468 THE BESPIBATOBY APPARATUS IN MAMMALIA. 



number of small polyhedral lobules by septa of connective tissue, which 

 appear to be prolongations of the corium of the external serous membrane. 

 This segmentation into lobules is a common feature in the organisation 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 Ruminants and Pachyderms. 



The organisation of these lobules resembles, in a striking manner, that 

 of the salivary lobules. Each receives a small bronchial tube (lobular 

 bronchial tube), which is prolonged into the lobule by several short terminal 

 branches, in which open a certain number of elementary vesicles. In 

 comparing, for the moment, the lung to a gland, it will be seen that this 

 organ should be ranged in the category of racemose glands. 



To demonstrate the vesicular structure of the lung, it may be inflated 

 and dried, and sections afterwards made to show the pulmonary vesicles. 

 But this procedure has the inconvenience of unduly extending the vesicles, 

 and thinning, and even destroying, their walls. A better method is the 

 following : leave the lung in the intact thoracic cavity ; by the jugular vein, 

 pour into' the right side of the heart an injection of very hot tallow, 

 employing a certain amount of force to propel it from the pulmonary artery 

 into the veins ; when this injection has cooled, open the thoracic cavity, and 

 take out the lungs. These, being impregnated with solidified fat, do not 

 collapse on contact with the air, and sections made in different directions 

 then exhibit innumerable perfectly-circular porosities, which are the open 

 pulmonary vesicles. 



In this way it is easy to demonstrate the presence of the air-cells ; but, 

 in order to conveniently study their arrangement, it is necessary to take a 

 cast of them by means of a solidifiable material introduced by the bronchii, 

 and afterwards destroyed by the maceration of the pulmonary tissue. The 

 Darcet alloy, employed in this manner, often gives very good results. It is 

 then found that the pulmonary vesicles form, in each lobule, saccular 

 dilatations or culs-de-sac, from l-70th to l-200th of an inch in diametei', 

 grouped around the infundibuli, of which they are only 

 Fig. 238. diverticuli ; these infundibuli communicate with the 



terminal bronchule of the lobule through the medium of 

 a narrow central cavity, into which they all open. Such 

 are the principal histologic details relative to this im- 

 portant point in the history of the lungs ; and it must 

 necessarily be followed by a notice of the structure of 

 the pulmonary vesicles (or air-cells). 



The pulmoiMry vesicles comprise, in the organisation 

 of their walls : a proper membrane ; epithelium ; and 

 capillary vessels. 

 [plan of a pul- 1- thQ proper memhrane is thin and homogeneous; it 



MONARz LOBULE. coutains the nuclei of connective tissue and elastic fibres, 

 a, Bronchule tevmi- and its external face is applied to that of the neighbour- 



duSo" 6!- ;, 'it- ^"" rit' ' ^'1 ^?*'™^^ ^"-'^ '^ ^""^^ ^y epithelium. 



sacs, or infundibuli; "'• -"-"^ epithelium IS Simply tesselated, and is com- 



d. Air or pulmonary posed of extremely thin cells. It is continuous through- 



■''^s''=i"- out the vesicles, and with that of the terminal bronchule. 



,. . , , ^^ ^ properly-prepared section, the polyhedral cells 



lining the latter can be seen changing gradually and rapidly into 



squamous epithelium in the cavity of the lobule, at the entrance to the 



infundibuli. 



