STRUCTURE OF THE LUNGS. 159 



and the highly subdivided respiratory organ of the mammals, the condi- 

 Fig. T6. tion of things is well illustrated by the lungs 



of the frog. In Fig. 76, a is the hyoid appa- 

 ratus ; b, cartilaginous ring at the root of the 

 lungs ; c, the pulmonary vessels ; and d, d, the 

 pulmonary sacs. 



Of all tribes, the respiratory mechanism is 

 most highly developed in birds, Respiration of 

 which, besides being provided with birds - 

 lungs, have air-sacs between the muscles, and 

 respiratory membranes spread on the interior 

 of the hollow bones. It is in consequence of 

 this that a bird is killed so readily, even by a 

 Lungs of frog. Yerv small shot, since it is scarcely possible to 



make a perforation into any part of the body without opening the respi- 

 ratory cavity. 



In man, the bronchial tube, as it passes into each lung, branches forth 

 like a tree, the walls of the tubelets thus arising having car- 



, .-, . - . (, , . Lungs of man. 



tilagmous rings to preserve their form under compression, 

 circular organic muscular fibres to enable them to contract, and longitu- 

 gitudinal fasces of elastic tissue to shorten them after extension. In 

 their interior they are covered with mucous membrane provided with 

 cilia. When the proper degree of minuteness, about -^ of an inch, is 

 reached, they consist alone of elastic membrane, interspersed with mus- 

 cular fibres, and upon their sides the air-cells open ; sometimes single 

 ones, or sometimes many cells communicating with one another, discharge 

 through the same orifice, the tubelet itself ending in a cell. The air- 

 cells have various dimensions, from JL to 1 2 * tf of an inch. Their struc- 

 ture is like that of the tubelet. The pulmonary capillaries are spread 

 so closely upon them that the spaces between them are less than their 

 own diameters, which, on an average, are 3-^-3- of an inch. As the cells 

 are close together, the blood-vessels passing between them are brought 

 in communication with the air. on both sides, and arterialization is thus 

 rapidly and completely performed. Each tubelet, with the air-cells thus 

 clustered upon it, is a miniature representation of the lung of a reptile. 

 These cells themselves communicate by lateral apertures with one an- 

 other. The membrane which lines their interior is sharply folded at the 

 apertures, and there are reasons for supposing that it contains organic 

 muscular fibres. It is stated that each terminal bronchus has nearly 

 20,000 air-cells clustered upon it, and that the total number is 600 

 millions. 



The mode of distribution of the air-tubes is represented in Fig. 77. 

 a is the larynx ; b b, the trachea, the upper letter corresponding to the 



