BOOK III. 



Respiratory Apparatus. 



The maintenance of life in animals not only requires the absorption of the 

 organizable and nutritive matters conveyed to the internal surface of the 

 digestive canal, but demands that another principle — the oxygen of the atmo- 

 sphere — should enter with these materials into the circulation. In animals with 

 red blood, this element, in mixing with the nutritive fluid, commences by 

 expelling an excrementitial gas — carbonic acid — and communicating a bright red 

 colour to that fluid, with which it circulates ; it is brought into contact, in the 

 general capillary system, with the minute structures of the various apparatuses, 

 exercising on the organic matter composing them a particular stimulating 

 influence, without which the tissues could not manifest their properties, as well 

 as inducing a combustible action which evolves the heat proper to the animal 

 body. 



This new absorption constitutes the phenomenon of respiration. In the 

 Mammalia, this is effected in the lungs — parenchymatous organs chambered into 

 a multitude of vesicular spaces, which receive the air and expel it, after depriving 

 it of a certain quantity of oxygen, and giving, in return, a proportionate quantity 

 of carbonic acid. These organs are lodged in the thoracic cavity, the alternate 

 movements of dilatation and contraction of which they follow. They com- 

 municate with the external air by two series of canals placed end to end : 1. A 

 cartilaginous tube originating in the pharyngeal vestibule, and ramifying in the 

 lungs. 2. The nasal cavities, two fossae opening into that vestibule, and com- 

 mencing by two openings at the anterior extremity of the head. 



CHAPTER I. 



RESPIRATORY APPARATUS IN MAMMALIA. 



In this apparatus we will first study the organs external to the thoracic cavity — 

 the nasal cavities, and larynx and trachea; then the chest and the organ it 

 contains — the lung. 



To this study will be added that of the two glandiform organs, the uses of 

 which are unknown, but by their anatomical connections they belong to the 

 respiratory apparatus. These are the thyroid bodies and the thymus gland. 



