////.' lil-XPIRATOIlY .!/T,l/M77> /V .W.I M U I /,/.!. 



Tho deep face is united to tho subjacent parts by connective tissue 

 destitute of fat ; the adherence of tho visceral pleura is most intimate. 



The pleura has plexuses of vessels : one, tho suliserous, has largo meshos ; 

 but a second, tho subcpithclial, lias a closer network. 



The nerves are from tho sympathetic and pneumogastric for the pul- 

 monary pleura : from tho diaphragmatic and intercostal nerves for tho 

 parietal pleura. 



FUNCTIONS. The thorax is not a mere receptacle, but, on tho contrary, 

 performs a very important part in the act of respiration. We know, in tart, 

 that it is dilated and contracted by the movements of the diaphragm uiul tlir 

 ribs (see pp. 142, 248). The lung being applied immediately against the 

 thoracic walls, and never at any time separate from them, follows this cavity 

 in its movements, dilating in inspiration and contracting in expiration, after 

 a certain quantity of the oxygen of the inspired air has been removed and 

 replaced by an equivalent amount of carbonic acid. 



The movements of the thorax are, therefore, of capital importance, con- 

 stituting, as they do, the initial phenomenon of respiration, and having 

 dependent on them all the other acts of this function. 



DIFFEBENTIAL CHABACTERS IN THE THORAX IN OTHER THAN SOUPED ANIMALS. 



In the Ox, the thorax is not so long, particularly in its superior part, ns in Sc>li|M 'ds> 

 by reason of the slight obliquity of the diaphragm, and of its mode of attachment to tin- 

 ribs. 



The total capacity of tl-is cavity is also certainly inferior to that of th- Hi -< '- ln>t. 

 It is the same, though relatively more extensive, in the /</'''""'. an I /'A/; while 

 the Doj possesses in tli is resect an Incontestable superiority over Solipeds. li i< t > ! 

 noted that nil these animals, without exception, are distinguished from the Horse, Ass, 

 end Mule by the conformation of the posterior mediastinum. In them it is not r>ien in 

 its lower part, but as solid, thick, and complete there as elsewhere. Therefore it i- that 

 the consecutive effusion of pleuritis is readily localised in one of the pli-ural sacs in the 

 first-named animals, while this localisation is impossible in the second. (This is nn 

 important observation, in a pathological point of view.) 



THE LUNG (OR LUNGs). (FigS. 230, 234.) 



Preparation. The disposition of the lung in the thoracic cavity is best studied by 

 placing the subject in the second position, opening the chest by excision of the ribs, as 

 in Fig. 234, and inflating the organ by the trachea. To study its external conformation. 

 it should be' removed from the cavity, with the heart and large vessels, and inihited a> 

 before. 



Situation General disposition. This essential organ of respiration is a 

 spongy viscus, lodged in the thoracic cavity, and divided into two lateral, 

 but independent, moieties, each of which occupies one of tho two serous 

 formed by the phme. It is also described as two jiiiln/<>ii<ir// A-/X.S-, or ///-, 

 lungs a right and left, the latter a little less voluminous than the former. 



Form and Relations. Together, the lungs affect tho outline of tho 

 thoracic cavity ; each represents the moiety of a cone, and offers for study : 

 tin external and internal face, a base and summit, and a superior, infi-rim; and 

 posterior border. 



The external or costal face is convex (and smooth), and moulded to the 

 external wall of the thorax. 



The iiiti'i-mil, or nn ilin.tiiiinf fn<'<; forms a vertical plane, separated from 

 the opposite lung by the mediastinum. It shows : 1, An inextensivo anterior 

 part, in contact with the anterior mediastinum ; 2, At the level of the heart, 

 an excavation in which that organ is lodged ; 3, Immediately behind this 

 excavation, and a little above it, the root of tin- lung (JiiJnm-pulmonis), a fasci- 





