470 THE BESPIBATOBY APPARATUS IN MAMMALIA. 



gical functions of the lung, like the vena portte with the liver, are very 

 properly distinguished from the other arteries or veins of the organ by the 

 designation of functional vessels. But though they are so named, it must 

 not be inferred that they are excluded from all participation in the acts of 

 nutrition. It is now admitted by competent authorities that the blood 

 of these vessels concurs to sustain vitality in the tissue of the lung, in 

 common with the nutritive fluid carried by the arteries and veins now to be 

 described. 



Nutrient vessels. — By this name is designated the divisions of the 

 hronchial arteries and veins, whose terminal ramifications anastomose with 

 the capillaries of the pulmonary vessels at the ultimate bronchules. 



Lymphatics. — These vessels are divided into mperfidal and deep. The 

 first form a network beneath the pleura , the second exist in large numbers 

 around the lobules. They mix together, and terminate in the bronchial 

 glands. (Lymphatics of very small size have been described as commencing 

 in the alveolar spaces, on leaving which they gain a proper coat or internal 

 tunic, and are subsequently supplied with valves.) 



Nerves. — The nervous branches supplied to the tissue of the lung come 

 from the same source as those of the bronchial tubes — the pneumogastric and 

 great sympathetic nerves. Their ramifications accompany the pulmonary 

 vessels and bronchia3, and they show small ganglia on their course. 



Functions. — To know that the lung is the seat of the absorption of 

 oxygen and the expulsion of carbonic acid from the nutritive fluid — 

 phenomena accompanied by the transformation of the dark into red-coloured 

 blood, and probably of several other metamorphoses yet doubtful or un- 

 known — is the only authentic fact necessary to remember with regard to the 

 functions of this organ. It must be added that the subtle molecular opera- 

 tions from which all these phenomena result, take place in the lung by the 

 mediate contact of the atmosphere introduced into the air-cells during 

 inspiration, with the blood traversing the walls of these cells. With the 

 intimate mechanism of these molecular actions we have nothing to do here, 

 however 



Development. — ^Although the lung is in a state of inactivity in the 

 foetus, yet it is one of the organs early developed. During the whole period 

 of foetal existence, its lobular texture is much better defined than in the 

 adult, and it then appears to be formed exactly like a racemose gland. 

 Sections of it prepared for microscopical examination distinctly show the 

 vesicles and their arrangement. We have already made known the differ- 

 ences in colour and density which distinguish the pulmonary tissue of the 

 foetus and that of the adult. It only remains to repeat what has been said 

 as to the slight vascularity of the first, and to note that the blood of the 

 pulmonary artery passes almost entirely into the posterior aorta by the 

 arterial canal (or ductus arteriosus). 



DUTEBENTIAL CHAEACTEBS IN THE LUNGS OP OTHER THAN SOLIPEB ANIMALS. 



The lunga of the Ox, Sheep, and Goat are remarkahlq for the distinctness with which 

 the lobules are defined. They are, in fact, separated by thick layers of cellular tissue 

 continuous with the internal face of the visceral pleura. ■ These thick septa are rather 

 the interlobular ramifications sent off from the subserous envelope.; Dietrichs who was 

 the first to draw attention to this peculiarity in the larger Euminants, has justly remarked 

 that it perfectly explains the altogether special characters of the lesions of pneumonia in 

 these animals. 



The general figure of the lungs of Euminants does not differ from that observed iu 

 the Horse ; the left lung, however, is divided into two lobes, and the right into four. 



