TBE THORAX. 545 



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

 on the parietes of the thoracic cavity. The deep face is united to the su])jacent 

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

 is most intimate. 



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

 a second, the endotheUal, has a finer network. 



The ncrt'es are from the sympathetic and pneumogastric for the pulmonary 

 pleura ; from the diaphragmatic and intercostal nerves for the parietal 

 pleura. 



FuNCTFONS. — 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 lungs, being applied immediately against the thoracic 

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

 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 amoimt 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 Characteks in the Thorax of the other Animals. 



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

 reason of the slight obliquity of the cliaj)hragm, and of its mode of attachment to the ribs. 



The total rapacity of this cavity is also certanily inferior to that of the Horse's chest. It is 

 the same, though relatively more exten.sivf, in the Sheep, Goat, and Pig ; while the Dog 

 possesses in this respect an incontestable superiority over Soliiieds. It is to be noted tiiat all 

 these animals, without exception, are distinguished from the Horse, Ass, and Mule by the con- 

 formiitioh of the posterior mediastinum. In them it is lot open in its lower part, but as solid, 

 thick, and complete there as eisewliere. Therefore it is that the consecutive elfnsion of 

 pleuritis is readily localized in one of tlie pleural sacs in the fir»t-named animals, while this 

 localization is impo.-i.^ible in the second. (This i.'^ an injportant observation, from a pathologicjil 

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

 the posterior mediastinum of the horse has bi en found imperforate. 



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



Preparation. —The disposition of the lung in the thoracic cavity is bist stmlied by placing 

 the suliject 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 plurte. It is also described as tiro pulmonar// lobes, or two Jungs — 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 l)ronchi. 



