380 STRUCTURE OF THE TRACHEA. [SECT. 1 74. 



somewhat thinner layer, lines the posterior membranous wall of the 

 canals. Next to this layer come the cartilages in front and at the 

 sides, while posteriorly, the fibrous layer is in contact with a stratum 

 of muscular fibres. The cartilages \'" to-^'in thickness, present quite 

 the same characters as the larger laryngeal cartilages, except that 

 they have no tendency to ossify. The muscular fibres of the 

 trachea and its branches, on the other hand, differ in being no 

 longer transversely striated. They form a layer of transverse fibres 

 0'3"' thick, upon the posterior wall of the canals, and also bundles 

 of separate longitudinal fibres external to these. The component 

 elements of these fibres measure 0"03'" in length, and 0'002'" to 

 0004'" in breadth, and are connected in the form of small fasciculi. 

 These bundles are inserted by beautiful little tendons of elastic 

 tissue — partly into the inner surfaces of the extremities of the 

 semi-circular cartilages, and partly (especially the longitudinal 

 bundles), into the outer fibrous coat (see my Micr. Anat., ii. 2, 

 fig. 277). 



Internal to the cartilages and muscular fibres, which in a certain 

 manner are to be regarded as one layer, there follows a stratum of 

 dense connective tissue, of about 012'" in thickness, and then the 

 proper 7nucous membrane. The latter possesses two layers, an 

 outer one of connective tissue o-i-i" thick, and an inner yellow layer 

 009'" to O'l'" thick, almost purely composed of elastic fibres; these 

 fibres measure up to o - ooi5" in diameter, and their reticulations 

 run in the longitudinal direction : in some places, especially upon the 

 posterior wall, they appear in the form of thick, flat bundles, fre- 

 quently joining each other at acute angles. Upon the posterior wall 

 especially, the innermost portion of this elastic layer is more like 

 that of the larynx, being composed of connective tissue with fine 

 elastic fibres ; it can be separated from the coarser elastic layer as 

 a thin pellicle o'024'" to 0'03"' in thickness, and this itself is 

 divisible into strata, one of which, immediately beneath the epi- 

 thelium, is very homogeneous, and about cvoc^'" in thickness. 

 Upon them is seated the ciliated epithelium, which is laminated, 

 and differs in nothing from that of the larynx. In the mucous 

 membrane, there are numerous glands; those upon the anterior 

 wall being smaller ( T V" to !'"), and immediately external to the 

 elastic layer j those upon the posterior wall being larger (£'" to 1'"), 

 and situated external to the muscular fibres also. The larger of 

 these glands differ in no respect from those of the larynx ; but the 

 smaller, situated in the substance of the mucous membrane, are 

 often only simple or forked cul-de-sacs, composed of oval gland- 



