THE LUNGS. 605 



nal surface is connected, by cellular tissue, to the ad- 

 jacent parts, having varying degrees of attachment, being 

 with much more difficulty separated at some points than 

 others. The cellular tissue of the pleura is so condensed 

 as to take the form of a fascia, in some places, as on the 

 ribs, where it is strong and more readily detached than on 

 the diaphragm or lungs. On these latter it is so thin and 

 delicate as scarcely to admit of demonstration ; and, though 

 so extremely delicate upon the lungs, it is nevertheless 

 asserted to be strong, resisting, and elastic, and that this 

 transparent fascia can also be dissected off the air-cells. 

 The pleura, then, is really a fibro-serous membrane, and in 

 the healthy state has no perceptible blood-vessels. 



Function. To secrete or exhale a serous fluid upon their 

 internal surface, by which the cavity of each pleura is kept 

 in a constantly moist and lubricated condition, thus allow- 

 ing its parietal and visceral portions to glide readily upon 

 each other, and thereby giving both the lungs and walls 

 of the chest the greatest freedom of motion during respi- 

 ration. 



SECTION II. 



ORGANS OF RESPIRATION. 



These comprise the larynx, the trachea, bronchi, and 

 the lungs. 



The larynx and trachea have already been described in 

 another place ; we therefore proceed to examine the lungs. 



THE LUNGS, (PULMONES.) 



The lungs are two in number, right and left, and situ- 

 ated upon either side of the chest, having the mediastinum 

 and heart to separate them. When distended with air, the 

 pleura pulmonalis and pleura parietalis are in close juxta- 

 position; and, strictly speaking, there cannot be said to be 

 any thoracic cavity, as the distended lungs fill the whole 

 space, excepting the small part occupied by the heart and 

 thymus gland. 



