RESPIRATION 



105 



Its vibrissse (the hairs) strain out large foreign particles in the draughts 

 of air. The organ of smell not only warns us of some sorts of 

 unwholesome air, but makes us breathe more deeply when the air is 

 sweet and fresh. The nasal fossse are larger and more complex than is 

 commonly supposed, and are lined with very thick vascular mucosa, 

 which warms to the body temperature and saturates with moisture the 

 incoming air. Both of these modifications are necessary to prevent 

 injury to the sensitive bronchi and the lungs. The larynx is the organ 

 of the voice, and is discussed later on. It is rather. a hindrance to 

 respiration in the long run than a respiratory organ, although the 

 closing-down of the epiglottis over the trachea and bronchi at each act 

 of swallowing protects the latter from frequent danger. The trachea 

 is ample in diameter, and kept so by strong cartilaginous rings. The 

 ramifying bronchi finally becomes muscular to a certain extent, and so 

 somewhat control the air-supply to the lungs. 



THE LUNGS. These are the proper organs of respiration, and must 

 be fully understood in their structure and mode of action. The two 

 lungs are separated by the mediastinum. The right lung weighs about 

 625 gm. and the left not far from 565 gm. The specific gravity of the 

 lung-tissue, according to Gray, is from 345 to 746, water being 1000. 

 To the touch, it is a highly elastic, 

 spongy, and crepitating mass; and in 

 appearance it ranges from a light pink 

 at birth to almost black in old age, 

 darkening largely from the coal- and 

 other dust slowly collecting in it. 



According to Rainey, the small bron- 

 chi after entering a lobule of the lung 

 subdivide from four to nine times con- 

 formably to the size of the lobule, the 

 smallest branches being about two- 

 thirds of a millimeter (0.63 mm.) in 

 diameter, or smaller. These smallest 

 subdivisions of the air-tubes then lose 

 their cylindrical shape and structure 

 and continue onward a short way as 

 infundibula. These are irregular tubes 

 from which the short alveoli dilate in 

 various places and directions. Indeed, 

 bronchioles of 2 mm. diameter often 

 have alveoli extending outward from 

 them, as shallow and rounded vesicles. 

 Those are the ultimate chambers 'for 

 air in the lungs, and it is from them and 

 through their walls that the two respir- 

 atory gases osmose and diffuse in their respective directions, inward and 

 outward. 



Scheme of the lung of a frog to show 

 its relatively simple tabulation: G, glottis; 

 globules; A, alveoli; e, lung-epithelium; 

 a, afferent artery; v, vein; s.p.p., serous 

 covering. (Renaut.) 



