176 



i 



In every effort of expiration, as in cough and vomiting, these muscles 

 re-act, not merely by their own elasticity, but they besides contract and 

 tend to approach towards the vertebral column, by pressing upwards the 

 abdominal viscera towards the chest. The triangularis sterni, the sub- 

 costales, and the serratus inferior pasticus, may likewise be ranked among 

 the agents of expiration; but they appear to be seldom employed, and to 

 be too slender and weak to contribute much to the contraction of the 

 chest. 



LXXIV. When the chest enlarges, the lungs dilate and follow its pa- 

 rietes, as these recede from each other. These two viscera, soft, spon- 

 gy, and of less specific gravity than water, covered by the pleura which 

 is reflected over them, are always in contact with the portion of that 

 membrane which lines the cavity of the thorax; no air is interposed be- 

 tween their surfaces (which are habitually moistened by a serous fluid 

 exuding from the pleura) and that membrane, as may be seen, by open- 

 ing, under water, the body of a living animal, when no air will be seen to 

 escape. As the lungs dilate, their vessels expand, and the blood circu- 

 lates through them more freely; the air contained in the innumerable 

 cells of their tissue, becomes rarefied, in proportion as the space in which 

 it is contained is enlarged. Besides, the warmth communicated to it by 

 the surrounding parts, enables it, in a very imperfect manner, to resist 

 the pressure of the atmosphere, rushing through the nostrils and mouth 

 into the lungs, by the opening in the larynx which is always pervious, ex- 

 cept during deglutition. 



LXXV. The pulmonary tissue into which the air is thus drawn in, 

 every time the capacity of the chest is increased, does not consist mere- 

 ly of air-vessels, which are but branches, of different sizes, of the two 

 principal divisions of the trachea, but is formed, likewise, by the lobular 

 tissue into which these canals deposit the air; it contains also a great 

 quantity of lymphatics and blood-vessels, of glands and nerves. Cellu- 

 lar tissue unites together all these parts, and forms them into two masses 

 covered over by the pleura, and of nearly the same bulk*; suspended in 

 the chest from the bronchia and trachea, and every where in contact 

 "with the parietes of the cavities of the chest, except towards their root, 

 at which they receive all their nerves and vessels. 



The pulmonary artery arises from the base of the right ventricle, and 

 divides into two arteries, one to each lung. On reaching the substance 

 of these viscera, these vessels divide into as many branches as there are 

 principal lobes. From these branches, there arise others, which again 

 subdivide into lesser ones, until they become capillary, and continuous 

 with the radicles of the pulmonary veins. 



These vessels, formed from the extremities of the artery, unite into 

 trunks, which progressively enlarging, emerge from the lungs, and open, 

 four in number, inio the left auricle. Besides these large vessels, by 

 means of which, the cavities in both sides of the heart communicate to- 

 gether, the lungs receive from the aorta two or three arteries, called bron- 

 chial arteries, these penetrate into their tissue, and follow the direction 

 of the other vessels, and terminate in the bronchial veins, which open in 

 the superior cava, not far from its termination into the right auricle.- 

 These bronchial vessels are sufficient for the nourishment of the pulmo- 



* It is well known that the right lung is larger than the left, that it is divided into 

 three principal lobes, while the latter has only two. Jiuthm's J\"ote. 



