SECT. 19, 20.] CELL-TISSUES. 33 



I. Cell-Tissues. 



§ 19. The epidermic and the glandular tissues, which I include 

 under the cell-tissues, have this in common, that they both, with 

 but few exceptions, arise from the continuous layer of cells lining 

 the inner and outer surfaces of the embryonal body ; and even in 

 the fully developed condition, essentially consist of cells, which, in 

 the one case, appear in compact masses, but in the other, generally 

 enclose spaces. In both tissues homogeneous structureless mem- 

 branes arc more or less extensively present. These are to be re- 

 garded as products of excretion from the cells; in the glands, 

 they surround the glandular elements as membrance proprice, 

 whilst in the epidermic tissue they are spread out in form of a 

 layer between the cells and subjacent vascular parts, with which 

 they are often intimately blended. Both tissues very closely 

 agree in respect of the form and chemical composition of their 

 cells ; and the close relationship between them is even more de- 

 cidedly indicated by physiological considerations, seeing that at 

 least the main function of glands, excretion, also belongs to very 

 many epidermic tissues. The latter also participate, it is true, in 

 the process of absorption, which can be ascribed to but a small 

 number of glands, and present other and altogether special rela- 

 tions ; circumstances which, however, do not affect the affinity of 

 these two tissues. 



§ 20. Epidermic Tissue. — The morphological character of epi- 

 dermic tissue consists in its being entirely made up of independent, 

 generally nucleated cells, intimately connected, without any visible 

 intermediate substance, which in part retain their perfect vesicular 

 character, and in part are converted into solid scales and fibres. 

 In a chemical point of view, this tissue is still but little known, 

 yet this much has been made out, that the cells principally contain 

 an albuminoid substance, partly, also, mucus; and that, at first, 

 all possess readily soluble protein-membranes, which, however, sub- 

 sequently become partly transformed into a substance which resists, 

 more or less, the action of acids and alkalies, and is named horn- 

 siubstance, or keratin. With the exception of the lens and the 

 enamel of the teeth, which are destined for altogether special ends, 

 the physiological purpose of the epidermic tissue is chiefly to serve 

 as a protective covering to parts rich in vessels and nerves, and by 

 the activity of its elements, to take part in the processes of secrc- 



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