In disease, however, these cells abundantly desquamate. (See Pathological Anatomy of the Lungs).
It thus appears that the cells of the tracheal epithelium proceed from the basement mem brane (b, fig. 204.) in the direction of the surface in successive generations, the youngest (cytoblasts) being the deepest and the next to the blood-supply, and the oldest, the highest and cilia-bearing. It is stated by Hannover t, that this epithelium may be indefinitely pre served in chromic acid.
The tracheal glands are productions of the mucous membrane of the trachea. The largest and most numerous are situated in the posterior wall of the tube, and possess long excretory ducts (i,fzg. 206.), which traverse the whole thickness of the muscular and elastic layers. The glandules themselves are best examined from the outside, and rest on the posterior surface of the trachealis muscle ; they exhibit a slightly reddish colour, and belong to the compound order of glands(f) ; they do not all belong to the same variety ; some coincide in structure with the salivary glands, others with the sudoriferous. Those which are found over the cartilaginous an terior three fourths of the tracheal walls, are thinly distributed, and penetrate into in tervals between the ring-cartilages. This class of glands rneasures from to* of an inch : they are smaller than those which cluster on the posterior surface of the tracheal muscle and which measure from * of an inch to an inch. In external characters these glands corre spond with the grape-like compound glands : from the latter, however, they differ in some particulars. It is only the larger variety of these glands which is lined at the terminal gland-vesicles, with globular epithelium, the ducts being clothed with the cylinder variety. The smaller sort, conforming in this respect with ordinary sudoriferous follicles, are lined throughout with cylinder epithelium. In these latter there is no distinction (except that of size) between the epithelium which clothes the blind extremities of the follicles and that which covers the ducts. These
follicles fork at their ccecal extremes, each branch measuring from 0.02 to 0.03"/ in diameter, the parietes of which are composed almost exclusively of fine small cylinder epi thelium.
The cylinder epithelium of the ducts of these glands bears no cilia; a character in which it differs from that of the mucous mem brane of the trachea in general. The ciliated variety ceases at the orifices of the ducts. The secretion of these glands is a limpid, non-corpusculated fluid. Here it is certain that the act of secretion is not synonymous with that of the shedding of the epithelium. Under certain pathological conditions the tracheal glands augment in size, and become choked with epithelial cells.* Fibrous Structures. — In the order from within outwards is next observed a remark able layer of elastic tissue. It lies imme diately underneath the mucous membrane (a, fig. 201. ; e,fig. 206.). It consists of two varieties of fibre, the yellow and the white. The former lies chiefly on the posterior wall, over the trachealis muscles. Its fibres are here disposed in a regular longitudinal direc tion. They are gathered into thick bundles readily seen with the naked eye, even through the mucous inembrane, of of an inch in thickness. They descend in a ser pentine manner along the posterior aspect of the trachea, and will be afterwards traced on the bronchi. They frequently anasto mose. Smaller fibres, forming a thinner layer of the same tissue, are distributed over the anterior walls of the trachea ; like the fornier portion, running under the mucous membrane and preserving uniformly a lon gitudinal course. Another order of elastic tissue lies between the ring-cartilages, tying them together cylindrically. Of this tissue the fibres are more slender than those of the former, and belong to the white variety (Bowman). It is to the elastic property of this tissue that the trachea owes its power of lengthening and shortening, a power which in birds is more remarkable than in mammalia.