378 THE SOLIDS. 



ART. XX. ORGANS OF RESPIRATION. 



THE organs of respiration seem to stand alone in the animal 

 economy, and not to exhibit any strong structural relation- 

 ship or affinity with any other organ or organs comprised in 

 that economy. In their position and general form they bear 

 some resemblance, in the mammalia at least, to glands : this 

 resemblance, however, is more apparent than real, as becomes 

 evident from the single consideration that the lungs are es- 

 sentially destitute of the innumerable granular cells which 

 constitute the chief bulk of true glands and form their dis- 

 tinctive and essential element. In a strictly natural ar- 

 rangement the lungs would be more properly and correctly 

 described in connexion with the circulatory apparatus ; for 

 essentially they are vascular organs, their only indispensable 

 constituent being blood-vessels, so situated indeed as to admit 

 of their being brought continually into contact either with 

 air, or with water impregnated with air. 



The great object aimed at in the formation of the lungs 

 of mammalia is the construction of organs which shall occupy 

 but little comparative space, and which shall yet present a 

 greatly extended surface, throughout which the blood and 

 air may be brought into close approximation. We shall now 

 proceed to describe the manner in which this purpose is ac- 

 complished. 



The tissue of the lungs of man and other mammalia may 

 be divided into two systems of apparatus : the first comprises 

 the circulatory or vascular apparatus, consisting of arteries 

 and veins together with the plexuses formed by the union of 

 the capillaries proceeding from both sets of vessels ; the 

 second comprises the respiratory or aeriferous system, which 

 includes the bronchial tubes, the air cells and the ciliated 

 epithelium. The intimate structure of the lungs will be best 

 understood by an examination in the first place of the aeri- 

 ferous apparatus. 



