260 



from 0-015 to 0-02"', and in breadth from 

 0-0025 to 0-004'", according to the measure- 

 ments of Kolliker.* This order of ciliated 

 cells is disposed upon a bed of cytoblasts in a 

 double stratum of about 0'006 to O'OT", in 

 thickness : they differ from ordinary cylinder 

 epithelia in the remarkable length to which 

 the attached extremity is prolonged ; the tail 

 of each cell exhibits quite the character of a 

 yellow filament, and measures from 0*024 to 

 0-027'" in length. 



In the centre of the broad end of each of 

 these cells is contained, without exception, a 

 clear, bright, oblong nucleus, of from O'OOS 

 to 0-0045'" in length ; and, further, each 

 nucleus bears a very visible nucleolus (e, fig. 

 204.). The cell contents consist of minute 

 granules and fat molecules. Valentin de- 

 scribes a double nucleus in many of these cells. 



The cilia attached to the tracheal epithelium 

 are clear, fine continuations of the cell-mem- 

 brane: they measure 0*0016 to 0-0022'" in 

 length : each cilium tapers to its free extre- 

 mity, that is, it is broader at its base than at 

 its apex. The cilia of the tracheal epithelium 

 are longer and more conspicuous in the em- 

 bryo than in the adult : the current excited 

 by their vibration tends in the direction of 

 the laryngeal outlet of the tube. Proofs will 

 be afterwards adduced that these motive or- 

 ganules enact no part in the office of re- 

 spiration : they subserve a merely mechanical 

 purpose in the process of mucous excretion. 

 These epithelia swell in water, while chromic 

 acid restores them to their original characters 

 (Hannover). By Biermer and Gosselinf, the 

 tracheal cilia have been detected in motion 78 

 hours after death in man. Under normal 

 circumstances no shedding occurs in the epi- 

 thelium of the air-passages. 



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 y 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 J, 

 that this epithelium may be indefinitely pre- 

 served in chromic acid. 



The tracJieal 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,fig. 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(/) ; 

 they do not all belong to the same variety ; 

 some coincide in structure with the salivary 



* Handbuch der Gewebelehre, &c., p. 450. 

 t Verb, der VVurzb. Pbys. et Med. Gesellsch. i. 

 212. 

 J Mttller's Archiv, 1840. 



RESPIRATION, ORGANS OF. 



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 measures from -^L- 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 tarminal 

 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 coecal extremes, each 

 branch measuring from 0'02 to 0-03'" in 



Fig. 206. 



i 



Ideal transverse section of the wall of the trachea. 



(After Kolliker.) 



a, areolar tissue embracing the cartilages ex- 

 ternally ; b, c, d, ring cartilage seen in section ; b, 

 outermost layers having flat cells ; c, soft and mid- 

 portion cells oval ; d, innermost layers with flat 

 cells ; e, sub-mucous tissue ; /, part of a tracheal 

 gland ; g, elastic tissue with longitudinal fibres j h, 

 ciliated epithelium. 



