250 
An Essay on Fat and Muscle. 
animals. Dr. ]\Iarcet imagines that the former is less abundant 
in albumen than the latter, and Liebig informs us that all the com- 
pounds of proteine, absorbed during the passage of the chyme 
through the intestines, take the form of albumen. The chyle is 
absorbed from the inner surface of the intestines by a set of vessels 
termed lacteals, \vhich commence by very minute orifices in incal- 
culable numbers, and unite successively into larger and larger 
vessels till they form trunks of considerable size, which empty their 
contents into a receptdcle for that purpose, forming the entrance 
of the thoracic duct. In this receptacle, other absorbent vessels 
termed lymphatics also terminate, and empty their contents. The 
fluid they convey is of nearly the same character as that brought 
by the lacteals. Almost every part of the body is in continual 
decay, so that we may justly say that death and decay are con- 
stantly going on in every living body, and are essential to the 
activity of its functions — a quantity of organised matter being 
continually removed, and replaced by that which is newly formed. 
Of this, a portion is doubtless unfit to be retained within the body, 
and is cast out l)y the various processes of excretion; but it appears 
that another portion of it may again be made use of, and is, 
in fact, taken up by the lymphatics, and brought to the central 
receptacle to be mixed with the newly absorbed ch\le : so that an 
animal may be said in a certain sense to live upon its oAvn flesh. 
The chyle and lymph thus mixed together flow into the thoracic 
duct, by which they are conducted into a large vein — the jnyular — 
and together are sent by a direct and short course to the lungs. 
7. i lie changes which result from the passing of the blood 
through the lungs form a very important part of the process 
of nutrition. The blood, the newly formed as well as that which 
has been returned by the circulation, after repairing and reno- 
vating the tissues, here comes in contact with the atmospheric 
air, which is principally composed of two gases, oxygen and nitro- 
gen, in the proportion of 21 parts of the former and 79 of the 
latter, besides the watery vapour with which the atmosphere is 
always charged more or less; and a change immediately takes 
place, from the dark purple hue which the blood has when it is 
brought to the lungs, to a bright vermilion colour. \\ hen the 
air has produced this effect, it is found that a certain proportion 
of oxygen which it had contained has disappeared, and tliat the 
place of the oxygen is almost wholly supplied by an addition of 
carbonic acid gas, together with a quantity of watery vapour. 
With regard to the nitrogen of the atmosphere, Liebig savs it is 
applied to no use in the animal economy except diluting the oxy- 
gen ; whilst other chemists, Mulder for instance, affirm that 
nitrogen is both absorljcd and exhaled by the blood in res])iration. 
The question next arises, what becomes of the oxygen which dis- 
