CHAPTER XI. 

 THE RESPIRATORY SYSTEM. 



GENERAL CONSIDERATIONS. 



The lungs : One function of the lungs, analogous to that of 

 the alimentary system, is to furnish to the circulation material 

 (in this case oxygen) required in the nutrition of the body. 

 Their other function, the excretion of carbon dioxide, is a 

 glandular process, and in view of it the lungs can be regarded 

 as a gland ; they certainly approach the glandular type as 

 closely as does the liver. 



The structure of the lungs is strikingly that of racemose 

 glands ; the main departure from the ordinary glandular type 

 is that the substances interchanged are gaseous rather than 

 liquid or solid, and the medium of excretion consists not of 

 spheroidal cells, but of squamous cells adapted to the ex- 

 change of gases. 



The analogy of the lungs to a gland is increased if their 

 alimentary or oxygen-furnishing function be regarded as a 

 sort of internal secretory process, in which the separated 

 product is discharged into the blood. 



The other portions of the respiratory apparatus, the nose, 

 pharynx, larynx, trachea, and bronchi (the ducts or air-pas- 

 sages of the lungs), and the pleura, for the most part simply 

 serve tributary and subordinate purposes to the lungs. 



The Nasal Fossae. 



The mucous membrane lining the nasal fossae rests upon the 

 periosteal and perichondrial coverings of the surrounding 

 bones and the hyaline cartilaginous septum, and is continuous 

 with the skin and the mucous membrane of the nasal ducts, 

 pharynx, and Eustachian tubes. It is composed of two dis- 

 tinct areas, the respiratory and the olfactory. The olfactory 

 area comprises the upper part of the fossaB, namely, the upper 



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