88 NATURE AND LIFE. 
in the system of vital operations. The constituent humors, 
the blood, chyle, and lymph, conveying throughout into 
the inmost parts of the tissues and organs those materials 
of nutrition designed to be assimilated, and that oxygen 
fitted to aid the work of assimilation, are eminently the 
vivifying fluids. They bathe the whole system, they pour 
into it ceaselessly new stores of strength and warmth, 
they maintain it in its harmonious and perfect working. 
They are true organic media, intervening between the ex-. 
ternal medium surrounding the individual, and the ana- 
tomical elements lodged deep within the body. They are 
organized, and have the faculty of nutrition, that is, their 
substance is molecularly renewed in a continuous way. 
While the secretions, and the excretions particularly, are 
liquids devoid of life, made by the glands and the paren- 
chyma at the expense of the blood, the blood, so to speak, 
creates itself with the materials it receives as well by way 
of the lungs as by that of the whole digestive canal. The 
blood is a laboratory in which the most varied and elusive 
transformations take place, in very minute intervals of 
time—so minute that it is impossible for the biologist’s 
vision to seize all their phases, and follow their headlong 
successive course. The whole of chemistry of which we ~ 
have any knowledge unfolds itself in this laboratory; but 
another chemistry also moves there in incessant action, of 
whose laws we can but gain a glimpse. In fact, those im- 
mediate principles which pass into the blood in the form 
of fatty substance, of sugary and of albuminoid matter, 
and pass out of it under the form of cholesterine, leucine, 
tyrosine, urea, creatine, etc., do not pass instantly from 
one state to another. During all the course of combustion 
sustained by breathing they undergo a thousand isomeric 
modifications and specific changes, of which we know 
nothing. We seize only the beginning and the end of the 
phenomenon, but the middle course of it evades our view. 
