600 AN ESSAY ON FAT AND MUSCLE. 
receptacle, other absorbent vessels termed lymphatics also termi- 
nate, and empty their contents. The fluid which they convey is of 
nearly the same character as that brought by the lacleals. Almost 
every part of the body is in continual decay, so that we may justly 
say death and decay are constantly 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 by 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 chyle : so that an animal may be said, in a certain sense, 
to live upon its own flesh. The chyle and lymph thus mixed 
together flow into the thoracic duct, by which they are conveyed 
into a large vein — th e jugular — and are sent together by a direct 
and short course to the lungs. 
The 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 re- 
turned by the circulation after repairing and renovating the tissues, 
here comes in contact with the atmospheric air, which is principally 
composed of two gases, oxygen and nitrogen, in the proportion of 
21 parts of the former and 79 of the latter, beside the watery 
vapour with which the atmosphere is always more or less 
charged. A change immediately takes place, from the dark purple 
which the blood has when it is brought to the lungs, to a bright 
vermilion colour. When the air has produced this effect it is 
found that a certain proportion of oxygen which it had contained 
has disappeared, and 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 says that it is applied to no use in the animal 
economy except diluting the oxygen ; while other chemists, Mulder 
for instance, affirm that nitrogen is both absorbed and exhaled by 
the blood in respiration. 
The question next arises, what becomes of the oxygen that dis- 
appears in respiration, and what is the origin of the carbonic acid 
gas I The blood consists of the same elements as the food which the 
animal consumes, containing, as we have seen, a very large pro- 
portion of carbon and hydrogen : and as carbonic acid gas consists 
of oxygen and carbon, it is evidently the result of the combination 
of the oxygen with the carbon of the food. According to Bous- 
singault, “ a horse consumes in this manner in twenty-four hours 
97j| oz. of carbon, and a milch cow 69-| 9 rt oz. ; and the former requires 
