THE LUNGS. 467 



cuius formed by the air-tubes and pulmnary vesse-S in entering the visciis ; 

 4, A posterior portion, more extensive than the other two put together, 

 corresponding to the posterior mediastinum, and attached to that septum by 

 means of a fold developed around the organ, to form the pulmonary pleura ; 

 this fold constitutes, posteriorly, a small serous ligament [ligamentum latum 

 -pulmonis), attached at once to the mediastinum and the posterior face of the 

 diaphra<5m. On this portion of the lung are remarked two antero-posterior 

 fissures : one, hollowed near the upper border of the organ, to receive the 

 thoracic aorta ; the other situated lower, but not so deep, more marked in 

 the left than the right, and lodging the oesophagus. In the right lung this 

 mediastinal face offers a small particular lobule, which is absent in the left. 



The hase, or diaphragmatic face of the lung, out obliquely from above to 

 below, and before to behind, is concave, and moulded to the anterior face of 

 the diaphragm. On the right lung is seen the posterior face of the small 

 lobule noticed on the inner side, and a deep fissure excavated between it and 

 the principal lobe, for the passage of the posterior vena cava. 



The summit of the viscus, situated behind the first rib, presents a kind of 

 detached appendix, designated the anterior lobule of the lung. 



The superior border, thick, convex, and rounded, is lodged in the vertebro- 

 costal channel or concavity. The inferior, much shorter and thinner, is 

 deeply notched at the level of the heart, and more so at the left than the 

 right side. The posterior is elliptical, and everywhere circumscribed by the 

 face of the diaphragm, which it separates from the costal and mediastinal 

 faces. 



Structure. — An external serous envelope, proper fundamental tissue, 

 functional and nutrient vessels, lymphatics, and nerves : such are the elemeats 

 which enter into the organisation of the lung. 



Serous Envelope. — This is the pleura pulmonalis already described. 

 (There has also been described a subserous connective tissue, containing a 

 large proportion of elastic fibres ; it invests the entire surface of the lung, 

 and extends between the lobules.) 



Fundamental Tissue. — Physical characters. — The pulmonary tissue in 

 the adult is of a bright rose-colour ; it has a deeper hue in the foetus which 

 has not respired. Although soft, it is yet very strong and resisting, and 

 can with difficulty be torn. Its elasticity is remarkable ; it concurs in the 

 collapse the lung experiences when air is admitted to the pleural sacs. It is 

 very light: plunged in water, if healthy, it floats: this specific lightness 

 ought to be attributed to the air imprisoned in the pulmonary vesicles. 

 This may be proved by what takes place when the lung of a foetus is 

 inflated : heavier than water before that operation, it then becomes lighter, 

 because, notwithstanding all the manipulation that may be employed to expel 

 the air introduced into the pulmonary vesicles, a certain quantity always re- 

 mains. On the other hand, the absolute weight of the lung is relatively more 

 considerable in the adult than in the foetus, the first representing l-30th of 

 the total mass of the body, while it is only l-60th in the second. 



A knowledge of these facts may be utilised in determining whether a given 

 lung has belonged to an animal which has respired or has died before birth. 

 If the tissue is plunged in water, this test is called hydrostatic pulmonary 

 docimacy • if its relative weight is to be ascertained, it is designated 

 pulmonary docimacy by weight. 



These are the physical characters of the fundamental tissue of the lung ; 

 we will now study its anatomical characters. 



Anatomical characters. — The pulmonary tissue is partitioned into a great 



