192 CIRCULATING FLUIDS: 



albuminous constituents, in fats, and salts, in proportion to the 

 riclniess of the nutriment that has been taken, and of the chyle 

 that has been evolved from that nutriment. In order to coun- 

 teract the evils that might arise from an excess of water in the 

 blood, (which, if allowed to remain unchecked, would induce 

 too rapid a solution of the blood-corpuscles,) the kidneys, skin, 

 and lungs exert an active agency; while, on the contrary, if 

 there be a deficiency in the proportion of the water, caused 

 either by too great exhalation, dependent upon excessive fatigue, 

 or by a direct accumulation of the salts (which might impede 

 the solution of the corpuscles) it is immediately indicated by 

 an urgent desire for drink. 



When substances, injurious to life, are taken into the sto- 

 mach, only small quantities enter the blood, the great pro- 

 portion being usually carried off by the intestinal canal, and by 

 the organs of excretion and secretion. If the organism be 

 unequal to the task of rejecting the injurious agent, the equi- 

 librium of the system is destroyed, and death ensues. 



Another cause of the vai'jdng nature of the blood, inter- 

 esting equally to the physiologist and the physician, may be 

 referred to the modifications that it undergoes in the nutrition 

 of the organism, and to the changes undergone by the cor- 

 puscles, in connexion with the processes of secretion and 

 excretion. 



On the distinctive characters of arterial and venous blood. 



The distinctive colours of arterial and venous blood are too 

 well known to require any observation.' 



' [From Scherer's experiments it appears that, when fresh red ox-blood is deprived 

 of its fibrin and diluted with twice or thrice its volume of water, it assumes a dark 

 venous tint, which is not affected by the passage of a current of oxygen through it. 

 On the addition, however, of a little milk, oil, finely-powdered chalk or gypsum, the 

 original bright red colour is evolved. These experiments are sufficient to prove that 

 the bright red colour is dependent on other causes than oxidation, and that the dark 

 venous tint does not arise from carbonic acid or carbon ; in fact Scherer conceives 

 that they prove that the former is dependent on the presence of white particles of 

 chyle suspended in the fluid, an opinion confirmed by the microscope. It was observed 

 by Hewson that, when the colour of the blood is bright red, the corpuscles are always 

 biconcave ; they reflect a large amount of light, and in this respect act as the chalk, 

 milk, &c. in Scherer's experiments. When, on the other hand, the blood is of a 

 dark colour, the corpuscles are biconvex, and the capsule is so thin as to admit Ox 



