The internal or acute margin of the bran chial process conceals, in the recent structures, under the labyrinth of the superinmosed re spiratory membrane, a skeletal mechanism of singular novelty and beauty (f, fig. 238.) From the innermost side (near its root) of the denser piece which lies parallel with the outer margin of the lamella, a less dense, more transparent, and more cartilage-like process (d,fig. 238.) rises, to advance along the in ternal margin of the lamella from its base to its extreme apex. From either side of this process, at right angles, processes (f and c c), still more slender, delicate, and trans parent, are detached, to follow for some dis tance the circumference or free margin of the membranous leaflet on which the vascular network is outspread.
This part of the skeletal fabric of the branchial lamella is actively and importantly concerned in the mechanism of the respira tory function. It is to the branchiae what the ribs are to the mammalian lungs. The fine extremities of these transverse portions, as seen in c, c, taper off into a species of curly fibre, which travels accurately along the extreme margin of the membranous leaflets to the point (a) at which the latter rest upon the flat surface of the osseo-cartilage of the obtuse margin. The axis which sup ports this system of elastic "ribs" (in sec tion at c and d) exhibits under the mi croscope a median transverse line appa rently filled with an oleaginous fluid, which communicates with the moniliform system of cells, occupying the axes of the curved pieces (c, c), where probably it performs the two-fold office of mechanically distending and nourishing the parts. IsTo vestige of an Ha versian order of canals can be discovered in any portion of this branchial framework. The calcareous granules of Tomes are dis tributed irregularly over the parietes of the cartilage cells. It is not easy to misappre hend the office which devolves upon this apparatus. By means of two needles the recent branehial ray may be separated into two longitudinal halves, to the inner of which, exclusively, the membranous leaflets remain adherent, a circumstance which illustrates the anatomical connection between these softer parts and the delicately' adjusted framework of the internal border of the leaflet. The elastic transverse processes (c, c), from their constrained curved position, constantly tend to straighten themselves, and to convert the curved into a direct line of action. This straightening tendency, which is a constantly and unremittently operating force, constitute the immediate agency under w hich the tang ling, folding, or crumpling of the leaflets bearing the capillary network is rendered impossible.
Under the ceaseless operation of this resilient property, the true breathing surface is regu larly maintained in a state of uniform exten sion, and that at a degree of lightness, measured v:ith wonderful precision, to suit the exigencies of structures so surpassingly refined. A superficial consideration indeed might have sufficed to render it improbable, that a system of blood-channels of such ex treme delicacy as that which constitutes the breathing apparatus of fishes could exist un injured, unless by aid of a basis of support at once appropriate in its physical qualities and mechanical disposition.
Aforbid anatomy of the lungs and air pas sages. —it is possible in the space here al lowed to do ldtle more than to enumerate the pathological conditions to which these parts are liable. The diseases of the lungs and bronchi since the era of Laennec have re ceived a considerable share of the attention of pathologists. The normal anatomy of the lungs is now known with precision. The characteristics of the alterations of structure which in disease they undergo, within recent years have also been defined with correspond ing precision. The community between the bronchial and pulmonary circulation established by the recent researches of Dr. Ideate, will probably oblige pathologists to modify their views with respect to the supposed distinct ness and independence of the diseases of the lungs and the bronchi. The following tabu lated arrangement, as given by Rokitansky, in the 4th vol. of his Pathological Anatomy, ex hibits the abnormal conditions of these parts in lucid suminary.
small bronchi and those near the surfaces and borders of the lungs are most liable to stiffer this change. The walls of the tubes at the dilatations are hypertrophied and thickened. Sometitnes, with the saccular variety, the same parts are relaxed and attenuated.
The bronchitic collapse of the lungs occurs under two distinct aspects, the diffused form, and the limited or lobular form. Of these the latter variety is the more striking or cha racteristic, and has been, especially in the lungs of children, the subject of more discus sion than the former. But the diffused form is by far the more common, and is of frequent occurrence in its slighter degrees. In both conditions the pulmonary tissue presents a dark violet colour as seen beneath the pleura ; internally it is red.