39G RESPIRATION. [CHAP. XXIX. 



Mr. Rainey's paper, and is drawn from the lung of the fowl, highly 

 magnified. 



The gradations of perfection in the respiratory surface, considered as to its 

 minute organisation in Reptiles, Mammalia, and Birds, may be thus recapitulated. 



In Reptiles, the cells are large and few. The air has access to the respira- 

 tory capillaries only on one surface, viz., that towards the cavity of the cell 

 which they line ; and where two cells are contiguous, but separated by a sep- 

 tum, the septum has a capillary web on each of 4ts surfaces. 



In Mammalia, for the most part, the cells are much multiplied, and where 

 two lie side to side, the septum between them has but one layer of capillaries, 

 which is, in a great measure, common to them both, and aerated on both its 

 surfaces. And here we may remark the reason of the non-isolation of the 

 lobules by areolar tissue in the mammalian lung, except at the borders and 

 thinner parts, where such isolation is necessary for the movements of the 

 lung in respiration. When the lobules are thus isolated, the capillaries on 

 the cell- walls which bound the lobule, can have air only on that surface which 

 is towards the cavity of the cells ; whereas, when the contiguous lobules are 

 blended by the fusion of the cell-walls which form their exterior, these cell- 

 walls, like the rest, contain but a single respiratory plexus, which is aerated 

 on both its surfaces. 



In Birds, the respiratory capillary plexus is rendered the most perfect pos- 

 sible, both by the omission of the elastic supporting tissue, and also by the 

 vascular plexus being no longer lamellar, but solid, or extended alike in all di- 

 rections. It is manifsst, that by this arrangement, a given pulmonary space 

 is made to contain the greatest possible quantity of capillary wall, and that this 

 wall is exposed most completely to the action of the air admitted around it. 



Movements of Respiration. The thorax is a moveable frame- 

 work, composed of a nearly fixed column, the dorsal spine, on 

 which are moveably articulated twenty-four rihs, curved and in- 

 clined, so as to embrace a large conical space, and ending in carti- 

 lages, which, with three exceptions on each side (the floating ribs), 

 are joined in front through the medium of a flat piece, the sternum, 

 the inclination of which is downwards and forwards. The upper 

 orifice, embraced by the vertebral column, first ribs, and sternum, 

 is closed by a fascia of dense areolar tissue, rendered extremely 

 irregular by the apices of the lungs rising slightly into the neck, 

 and by the various structures, the resophagus, trachea, great blood- 

 vessels, muscular and other parts, that pass through it. The lowe 

 orifice, which is very much larger, is closed by the diaphragm, 

 an arched and sloping musculo-tendinous septum between the 

 thorax and abdomen, through which pass the inferior vena cava, 

 the aorta, and great absorbent trunk, as well as the oesophagus and 

 the pneumogastric and sympathetic nerves. Between the ribs are 

 placed the two layers of intercostal muscles, with oblique fibres op- 

 positely crossing, and chiefly on the exterior of the bony frameworl 



