110 SIMPLE HISTOLOGICAL ELEMENTS. 



stance, containing granules in it; and the third form is an instance 

 of the development of scales from germinal points, or nuclei, as cells 

 are normally developed (Chap. IV.), and which scales coalesce at 

 their borders to form the continuous layer. In all these cases, how- 

 ever, it is organized, and manifests vital properties. 



Simple membrane is found only in two conditions, except when 

 an element of the compound tissues, viz., in the form of a mem- 

 branous expansion, to constitute 



1. Basement membrane, which will be described at the end of 

 this chapter. 



2. As constituting the walls of cells, whether secreting, absorbing, 

 or primordial, &c. &c. 



Properties of Simple Membrane. Though the simplest of the histo- 

 logical elements, it manifests important vital properties. Since it 

 forms the walls of cells (secreting and otherwise), simple membrane 

 is the direct agent of secretion in all cases. It is also, in many in- 

 stances, of absorption. These are, therefore, its two vital properties. 



The physical property of endosmosis also inheres in it in a re- 

 markable degree. 1 



Distribution of Simple Membrane. The basement membrane of 

 the skin, and of serous and mucous membranes, is formed of this 

 element. It therefore enters the lobes and follicles of all glands. 

 It forms the capsule of the crystalline lens, and the posterior layer 

 of the cornea. It lines the bloodvessels and the lymphatic vessels, 

 in the form of epithelium, if not of a basement membrane also. 



It forms the walls of all cells, whether blood-corpuscles, secreting 

 cells, fat-cells, &c. &c. 2 



As an element of compound tissues, it also constitutes the myo- 

 lemma of the muscular fibres, and the neurilemma of the nerve- 

 fibres or tubes. 



Simple membrane will, therefore, constantly recur in the descrip- 



1 By endosmosis is meant the property inherent in animal membranes, of trans- 

 mitting fluids through them. If two fluids of different specific gravity are placed 

 on opposite sides of them, they are transmitted in opposite directions, and thus 

 mixed one current being termed endosmosis, and the other exosmosis. 



2 Simple membrane also forms the walls of the cells from which the tissues of 

 vegetables are developed ; it being identical in them, it is said, with cellulose, 

 C 24 H 21 21 , or C 8 -fAqua 7 . It is an interesting fact, if verified by futuve investiga- 

 tions, that the vegetable cell is lined by a "primary utricle" or cell, identical in 

 composition with proteine i. e. it is an albuminous compound, as is the animal 

 cell-wall. 



