NUTRITION AND WASTE. 13 



Serous Auid, which we find in recent serous cysts (p. 531), and which 

 is secreted by serous membranes, closely resembles, plasma. In health it 

 does not appear to contain any white corpuscles, and consequently, then 

 shows no tendency to coagulate. Under the influence of inflammation, or 

 even of congestion, there is a greater or less numerous migration of white 

 corpuscles, with the result that .their presence in serous fluid gives rise to 

 the deposition of fibrin. It thus differs from serum, which, being deficient 

 of fibrin-forming material, cannot coagulate. Excessive action of serous 

 membranes (t.g. as a result of inflammation) in the head, chest, and 

 testicles, for instance, produces respectively, water on the brain, hydro- 

 thorax (p. 352) and hydrocele {p. 289). 



NUTRITION AND WASTE.— Assuming for the present that immediately 

 before a contraction of the heart, the blood which is in the left ventricle is 

 in a state of comparative purity, it passes with but little alteration through 

 the arteries of general circulation, the walls of which are comparatively 

 impervious to fluids. Having arrived in the capillaries, the pressure on the 

 blood-stream forces a portion of the plasma through the thin walls of the 

 capillaries, so that plasma constantly bathes all the tissues, which take up 

 from it the materials required by them for nourishment and repair. Here, 

 the walls of the capillaries appear to act as a filter to the plasma, in 

 keeping back a large proportion of the fibrin constituents, so that transuded 

 plasma has no tendency, in health, to coagulate in the tissues.. In the 

 capillaries, the red corpuscles part with more or less of their burden of 

 oxygen to the tissues. Under normal conditions, the red corpuscles remain 

 in the blood-vessels ; but a few of the leucocytes pass, from time to time, 

 through the walls of the capillaries into the tissues, probably to furnish 

 fibrin-ferment to the transuded plasma, or to remove wast« matters. In 

 health, excess of plasma and waste products given off by the tissues, are 

 drained away by a system of vessels, called lymphatics, which pour their 

 contents into the veins. The absorbent action of the lymphatics is supple- 

 mented, to some extent, by that of the capillaries and veins.. When the 

 blood arrives in the right ventricle, it is accordingly in an impure condition 

 and has consequently changed the bright scarlet colour which it wore when 

 quitting the left ventricle, to that of a bluish purple. This impure blood, 

 after being pumped from the right ventricle, gives off into the air-cells 

 of the lungs the carbonic acid which it received from the tissues, and taking 

 up a fresh supply of oxygen from the air in the air-cells, arrives bright- 

 coloured and more or less pure in the left auricle, from which it flows into 

 the left ventricle, ready for another circuit. Besides the lungs, other 

 organs, such as the kidneys, skin and liver, remove impurities from the 

 system. i « | 



DISTRIBUTION OF BLOOD IN THE BODY.— Under normal circum- 

 stances, the amount of blood in the body is just sufficient for its require- 

 ments. Hence, if the blood supply be increased or decreased in one part, 

 it will, resipectively, be decreased or increased in the remainder of the 

 system. Thus mental work, or muscular exercise, if indulged in soon after 

 a full meal, will be liable to more or less interfere with the process of 

 digestion, by " drawing " the blood away from the internal organs, to the 

 brain or muscles, as the case may be. The feeling of drowsiness (due to 

 a state of comparative bloodlessness of the brain) experienced after eating, 

 is caused, to a greaA extent, by the blood-vessels which supply the organs 

 of digestion, being unusually full at the time. The feeling of sleep or even 

 faintness when taking a hot bath and the relief of " fulness of blood in the 

 head " by placing the feet in hot water, illustrate the same fact. Here we 

 have an explanation of the benefit — iii cases of inflammation of the organs 

 of breathing, for instance — of warm fomentations to the surface of the body 

 and of counter-irritation (p. 17). 



