418 THE HORSE. 
and axillary veins correspond with the arteries of the same name, the 
divisions of the latter contained within the foot being described with that 
organ. The left axillary vein receives the contents of the thoracic duct 
which opens into it close to its junction with the vena cava. 
THE POSTERIOR VENA CAVA commences by the junction of the two 
common iliac veins (each made up of an external and internal iliac cor- 
responding to the arteries of the name). It is then joined by the lumbar 
veins, the spermatic and renal veins, after which, and close to its termina- 
tion, the hepatic and phrenic veins empty themselves into it. 
THE PULMONARY VEINS, commencing with eight trunks as they emerge 
from the lungs, soon unite into four, in which number they enter the left 
auricle. They carry arterial blood, and differ in this respect from all the 
other veins of the body, as has been already mentioned. 
PHYSIOLOGY OF RESPIRATION. 
Tur ESSENCE of the act of breathing consists in the absorption of 
oxygen from the air, and the excretion of carbonic acid from the blood 
which is circulated through it. In a state of rest this interchange must go 
on with regularity, for carbonic acid is constantly developed by the decay 
of the tissues, arising from the peculiar necessities of the muscular and 
nervous tissues, and by the conversion of the carbon of the food which 
appears to be required for the development of heat. But when the 
muscles of the whole body are called into play with unusual rapidity ane 
force, the development of carbonic acid is largely augmented, and thus, 
uot only is there a necessity for extra means of excreting the carbonic 
acid, but there is also a demand for more oxygen to unite with the carbon, 
which is the result of the disintegration of the muscular fibres employed. 
Hence the acts of respiration are more complete and rapid during exercise 
than in a state of rest, and while much more carbonic acid is given off, a 
greater volume of oxygen is absorbed from the air which is inspired. 
Tr IS FOUND BY EXPERIMENT that if venous blood is exposed to the 
action of oxygen, through a thin membrane such as bladder, it absorbs 
a portion of that gas, and changes its colour from dark red to a bright 
scarlet. This is in accordance with the recognised laws of endosmose and 
exosmose; and as the blood circulates in very fine streams within the 
vessels of the lungs, whose walls are much thinner than an ordinary 
bladder, it may readily be understood that it is placed in more favourable 
circumstances for this interchange of gases than when tied up in a large 
mass within a comparatively thick membrane. On examining the struc- 
ture of the lungs, they are found to be made up of a pair of cellular sacs, 
communicating with the trachea, which admits air into them; and 
these sacs are furnished with a fine network of capillary vessels distri- 
buted on their walls, and on those of the numerous cellular partitions of 
which they are composed. Thus the blood, as it enters the lungs im a 
venous state, is submitted under very favourable circumstances to the 
agency of atmospheric air ; it readily absorbs the oxygen while it gives off 
large volumes of carbonic acid gas, the result of the combination of pre- 
viously absorbed oxygen with the carbon given off by the various organs 
of the body already alluded to. 
THE EXACT CHEMICAL CHANGES which have taken place in the atmo- 
spheric air exhaled from the lungs and in the blood itself are believed to 
be as follows—1. A certain portion of oxygen has disappeared from the 
air. 2. It has received a considerable volume of carbonic acid. 3. It 
