506 OF RESPIRATION. 



by its contraction, of largely increasing the capacity of that cavity. In fact, many 

 Reptiles are incapable of draining in air, and can only force it in by a process 

 resembling deglutition. 1 



541. The size of the Lungs in Man and the Mammalia is far smaller in propor- 

 tion to their bulk, than it is in most Reptiles ; but this diminution is more than 

 compensated by the minute subdivision of their cavities, by the peculiarity of the 

 distribution of their bloodvessels, and by the arrangements whereby a continual 

 and rapid interchange, both of the blood and the air, is provided for. The fol- 

 lowing are the points of most importance in the structure 

 Fig. 143*. O f the Human Lung. 2 The walls of the bronchial tubes con- 



tain distinct longitudinal and circular layers of fibrous struc- 

 ture ; but the latter alone, according to Prof. Kolliker, con- 

 tain muscular fibre-cells. [The muscular fibres which exist 

 in the trachea are continued down even to the terminal bron- 

 chi, but instead of filling up the gap in the cartilaginous 

 framework, posteriorly, as in 'the trachea, they form a uni- 

 form layer encircling the canal, but excessively thin. Fig. 

 143*. ED.] These tubes divide and subdivide, like the 

 branches of a tree, still retaining their ordinary characters, 

 until they are no more than from l-50th to 1-3 Oth of an 



Small bronchial tube i_ i ,1 ,11 -TT i i 



laid open, showing the incn m diameter ; and in these the longitudinal and annular 



transverse piexiform ar- fibres, together with the ciliated epithelium, come to an 



rangement of the muscu- abrupt termination. Beyond this boundary, the tubular 



iar layer, and its disposi- f orm O f the air-passages continued from the bronchi is 



tion at the orifice of a retaine( j f or some distance: but it is gradually changed by 



branch. From a man set. .* i > i /..{ D 111- 



fifty. Magnified 2 diam. * ne irregular branches ot the passages, and by the increase 

 From Todd <& Bowman, of the number of apertures in their walls, which lead to the 

 air-vesicles. Thus, at last, each minute division of the air- 

 passages becomes quite irregular in form ; air-vesicles opening into every part 

 of it, and almost constituting its walls; until it terminates, almost without 

 dilatation, in an air-vesicle. This terminal portion of the air-passage, with 

 its surrounding cluster of air-vesicles, may be regarded as forming a sort of 

 lobule, and as representing the entire lung of a Frog or other Reptile; the 

 whole lung of the Mammal being made up of a multitude of such lobules, 

 which are almost exact repetitions of each other. Those vesicles which com- 

 municate directly with the bronchial tubes and intercellular passages, open 

 into them by large circular apertures ; and they are themselves similarly 

 opened into by other vesicles, which again communicate with others beyond 

 them ; so that each of the openings in the air-passage leads to a series of air- 

 vesicles extending from it to the surface of the lobule. The vesicles which 

 communicate most directly with the air-passages, are more minute, and have 

 a closer vascular network, than those which lie nearer the surface of the lobule ; 

 an arrangement which is in beautiful harmony with the relative facility of reno- 

 vation of the air which they respectively contain. The air-vesicles have also 

 lateral apertures into each other ; so that all the parts of any one lobule freely 

 communicate together. The walls of the air-vesicles are formed of a very thin 

 and transparent membrane, which is folded sharply at the orifices of communi- 

 cation, so as to form a very definite border to them ; and which is lined by an 

 epithelial layer, composed of minute polygonal cells of from l-1600th to 1 -2250th 

 of an inch in diameter, and from l-2800th to l-3800th of an inch in thickness 

 (Kolliker). This lining membrane is distinctly fibrous ; and its fibres are par- 



1 See "Prin. of Phys., Gen. and Comp.," g 324r, 526, 527, Am. Ed. 



2 See especially the Memoir by Mr. Rainey in the " Med-Chirurg. Trans.," vol. xxviii. 



