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 potassm. They are most visible in the lungs of the ce air and the blood. These two elements are separated only by a slender hyaline lamina 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 trabecukc. 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 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 Ki3lliker 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-eells, 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 alruost 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
Promotor 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 system : " the secori-1, pulmonary, or small circulation." The-nutrition of some parts of these organs is sustained by another order of vessels, distinct from these.
The pulmonary artery, conducting venous blood, and proceeding from the right ventricle, 13 the channel by which the blo9d destined to arterialised is conveyed to the lungs. It is circumscribed in its distribution to the area of the true pulmonary tissue as distinguished from the bronchial. The plexus formed by its branches is emphatically the rete mirabile. The branches of the pulmonary artery follow those of the bronchi as far as the origin of the'intercellular passages ; a point at which they assume an irregular course over and between the cells. A lobule of the lung receives, with great regularity, only a single ramusculus from the pulmonary artery. It is not so large in size as the bronchial tube which it accompanies. Within the lobule, the artery coincides with the tube in its divisions, which are here more intimately bound to gether than at the extralobular stages of their course. It was supposed by Bourgery that the artery formed a framework of vessels around the tube. This is not the case. Of course, many of the branches of the pulmonary artery course between the lobules in order to reach others more distantly situated. Reisseissen conceived that he had traced a rarnuscule ot the pulmonary artery to the root of each " vesicle," describing a venule on the other side. Krause supposed that each individual cell, with unvarying constancy, had its artery and vein, and intermediate plexus.* Berres believed that each cellule presented, on its circumference, a great many facets, like the eyes of insects, each facet having its own plexus. Rossignol t embraces the views of Krause which assign a separate arteriole and vennle to each cell. In the accompanying figure, taken from the essay of Adriani, and drawn from a preparation by Schroeder van der Kolk himself, the branches of the puhno nary artery are seen to run, not only between the ultimate air-cells, but in many instances through the very centre of the walls.