SECT. 21.] TISSUE OF THE LENS. 37 



organ ; probably, however, not by the direct calcification of these 

 cells, but the excretion from them. (Sec below.) 



B. As Tissue of the Lens. 



The lens, as its development shows, is an epidermic structure ; 

 and its long, partly tubular, partly solid fibres, are each developed 

 by the elongation of a single epithelial cell of the capsule. 

 Nevertheless, it deserves a distinct place, partly on account of its 

 chemical composition, and partly on account of the peculiar form 

 of its elements. 



Literature. — Purkinje et Valentin, De Phcenomeno generali et funda- 

 mental i Mot us Vibratorii continui. Vratisl. 1833. (Discovery of ciliary 

 motion in the higher animals.) Sharpey, Art. Cilia, in Cyclop, of Anatomy. 

 Henle, Symbolce ad Anatom. vill. int., Berol. 1837. On the Distribution of 

 Epithelia in the Human Body, Berlin, 1838 ; and On the Formation of Mucus 

 and Pus, and its Relation to the Epidermis (first accurate description of the 

 different epidermic cells). Valentin, Art. Flimmer-Bewegung (Ciliary Move- 

 ment), in Ilandwor. d. Physiol., Jdsche de Telis epithelialibus in specie et de Us 

 Vasorum in gene re. Dorp. 1847. 



§ 21. Tissue of Glands. — The glands possess, as their most es- 

 sential part, the secerning elements, which appear as cellular 

 masses, shut and open vesicles, and tubes, and contain, as the 

 most important constituent, the gland-cells, or gland-parenchym- 

 cells, as they are called. These cells are mostly polygonal or cy- 

 lindrical, and completely resemble certain epithelial cells; on the 

 other hand, they are very frequently characterised by peculiar 

 contents. The union of these cells, to form the secerning parts of 

 glands, takes place either directly or with the co-operation of 

 homogenous membranes, secreted by the gland-cells (membrance 

 propria?, as they are called), and of connective tissue. There thus 

 arise, according to the different glands, different secerning glan- 

 dular elements, which are surrounded by vessels and nerves, and 

 connected together by areolar tissue, with which elastic fibres, fat- 

 cells, and even muscular fibres are intermingled, to form the larger 

 and smaller divisions of the glands. The chief forms of secerning 

 glandular elements in man are the following : — 



1. Solid cell-neticorks, without rudiments of an investing mem- 

 brane. In the liver. 



2. Shut vesicles, with a fibrous coat and an epithelium, Graafian 

 vesicles of the ovaries ; follicles of the thyroid. The thymus, which 

 consists of a common cavity beset with numerous vesicles, per- 

 haps belongs to this division (Fig. 11). 



