GENERAL STRUCTURE OF THE RESPIRATORY ORGANS. 363 



duction of Carbonic acid iu the animal body are threefold. i. The con- 

 tinual decay of the tissues common to all organized bodies, which is favored 

 by whatever promotes their vital activity, and is retarded by every influence 

 that depresses it. n. The metamorphosis peculiar to the Nervous and Mus- 

 cular tissues, which is the very condition of the production of their power, 

 and which therefore bears a direct relation to the degree in which they are 

 exerted. in. The direct conversion of the carbon and hydrogen of the food 

 into carbonic acid and water, which is peculiar to warm-blooded animals; 

 and which varies in quantity, in accordance with the amount of heat to be 

 generated. 



284. The activity of the process of Respiration in any Animal is, as a 

 general rule, in direct proportion to the smallness of the Corpuscles of its 

 blood; for these, as has already been shown ( 193), are the carriers of oxy- 

 gen ; and it is evident that a given weight of smaller globules will offer a 

 larger surface for the absorption of oxygen, than an equal weight of larger 

 Corpuscles. Hence in Fishes and Reptiles, which possess large Corpuscles, 

 the provision for the aeration of the blood is much less perfect, and the 

 amount of oxygen absorbed, and of carbonic acid eliminated, is considerably 

 smaller than in the case of Mammals and Birds, the corpuscles of whose 

 blood are remarkably minute. At the same time the size of the lungs in 

 the latter classes is far less in proportion to their bulk than 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 of the air, is provided for. The following are the points of 

 most importance in the structure of the Human Lung. 1 The walls of the 

 larger bronchi are composed independently of the tunica adventitia of four 

 structurally distinct layers. Of these the outermost (a, Fig. 157) is a fibrous 

 layer of dense connective tissue in which cartilaginous laminae conferring 

 firmness, and elasticity on the tubes, ai;e imbedded. The next (6) is a mus- 

 cular layer composed of compact circularly arranged fasciculi of smooth 

 muscular tissue. The third is the internal fibrous layer (c), composed 

 of tolerably regularly-arranged longitudinal 

 bundles of thick elastic fibres, which on sec- 

 tion give a sinuous outline to the interior of 

 the tube. The innermost part of this layer is 

 condensed into a hyaline layer, and upon this 

 last or so-called basement-membrane rests the 

 ciliated columnar epithelium that forms the 

 fourth layer. Intervening here and there 

 amongst the ciliated columnar cells are E. 

 Schuke's cup, goblet, or chalice celk (b, Fig. 



156). These are filled with a mUCOUS maSS Ciliated Epithelium from a bron- 



through which numerous highly refractile chiai twig i-6th of an inch in diameter 



, . ., -, T ' 111- -i a, ciliated columnar epithelial cells; 



granules are distributed. Imbedded in the 6i goblet . ceUa . From the Dog, mag- 

 outer fibrous layer of the larger bronchi are nified 320 diameters. 

 numerous mucous glands, the ducts of which 

 run inwards, are lined with columnar epithelium, and open by trumpet- 



.mt of the principal forms of Kespiratory apparatus among the lower 

 rinc. of Comp. Pliys.-, chap, vi ; also the Memoir by Mr. Rainoy in 

 'o-. Trans., vol. xxviii; and Prof. Kolliker's Manual of Human Mi- 



& " ' s* 



1 For an account. 

 Animals, see Princ. 

 the Med.-Chirurg. 1 



croscopic Anatomy, 1860, Lond. For excellent general accounts of the structure of 

 the Human Lung, see Waters's Fothergillian Essay, London, 1860; Williams in the 

 Supplement of the Cycl. Anat. and Physiology ; and F. E. Schulze in Strieker's 

 Manual of Histology (Syd. Soc. Trans.), vol. ii, 1872, p. 63, on whose essay the fol- 

 lowing description is founded. 



