THE STRUCTURE OF THE ELEMENTAKY 1 ISSUES. 29 



The epithelium of the various glands, and of the whole intestinal 

 tract, has the power of secretion, i.e., of chemically transforming certain 

 materials of the blood; in the case of mucus and saliva this has been 

 proved to involve the transformation of the epithelial cells themselves; 

 the cell-substance of the epithelial cells of the intestine being discharged 

 by the. rupture of their envelopes, as mucus. 



Epithelium is likewise concerned in the processes of transudation, 

 diffusion, and absorption. 



It is constantly being shed at the free surface, and reproduced in the 

 deeper layers. The various stages of its growth and development can 

 be well seen in a section of any laminated epithelium such as the epi- 

 dermis. 



The Connective Tissues. 



This group of tissues forms the Skeleton with its various connections 

 bones, cartilages, and ligaments and also affords a supporting frame- 

 work and investment to the various organs composed of nervous, muscu- 

 lar, and glandular tissue. Its chief function is the mechanical one of 



Fi3. 28. Horizontal preparation of cornea of frog, stained in gold chloride ; showing the net- 

 work of branched cornea corpuscles. The ground substance is completely colorless, x 400. 

 (Klein.) 



support, and for this purpose it is so intimately interwoven with nearly 

 all the textures of the body, that if all other tissues could be removed, 

 and the connective tissues left, we should have a wonderfully exact 

 model of almost every organ and tissue in the body, correct even to the 

 smallest minutiae of structure. 



Classification of Connective Tissues. The chief varieties of con- 

 nective tissues may be thus classified : 



I. The Fibrous Connective Tissues. 



A. Chief Forms. 

 . White fibrous. 

 I. Elastic. 

 c. Areolar. 



