Title: Compendium of histology
Identifier: compendiumofhist00frey (find matches)
Year: 1876 (1870s)
Authors: Frey, Heinrich, 1822-1890; Cutter, George R. (George Rogers), b. 1840, tr
Subjects: Histology
Publisher: New York, G. P. Putnams sons
Contributing Library: Boston Public Library
Digitizing Sponsor: Boston Public Library

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30 THIRD LECTURE. retina. They were formerly called polyhedral pigment cells (Fig- 35)- Their surface presents a delicate mosaic, as a rule, of a hexagonal form, of 0.0135 to 0.0204 mm. The quantity of the pigment granules embedded in the soft, homogeneous substance of the cell body varies, so that the nucleus is some- times visible, at others concealed. The outer portion of the cell remains free fig 35 -rigment epkhe- fr°m these melanine molecules, which IS,? .fort&jlJS appear to form small crystals (Frisch). ^nalone!1 ",alargerocta" The profile view shows, however, that our cell, far from constituting a flat struc- ture, possesses, rather, a certain, sometimes a considerable, height, which, at least, equals the diameter. It is thus in the lower vertebrate animals ; here their body sends downwards a number of filamentous and spiny processes. The latter, at first, still contain pigment molecules, and surround in a sheath-like manner the rods and cones, the marvellous termi- nal apparatus of the retina. This is less frequent in mam- malial animals. These pigmented cells extend beyond the limits of the specially nervous portion of the retina, over the so-called ora serrata, where they become smaller, more rich in pigment, and are in thin layers. In this manner they then cover the ciliary processes and the posterior surface of the iris. Certain of the mammalia (the carnivora and ruminantia) have a bright, glistening place in the interior of the eye, called the tapetum. Our retinal epithelium is here unpig- mented. In albinos this is the case throughout, and the cells present the appearance of a delicate pavement epithe- lium, as, for instance, in the white rabbit. Many mucous membranes present material, and sometimes very considerable, layers of pavement epithelium. Among these are the conjunctiva of the eye, the mucous membrane of the nostrils and of the anus, the mouth and pharynx, the oesophagus, the urinary passages, and the vagina. In the most superficial layers of the conjunctiva (Fig. 36, a)

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