BOOK III. 

 KESPIRATOKY APPARATUS. 



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

 organisable 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 

 apparatus, exercising on the organic matter composing them a special 

 excitory 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 hollowed out 

 into a multitude of vesicular spaces which receive the atmospheric air and 

 expel it, ufter 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, whose alternate movements of dilatation and contraction they 

 follow. They communicate 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 commencing by two openings formed at the 

 anterior extremity of the head. 



CHAPTER I. 



RESPIRATORY APPARATUS OF MAMMIFERS. 



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 

 organs it contains the lungs. 



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

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

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



THE NASAL CAVITIES. 



These cavities are two in number, a right and left, and oifer for study : 

 their entrance, or nostrils the fossae, properly called, which constitute 

 these cavities ; and the diverticuli named sinuses. 



