THE LUNGS. 



549 



Fig. 327. 



the conductive and the respiratory parts, properly speaking, of the lung have 

 an organic independence, just as they have a physiological and pathological 

 individuality. 



The elements of the pedicle, in ramifying in the lobule, caiTy with them a 

 certain amount of connective tissue. 



To demonstrate the structure of the lung, it may be inflated and dried, 

 and sections afterwards made to show 

 the pulmonary alveoli. But this 

 procedure has the inconvenience of 

 unduly distending these small cavities 

 by 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 solidi- 

 fied fat, do not collapse on contact 

 with the air, and sections made in 

 different directions then exhibit in- 

 numerable perfectly circular porosi- 

 ties, which are the open pulmonary 

 vesicles. 



In this way it is easy to demon- 

 strate 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 solidi- 

 fiable material introduced by the bronchi, and afterwards destroyed by the 

 maceration of the pulmonary tissue. The Darcet alloy, employed in this manner, 

 often gives very good results, as seen in figures 327, 328. 



"We will follow the preceding details on the interior of the lung, \\ith some 

 words on the structure of its different parts. 



The bronchiole, on entering the lobule, has all the elements of the larger 

 bronchi — cartilaginous rings, a layer of racemose mucous glands, muscular layer, 

 festooned mucous membrane, and ciliated epithelium ; but as it divides and 

 becomes smaller, its composition is modified. The intra-lobular bronchioles suc- 

 cessively lose their cartilages, the glandular layer, and the contractile layer, and 

 at last at the acini the mucous layer is reduced to its basement membrane, and 

 the epithelium becomes cubical. The walls of the infundibula are formed by a 

 thin amorphous membrane strengthened externally by some connective tissue and 

 an elastic network ; while the interior is lined by a row of flat cells, the outline 

 and nuclei of which can easily be made out in the adult ajiimal. The capillaries 

 form an extremely rich network spread outside the proper membrane — though 

 sometimes they enter its substance, and push the epithelium towards the centre of 

 the alveoliis. 



CAST OF A PRETERMINAL AND TERMINAL BRON- 

 CHIOLE, FROM THE LUNG OF THE ASS. 



1, Bronchiole of ]§ millimetre in diameter; 2, 2, 

 ramifications ^ a millimetre in diameter; 3, 3, 3, 3, 

 sublobular bronchioles ; 4, ramifications of an 

 intra-lobular bronchiole; 5, 5, infundibula ; 6, 6, 6, 

 respiratory canaliculi ; 7, pulmonary alveoli ; 8, 

 debris from the canaliculated part of a pulmonary 

 lobule; 9, acinus. A point has been chosen 

 where the moulding of a great number of alveoli 

 and acini failed, in order to show the divisions of 

 the bronchi better. Magnified 5 diameters. 



