450 



MANUAL OF HISTOLOGY. 



241. 



We turn now to the consideration of the lungs. 



These organs may, as regards form, be compared to racemose glands. This 

 resemblance is also seen in their mode of development. The excretory 

 canals are represented by the bronchial ramifications, and the acini by 

 the air- vesicles. Besides these numerous blood-vessels, lymphatics, nerves, 

 and connective-tissue structures enter into their composition. 



The two bronchi, which, as is well known, divide again into two before 

 their entry into the roots of the lungs, continue to sub-divide with the 

 factor two, and at acute angles, on entry into the organ, so that a multitude 

 of ever decreasing canals is soon formed. The cartilaginous supports lose 

 from this on the character of rings, and assume rather the form of irregular 

 plates and scales, which are no longer confined to the anterior wall, but 

 for the rest, as far as their texture is concerned, differ in no respect from 

 those of the trachea. The last traces of cartilaginous plates are only lost 



in bronchial twigs of extreme fineness, 

 Gerlacli having found them in those of 

 only 0*23 mm. diameter. The walls of 

 these tubules present, but in decreasing 

 strength, of course, the same fibrous layer 

 that we have already seen in the trachea, 

 and a mucous membrane with ciliated 

 cells, which loses gradually its laminated 

 structure, until there only remains at last 

 but one single layer, 0*0135 mm. in height, 

 of dwarfed cells (p. 149). Racemose 

 mucous glands, likewise, present them- 

 selves here until we pass into canals of 

 extreme fineness. The smooth muscular 

 layer which, as we have seen in the foregoing section, exists in the 

 trachea, forms around the bronchial passages a continuous tunic. It may be 



followed here, likewise, down to the 

 very finest tubes, and is present possibly 

 even in the neighbourhood of the air- 

 vesicles, but is certainly not to be found 

 on the latter. In the very finest tubes 

 the mucous membrane and external 

 fibrous layer become eventually fused 

 into one single thin coat, made up of 

 a homogeneous membrane surrounded 

 externally by elastic fibres. 



Owing to this progressive sub-divi- 

 sion, together with which small lateral 

 twigs are given off from the larger 

 bronchi, a very complex system of 

 branching passages is produced. 



At the end of the last bronchial twigs 

 (fig. 431, a), tubes of 0'3-0'2 mm. in 

 diameter, we come upon the true re- 

 spiratory part of the organ. This con- 

 sists, in the first place, of thin-walled 

 round canals of 0'4-0'2 mm. across. To these the name pf alveolar 



Fig. 431. A portion of the lung of an 

 ape (Cercopithesus) filled with quick- 

 silver (after F. E. Schulze). a, end of a 

 bronchial twig ; c, alveolar passage ; b, 

 infundibula. 



Fig 432. Two primary pulmonary lobuli 

 or infundibuli (a) with the air-vesicles, 

 6; and terminal bronchial tubes, c; which 

 also bear some of the pulmonary ve- 

 sicles, 6. 



