468 '////: BKPIRATOBjr Ari'AHATUS IN MAMMM.I.l. 



mimbor of small polyhedral lobules by septa of cminertive tissue, which 

 appear to be prolongations of the corium of tho external serous incnilmine. 

 This segmentation into lobules is a common feature in tho organisation of 

 tlir lungs in the mammalia, but it is more readily demonstrated in some than 

 others : not very evident in Solipcds, and less so in tho Carnivora, it is well 

 di tilled 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 vi ->ii -li -s. In 

 comparing, for tho moment, tho 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 bo 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 tho 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 dim-lions 

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

 pulmonary vesicles. 



In this way it is easy to demonstrate tho 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 tho maceration of tho pulmonary tissue. The 

 Dorcet 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 diameter, 

 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 pulmonary vesicles comprise, in the organisation 

 of their walls : a proper membrane ; epithelium ; and 

 capillary vessels. 



PLAN OF A PUL- 1- The proper membrane is thin and homogeneous ; it 



MIJXARY umuM-:. contains the nuclei of connective tissue and elastic fibres, 

 a, Bronchule termi- and its external face is applied to that of the ncighbour- 

 nating in a slight ing vesicles ; its internal face is lined by epithelium. 



2. Tho epithelium is simply tcsselated, and is corn- 

 sacs, or infundibuli ; , - , ,, . n TA * i 

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

 vesicles, out the vesicles, and with that of the terminal bronchule. 

 In a properly-prepared section, tho polyhedral cells 

 lining the latter can bo seen changing gradually and rapidly into 

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

 infundibuli. 



