212 THE DOG IN DISEASE. 



in a collection of nerve-cells or center in that portion of 

 the brain (medulla oblongatd) just anterior to the spinal 

 cord. This center is accessible to the blood, and may be 

 influenced by nervous connections of the most extensive 

 kind, so that the breathing reflects the changes that are 

 taking place elsewhere in the body. 



The essence of respiration is the interchange of the 

 oxygen of the air and the carbon dioxide (CO a ) of the 

 blood ; but beyond that there can be no doubt that many 

 poisonous substances are eliminated from the blood by the 

 lungs. 



The blood is exposed in the lungs in extremely minute 

 blood-vessels (capillaries), which are everywhere distrib- 

 uted over the air-cells, so that the whole process may be 

 said to be resolved into the exposure of blood to the air 

 by the intervention of the cells that compose the capillary 

 blood-vessels, the basement membrane of the lungs and 

 the cells covering it all of which are of the very thinnest. 



The retention of the carbon dioxide and the poisonous 

 excreta referred to stupefies and poisons the animal, while 

 the withholding of oxygen from the blood starves the tis- 

 sues, all of which constantly require it for their very exist- 

 ence, and gives rise to a feeling of distress which we can 

 ourselves artificially produce by holding the breath. The 

 oxygen is carried to all parts by the coloring matter of the 

 red blood-corpuscles, which is lighter or darker according 

 to the amount of oxygen it retains. Such facts enable us 

 to understand many of the phenomena of inflammation 

 and other affections of the respiratory tract. 



The principal inflammatory diseases of the tract are 

 laryngitis^ bronchitis, influenza, pneumonia, and pleurisy. 



