466 TBE RESPIBATOBY APPARATUS IN MAMMALIA. 



The deep face is united to the subjacent parts by connective tissue 

 destitute of fat ; the adherence of the visceral pleuras is most intimate. 



The pleura has plexuses of vessels : one, the subserous, has large meshes ; 

 but a second, the subepithelial, has a closer network. 



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

 monary pleura : from the diaphragmatic and intercostal nerves for the 

 parietal pleura. 



Functions. — The thorax is not a mere receptacle, but, on the contrary, 

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

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

 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. 



DIFPEKENTIAL CHAKAOTEBS IN THE THOKAX OP OTHER THAN SOLIPBD ANIMALS. 



In the Ox, the thorax is not so long, particularly in its superior part, as in Solipeds, 

 by leasou of the slight obliquity of the diaphragm, and of its mode of attachment to the 

 ribs. 



The total capacity of this cavity is also certainly inferior to that of the Horse's cheat. 

 It is the same, though relatively more extensive, in the Sheep, Goat, and Pig ; while 

 tlie Dog possesses in this respect an incontestable superiority over Solipeds. It is to be 

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

 and Mule by the conformation of the posterior mediastinum. In them it is not open in 

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

 the consecutive effusion of pleuritis is readily localised in one of the pleural sacs in the 

 first-named aniinals, while this localisation is impossible in the second. (This is an 

 important observation, in a pathological point of view.) 



THE LUNG (or ltjngs). (Figs. 230, 234.) 



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

 phicing 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 inflated as 

 before. 



Situation— General disposition. — This essential organ of respiration is a 

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

 but independent, moieties, each of which occupies one of the two serous sacs 

 formed by the plurae. It is also described as two pulmonary lobes, or fico 

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



Form and Belations.— Together, the lungs affect the outline of the 

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

 an external and internal face, a base and summit, and a superior, inferior, and 

 posterior border. 



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

 external wall of the thorax. 



The internal, or mediastinal face, forms a vertical plane, separated from 

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

 part, m 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 the lung {hilum-pulmonis), a, ia,sci- 



