36 NORMAL HISTOLOGY. 



the limiting membrane of the serous cavities. It 

 forms an encasing membrane for many organs, and, 

 extending into their interior, serves, under the name 

 of interstitial tissue, to support their parenchyma. 



Fibrillar connective tissue is composed of two dis- 

 tinct classes of structures : a, cells, and b, a substance 

 lying between the cells, the intercellular or basement 

 substance. We shall consider the latter first. 



b. INTERCELLULAR SUBSTANCE. 



The intercellular substance consists chiefly of two 

 distinct kinds of fibres : fibrillated fibres and elastic 

 fibres. 



The fibrillated fibres are tiny, grayish, translucent 

 cords, which, sometimes singly, sometimes in straight 

 or wavy bundles, lie nearly parallel to one another, 

 and again cross each other at all conceivable angles, 

 forming complicated networks. When examined 

 fresh, with high-magnifying powers, the individual 

 fibres are seen to be moderately refractive ; they 

 have a delicate longitudinal striation this striation 

 being, as we shall presently see, the expression of 

 the fact that each fibre is made up of a number 

 of still finer fibrillae. 



On boiling for a considerable time in water they 

 are converted into gelatin, and when treated with 

 acetic acid or dilute alkalies, they swell up, lose 

 their longitudinal striations, become very trans- 

 parent, and finally almost invisible. 



