elastic character of its wall. As in secreting glands, we have here to deal with epithelium lining the cavity, and with connective tissue outside it, supporting blood-vessels. In this case the epithelium is of the simple squamous variety, the connective tissue outside it being in the form of a network of elastic fibres. The blood-vessels form a capillary network immediately below the epithelium, to some extent between it and the elastic tissue, and to some extent buried in the latter. Thus it will be seen that, between the blood in the vessel and the air in the sac, there is the thickness of two layers of squamous epithelium; on the one hand, the epithelium of the capillary wall, and on the other, that lining the cavity of the sac. At the opening of the trachea the epithelium changes in character, becoming columnar and ciliated, in which form it is continued along the tube till it becomes continuous with that of the pharynx. In the wall of the sac non-striped muscle cells are to be found here and there, running in the outer part of the elastic tissue in a transverse or circular direction; and a circular layer of non-striped muscle is also found in the trachea. However complicated in detail of structure the lung may become, as we pass from this simple typical form to that found in the mammalia, the general principle of its structure is never departed from. We have always to deal with an elastic sac, consisting of a layer of squamous epithelium, resting on a basis of elastic fibres, supporting a network of capillary blood-vessels. The frog's lung may be regarded as an intermediate form. It largely resembles the newt's, but is broken up into compartments by the projection inwards of septa of the subendothelial tissue, bearing with them the capillary network, covered on either side with the layer of squamous epithelium. In the upper part of the lung especially, we have these primary septa giving rise to secondary, and these again to tertiary septa; but this subdivision gradually ceases to take place as we pass towards the lower end of the sac. Thus, whereas the newt's lung is a single sac, the frog's is split up into a number of intercommunicating compartments, by the infolding of the wall at many points; such an infolding consisting of a basis of elastic fibres, supporting a capillary network, covered on either surface with a layer of simple squamous epithelium. As in the newt, the squamous epithelium of the lung becomes columnar in shape, and ciliated at the point where it passes into the trachea. The lung of a mammal, such as the cat, corresponds with a compound racemose gland, such as the sublingual or submaxillary. We have the trachea corresponding with the main duct; the extra-pulmonary bronchi with the few larger branches of the duct, which coalesce to form it; the intra-pulmonary bronchi with the inter-lobular and intra-lobular branches; the bronchioles, or smallest bronchi, with the intermediate ducts; the infundibular and alveolar passages with the lumen of the alveoli of the gland; and the squamous epithelium of the one with the glandular epithelium of the other. If we follow the mammalian trachea towards the lung, we find that, before reaching it, it divides into two bronchi, the right and the left. After entering the substance of the lung, these divide again and again dichotomously, to form the first or larger interlobular, intra-pulmonary bronchi. These subdivide, in like manner, to form lobular bronchi, each of which enters a lobule of the lung. Within the lobule the bronchus divides into several smaller branches, the bronchioles, each of which terminates in an infundibular passage, surrounded by the air vesicles of the lung. The Structure of the Trachea. The trachea, or windpipe (Fig. 88), is a tube consisting from within outwards of the following layers:— (1) The Mucosa, lined internally by a layer of ciliated epithelium (a), which is always stratified; ie., there are several layers of cells, the superficial of which alone bear cilia. Those immediately below them are younger cells, which in time will, as the superficial ones require replacing, reach the surface and acquire cilia; the most deeply placed cells are the germinal or proliferating ones, by the division of which the supply is maintained. Here and there, amongst the ciliated, will be found goblet cells. Beneath the epithelial lining there is found, in the human trachea, a very distinct basement membrane, of a clear homogeneous appearance. In chronic bronchitis, when the surface has become denuded of epithelium, this membrane often becomes enormously thickened from the irritation to which it is constantly exposed. In animals, such as the cat and rabbit, the membrane is very much thinner. Outside the epithelium there is a narrow layer of adenoid tissue (b), with a proportion of intermingled elastic fibres; and outside this a very distinct elastic layer (c). The fibres of this run longitudinally, and the outer |