. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. RESPIRATION, ORGANS OF. 261 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.
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. RESPIRATION, ORGANS OF. 261 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. 204-. ; e,fig, 20G.). 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 membrane, of TV 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 former 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 remark
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