TEE THORAX. 545 



constantly lubricated by a serous fluid which facilitates the gliding of the lung 

 on the parietes of the thoracic cavity. The deep face is united to the subjacent 

 parts by connective tissue destitute of fat ; the adherence of the visceral pleurae 

 is most intimate. 



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

 a second, the endothelial, has a finer network. 



The nerves are from the sympathetic and pneumogastric for the pulmonary 

 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. 193, 310). The limgs, being apphed immediately against the thoracic 

 walls, and never at any time separate from them, follow this cavity in then- 

 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, constitut- 

 ing, as they do, the initial phenomenon of respiration, and having dependent on 

 them all the other acts of this function. 



Differential Characters in the Thorax of the other Animals. 



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

 reason 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 chest. It is 

 the same, though relatively more extensive, in the Sheep, Goat, and Pig ; while the 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 con- 

 formation 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 localized in one of tlie pleural sacs in tiie first-named animals, while this 

 localization is impossible in the second. (This is an important observation, from a pathological 

 point of view ; but it is to be noted that exceptional instances have been recorded, in which 

 the posterior mediaotinum of the horse has been found imperforate. 



The Lung (or Lungs) (Figs. 318, 321, 325, 326). 



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. 322, 

 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 

 viscus, lodged in the thoracic cavity, and divided into two lateral, but inde- 

 pendent, moieties, each of which occupies one of the two serous sacs formed by 

 the plurse. It is also described as fivo jnilmonary lobes, or tivo lungs — a right and 

 left, the latter a little less voluminous than the former. 



Form and Relations. — 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. 



* This is not the natural shape of the lungs, and is only assumed when the chest is opened, 

 and their elasticity is no longer counterbalanced by the atmospheric pressure, except at the 

 external surface of the bronchi. 



