24 ANATOMICAL CONSIDERATIONS. 



blood back to the heart, whence it is forced into the lungs, and there 

 revivified or rendered equal to its many forms of nutrition. This mighty 

 change is very simply effected. When the thorax expands, air merely 

 enters the lungs to anticipate the vacuum, which otherwise must be 

 occasioned by the enlargement of the chest. The air consists chiefly 

 of two substances in a gaseous state — of oxygen and of nitrogen. The 

 venous blood, being very near to the inhaled air within the lungs, ex- 

 tracts the oxygen from it, and in exchange sends forth a quantity of 

 carbonic acid, which is voided with the expired breath. 



This change will take place when blood is extracted from the body. 

 If the contents of some vein are exposed to the atmosphere, they will 

 in time change from a deep modena to a bright scarlet hue. There is, 



A DIAGRAM, EXPLANATORY OP THE SPECIAL PROVISIONS DISCOVERABLE IN THE HEAD OF A HORSE. 



1. The nostril leading direct to — 2. The larynx, situated at the commencement of the windpipe. 3 The 

 tongue. 4. The oesophagus or gullet. 5. The soft palate, which lies upon the tongue and affords a resting- 

 place whereon reposes the epiglottis, or the guardian cartilage to the entrance of the larynx (2). 6. The 

 guttural pouches, or large membranous and open sacs, containing nothing but atmospheric air. 7. Nasal 

 or frontal sinuses. 



however, this difference which marks the two processes. The altera- 

 tion, when quickened by vitality, is instantaneous; but the change, 

 when it ensues under human inspection, is slowly, and, as it were, labo- 

 riously accomplished. The size of the equine nostrils informs us of the 

 ample draughts of air which the animal is fitted to appropriate ; it like- 

 wise testifies to the high state of that vitality which could necessitate 

 such a provision. Creatures with small nostrils, for instance ox and 

 dog, are endowed with a limited capacity as respects nasal respiration. 

 Yet, as a recompense, such creatures are formed to inhale through the 

 mouth. • The horse, however, requires no such faculty, its nostrils are 

 ample; and, under ordinary circumstances, the mouth is closed by a 



