RESPIRATION. 



277 



muscular fibre, and that too of the involuntary 

 or unstriped kind. This muscular layer in birds 

 extends from the superior larynx to the com- 

 mencement of the bronchi : these latter are, 

 however, unsupplied by muscular fibres. They 

 are exclusively membranous.* The bronchi 

 in the case of birds, on entering the substance 

 of the lungs, divide and subdivide without 

 decreasing in diameter (a, a, a, jig. 225.) 

 Patches of cartilage appear in the parietes only 

 of the largest order of these tubes. They are 

 distinguishable into two principal classes : 

 those, first, which course superficially along the 

 inferior or sternal surface, and which terminate 

 by wide openings in the thoracic and abdo- 

 minal air-receptacles. This class of tubes is 

 perforated by the inter-cellular passages only 

 on one side, the other being strengthened by 

 cartilaginous semi-rings. The deep bronchi, 

 resembling cylindrical tubes, traverse the 

 lungs in many directions, and freely commu- 

 nicate with each other, not, however, to 

 form a network, for they run in nearly pa- 

 rallel directions. These tubes are always 

 patnlous on dissection, and seem incapable 

 of contraction and dilatation. They are 

 lined internally by a well-marked ciliated epi- 

 thelium. The submucous tissue in the true 

 bronchi is strong and dense, composed chiefly 

 of elastic fibres, none of a muscular character. 

 It constitutes a distinct fibrous layer, like that 

 which lines the trachea of quadrupeds. Those 

 bronchi which do not end in open orifices 

 on the surface of the lung terminate ccecally. 

 These coecal extremities are perfectly defined 

 by a prominent lining of fibrous and mucous 

 membrane. It was first proved by Mr. Rainey 

 that in the lungs of birds the mucous mem- 

 brane does not extend inwards in the direc- 

 tion of the interior of the lungs beyond the 

 limits of the bronchi. By the words mucous 

 membrane Mr. Rainey desires to indicate that 

 flocculent covering which is so well seen in 

 his injected preparations. By this observer 

 it is maintained that all parts of the lungs of 

 birds beyond the extremes of the bronchi are 

 literally devoid of all epithelial covering 

 whatever, the extreme capillary vessels being 

 included in nothing but their own proper 

 tunics. It has been already shown that 

 Mr. Rainey has mistaken the cessation of 

 the ciliated epithelium at the ends of the 

 bronchi for the termination of all the other 

 elements of this covering. The apparently 

 naked vessels of the air-cells are really in- 

 vested by a hyaline epithelium, coinciding with 

 that which, in the instance of reptiles, will 

 afterwards be described. The abrupt termina- 

 tion of the bronchial tubes marks the abrupt 

 commencement of the intercellular passages. 

 These passages contrast remarkably in struc- 

 ture with the bronchi. The membranous 

 walls of these parts are reduced to the ut- 

 most state of thinness ; those of the former 

 are furnished with cylindrical epithelium and 

 a dense fibrous coat. But, what is extra- 

 ordinary, the dense mass of vessels which 

 * See art. AVES, by Prof. Owen. 



bound these passages are not joined together 

 into a continuous partition. Each vessel is 

 separate from and unconnected with those 

 adjacent. " A wall" thus constructed is at 

 every point between the vessels permeable to 

 air. These "intercellular passages" (b, 6, /;, 

 fig. 225.) arise, with singular uniformity, from 

 the sides of the bronchi, at right angles to the 

 axes of the latter. This is so constant as to 

 become a characteristic point of structure in 

 the bird's lung. 



The "spaces" between the vessels forming 

 the walls of the intercellular passages lead to 

 no definitely bounded cells or chambers. 

 They lead only to the interval which divides 

 the contiguous bronchi from each other 

 (c, c). This interval is filled densely with the 

 ultimate pulmonary vessels. (n,fig. 225.) It 

 was first determined by Mr. Rainey that 

 these vessels, in the bird's lung, are arranged 

 in a peculiar manner. They do not form 

 plane reticular definitively bounded air- 

 chambers. Each ultimate capillary crosses 

 an air-space of its own. It is thus surroimded 

 by air. The ultimate vessels interlace and 

 interloop in every direction, forming a cubic 

 mass of capillaries permeated evert/where bit 

 the air. The apparently naked loops of the 

 ultimate vessels may be seen projecting into 

 the areas of the intercellular passages. No- 

 thing can be conceived more mechanically 

 perfect than this arrangement of the vessels 

 for the exposure of the blood to the opera- 

 tion of the air. The latter is in immediate 

 contact with each individual vessel (B, Jig. 

 226.) It surrounds the blood-current borne 



Fig. 226. 



Slightly ollique section through a branchial tube. 

 (After Rainey.) 



a, cavity of the tube; b, its lining membrane, 

 containing blood-vessels with large areolte; c, c, 

 perforations in this membrane, where it ceases at 

 the orifices of the lobular passages (rf, d); e, e, spaces 

 between contiguous lobules, containing the terminal 

 pulmonary arteries and veins supplying the ca- 

 pillary plexus (/,/) to the meshes of which the air 

 gains access by the lobular passages. 



by the latter. Every part of the circum- 

 ference of this current, less than ^nr f an 

 inch, is under the direct agency of the aerat- 



T 3 



