THE LUNGS. 547 



lung, this mediastinal face offers a small particular lobule, which is absent in 

 the left. 



The base, or diaphraf/mafic face of the lung, cut obliquely downwards and 

 backwards, 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 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 — 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 on 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, l//mphatics, and nerves : such are the elements which enter 

 into the organization of the lung. 



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

 Lymphatic stomata are observed on its surface. (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, and is the cause of 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 is due 

 to the air imprisoned in the pulmonary infundibula. This may be proved by 

 what takes place when the lung of a foetus is inflated : heavier than water 

 before, 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 remains. On the other hand, the absolute weight of the 

 lung is relatively more considerable in the adult than in the foetus, the first 

 representing -^^ of the total mass of the body, while it is only ^^ in the second. 



A knowledge of these facts may be utilized 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 divided into a great number 

 of small polyhedral lobules by septa of connective tissue, which appear to be 

 prolongations of the corium of the enveloping serous membrane. This segmen- 

 tation into lobules is a common feature in the organization of the lungs in the 

 Mammalia, but it is more readily demonstrated in some than others : not very 

 evident in Solipeds, and less so in the Carnivora, it is well defined in Rumi- 

 nants and Pachyderms. 



As the lobule constitutes the genetic unit of the lung, we will study its 

 organization in detail, as has been already done with regard to the hepatic 



