272 



RESPIRATION, ORGANS OF. 



then be accepted as a fixed conclusion in the 

 histology of the lungs that the air-cells are 

 lined internally by a single layer of "hyaline 

 epithelium." This conclusion is corroborated 

 by the minute structure of the respiratory 

 organs in all animals. In none are the vessels 

 absolutely naked. 



Elastic Tissue of the Air-Cells. The exist- 

 ence of this tissue is admitted by every ana- 

 tomist who has studied the subject. Its dis- 

 position amid the air-cells is less known. It 

 fulfils a part, though mechanical, of the high- 

 est consequence to the movement of the lungs 

 in respiration. The fibres of this tissue belong 

 to the yellow variety. They resist both the 

 action of acetic acid and liquor potassas. 

 They are most visible in the lungs of the ce- 



(After S. Van der Kolk.) 



a, b, elastic tissue (with thick yellow fibres) 

 bounding an air-cell in the lung of the whale ; c, a 

 small portion of the wall of the same, showing the 

 capillary web injected. 



tacei. They are readily detected in those of 

 all mammals. 



They are limited chiefly, in distribution, to 

 the edges and margins of cells. They encircle 

 foramina, and maintain them by their elasticity, 

 in a patulous state. They not unfrequently 

 arch over the roof of the air-cells, constituting 

 to the latter true trabeculcB. They pass from 

 cell to cell, and form an important connecting 

 tissue. They are everywhere arranged in 

 bands or fascicles, or in a large meshed net- 

 work of single fibres, as shown in the adjoined 

 figure. 



When they are distributed over the flat 

 surface of an air-cell, they are situated im- 

 mediately under the epithelium. As there are 

 two epithelial surfaces to each cell-wall, the 

 intermediate vascular plexus being single, it 

 follows that the elastic fibres must run over 

 and between this plexus on both of its sur- 

 faces. A real framework is thus constructed 

 which is well adapted to support the capil- 

 lary layer ; and this is an important function, 

 which devolves on the elastic tissue. The 

 framework formed by this tissue over the 

 walls of the air-cells is so large-meshed that 

 it does not obstruct the contact between the 



air and the blood. These two elements are 

 separated only by a slender hyaline lamina 



Fig. 222. 



View of a thin section of the lung of a Cat, vihich had 

 been injected by the pulmonary artery with gelatine, 

 so as to Jill blood-vessels and air-cells, and had been 

 sliced when cold. {After Todd and Bowman.) 

 a, a, a, air-cells and lobular passage in section ; 

 b, b, their fibrous wall in section ; c, their wall in 

 face ; d, extremely faint nucleus in the same ; e, e, 

 capillaries ; h, nucleus in wall of capillary ; n, small 

 pulmonary artery, or vein with simple wall. (Mag- 

 nified 250 diameters.) 



of epithelium and the coats of the blood- 

 vessels. At the edges, angles, margins of 

 cells and the foramina between the latter, the 

 fibres of this tissue are gathered into dense 

 and strong bands having an arched and cir- 

 cular disposition. It is suspected by Kolliker 

 that there may be muscular fibre-cells among 

 this tissue in the air-cells, like those already 

 described in the walls of the bronchial tubes. 

 But the long nuclei which occur in the walls 

 of the air-cells, seen also by Mr. Rainey and 

 frequently by the author, are situated in 

 the substance of the tunics of capillary blood- 

 vessels. They are neither so large nor so 

 long as the unstriped muscle nucleus. In 

 the walls of the bronchi the elastic fibres 

 were described as denser and stronger than 

 those of the walls of the air-cells, and as 

 observing almost exclusively a longitudinal 

 arrangement, the muscular fibres being dis- 

 posed circularly. Among the air-cells they 

 exhibit that order and plan which fit them 

 best to subserve the mechanical exigencies 

 of the part. Harting, S. Van der Kolk, and 

 Promoter have detected these fibres in the 

 sputum of phthisis, which they regard as cha- 

 racteristic of the existence of a vomica. A 

 failure in the mechanical property (elasticity) 

 of this tissue amid the air-cells is probably 

 one of the conditions of emphysema. 



Vascular System of the Lungs. The blood- 

 system of the lungs constitutes a separate 



