TISSUES OF THE BODY. 



141 



(fig. 130). According to our way of thinking they should be called 

 pigmentary epithelia. 



The extremely variable depth of this tissue just mentioned leads to 

 further variety. Besides epithelium, in which many layers are placed 

 one over the other, forming a heavy coating 

 (fig. 131), we find others made up of but one 

 single stratum of cells (fig. 132) ; and between 

 the densely laminated and non-laminated species 

 there exist many intermediate forms, in which 

 only a few strata are to be seen, disposed one over 

 the other. It must be borne in mind, from this Fig. iso. Pigmentary flattened 

 on, that it is only the flattened epithelia which are hSnS 

 capable of becoming laminated to any remarkable 8hee P- 

 extent ; but that they need not necessarily everywhere take on this form. 



87. 



The most widely distributed variety of the tissue under consideration 

 is the flattened or pavement epithelium. 

 Overlooking its more limited occurrence 

 in certain regions, it is met with on the 

 external skin, on many mucous mem- 

 branes, in serous sacs (true and false), 

 as well as on the internal surfaces of the 

 vessels of the circulatory system. Its 

 thickness is subject to the greatest varia- 

 tion, so that, at one time strongly lamin 

 ated, it represents the strongest of all 

 epithelia; at another it displays merely 

 a delicate coating of cells of the simplest 

 kind. 



Simple pavement epithelium (1), in 

 the first place, forms the internal coating 

 of the cavities of the heart, as also of 

 the blood and lymphatic vessels. (2.) 

 It makes its appearance, further, in the 

 true serous sacs on synovial membranes 

 (bursse, sheaths, and capsules for the 

 joints). (3.) Again, within the eye, on 

 the posterior surface of the cornea and 

 anterior of the iris ; on the internal surface of the anterior segment of the 

 capsule of the lens ; within the auditory apparatus, 

 namely, on the periosteum of the internal ear, the 

 inner surface of the semicircular canals and vesti- 

 bule. To what extent gland ducts possess such a 

 lining need not for the present be discussed. We 

 find sometimes a simple and sometimes slightly 

 laminated pavement epithelium in the canals of Fig. 132. simple coating of 

 exit of the sweat and ceruminous glands. The nTuc^me^bran^Fibrou* 

 infundibuli of the lung are likewise lined by the tissue of mucous membrane 

 same species of cell. (4.) Finally, the greater part 



of the ventricles of the brain in the adult is covered with a species of 

 pavement epithelium instead of the ciliated cells of early life. 



Fig. 131 Vertical section of the skin of a 

 negro. Thick laminated epithelium lying 

 on the elongated papillae of the dermal 

 tissue (a), with younger cells at b and c, 

 older at d. 



