THE FLUIDS OF THE BODY 59 



lymph contains more sugar than the blood. If an excess of 

 protein, whether of a kind foreign to the blood or its own 

 serum-albumin, be injected, it is removed by the kidneys. 

 The blood has various sources from which it can draw out 

 reserves of anything that is lacking, and various ways of 

 getting rid of anything that is in excess. It draws upon the 

 lymph in the tissue-spaces for water. It discharges salts into 

 the lymph. It also takes salts from the lymph. It draws 

 upon the liver for sugar, and probably for proteins also. In a 

 starving animal the blood still contains sugar long after fresh 

 supplies have ceased to reach it from the intestines. The 

 lungs remove its carbonic acid. The kidneys free it from 

 everything which cannot be otherwise removed. It is essential 

 to the well-being of the organism as a whole that a uniform 

 standard of composition should be maintained by the blood. 



Composition. The structural composition of the blood, and 

 the relation of its several constituents to each other, is best 

 studied under the microscope. A thin transparent membrane 

 in which blood is circulating through small vessels the web 

 between the toes of a frog's foot, the mesentery, the membrane 

 of a bat's ear affords an opportunity of observing blood in 

 circulation. In any of the smaller vessels, whether artery or 

 vein, a column of red corpuscles is seen moving in the axis 

 of the stream. This column is surrounded by a layer of clear 

 plasma. Amongst the red corpuscles a few leucocytes may be 

 detected floating placidly down the current. Others are seen 

 in the peripheral layer of plasma, tending to creep along the 

 wall of the vessel rather than submit to be moved forward, as 

 passive objects, by the current. If an irritant be applied to 

 the membrane, the vessels dilate ; yet, notwithstanding their 

 wider calibre, the current becomes slower. The red corpuscles 

 mass together. Apparently their constitution is slightly 

 altered by this commencing inflammation, in such a manner 

 that they cease to be clean, independent discs which slide past 

 each other like small boats on a river ; they exhibit a tendency 

 to stick one to another. In the capillary vessels leucocytes 

 may now be observed, not merely creeping along the inner 

 surface of the endothelium, but squeezing themselves between 

 its scales ; making their way out of the vessel into the tissue- 

 spaces through which the vessel passes. Such an observation 



