 

 rate hydraulic system which should be in mind before studying the 



