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, Jig. 20. ; 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 membrane, of r l ? 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 remarkable than in mammalia. 



The tracheal cartilaginous rings come next 

 to be described. Each cartilage forms a 

 little more than three quarters of a circle 

 ( e >fig- 207.). It embraces the anterior three- 

 fourths of the tracheal tube. The deficient 

 portion, comprising the remaining fourth of 

 the circle, is completed by muscle and mem- 

 brane and elastic tissue (k, Jig. 207.). This 



* " Die Driisen des Kehlkopfs und der Luftwege 

 liberhaupt warden bei Catarrhen haung verandert, 

 so dass ihre Blaschen bis 0'08 selbst 0-15'" messen 

 und mit kleinen rundlichen Zellen erfult sind, die 

 wohl den auf Schleimhautoberflachen sich bilden- 

 den Schleimkorperchen sich vergleichen lassen." 

 Kolliker, Anat. Mik., p. 452. 



" Interdum hse cryptae mucosae amplificatae, ita 

 ut pane V" latae sint, et sinubus secundariis in- 

 structae reperiuntur. In hoc casu illae saepe usque 

 in regionem plexuum vasorum postea et porrigun- 

 tur. In preparatis per longum tempus in spiritu 

 vini asservatis aperturae harum cryptarum muco- 

 sarum nudis oculis optime cerni poterant, atque 

 imprimis numerosaa in parietis anterioris interstitiis 

 non cartilagineis et in pariete posteriore animad- 

 vertebantur." Disq. de Struc. et Text. Canal, aerif. 

 S. E. Schultze, 1850. 



description applies only to the case of the 

 human subject. In the sheep the posterior 



Fig 207. 



Transverse section of the tracliea t/trough the middle 

 of (and parallel with} one of the cartilaginous rings. 

 (After Schrdtz.') 



a, ciliated epithelium, lining the inside ; 6, elastic 

 longitudinal tissue ; k, the tracheal muscles ; e, 

 ring cartilage ; /, external areolar tissue ; i, blood- 

 vessels ; 2, tracheal glands. 



ends of the tracheal cartilages meet, to pro- 

 ject behind as spinous processes, thus con- 

 cealing the trachealis muscle. In the horse 

 the same parts of the ring cartilages over- 

 lap. This arrangement prevails also in the dog. 

 In the ox the posterior ends of the tracheal 

 rings are everted. Each cartilage is embraced 

 in a fibrous perichondrium (f,Jig. 207.) which 

 is intimately united to the inter-annular 

 fibrous tissue. 



In ultimate structure these cartilages are 

 chiefly composed of cells interspersed through 

 a fibrous basis (b, c, d, c, b y Jig. 206.). The 

 cells are largest and doubly nucleated in the 

 centre of each cartilage (rf). At the inner 

 and outer surface these cells become flat- 

 tened and elongated (6, i). In different parts 

 of a vertical section of the cartilage they 

 present different directions as regards their 

 long axes (</). They are charged, in addition 

 to the nucleus, with minute fat molecules. 

 The first and last rings of the trachea are 

 figured differently from the rest. The first 

 constitutes a complete ring of cartilage, and 

 is frequently anchylosed to the cricoid. The 

 last is prolonged backwards over the mem- 

 branous interval ; consisting of more than 

 one piece, it arches over the angle where 

 the trachea bifurcates into the primary 

 bronchi. By such skilful adaptation of these 

 elastic rings, the occlusion of the air-tube 

 is prevented during the varied motions of 

 the neck and chest. Unlike the laryngeal 

 cartilages, the tracheal manifest no disposi- 

 tion to ossify. They amount, in man, to from 

 16 to 20 in number, and measure each in width, 

 from above downwards, about one line and a 

 half. They frequently fork at their posterior 

 s 3 



