40 HANDBOOK OF PHYSIOLOGY. 



pulsion from the body. In the case of the Fallopian tube, this agency 

 assists the progress of the ovum toward the cavity of the uterus. Of the 

 purposes served by cilia in the ventricles of the brain nothing is known. 



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 epidermis. 



II. 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, mus- 

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

 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 small- 

 est minutise of structure. 



Structure of Connective Tissues in General. 



Connective tissue is made up of two chief elements, namely, cells 

 and intercellular substance. 



(A.) Cells. The cells are of two kinds: 



(a.) Fixed Cells. These are of a flattened shape, with branched pro- 

 cesses, which are often united together to form a network: they can be 

 most readily observed in the cornea, in which they are arranged, layer 

 above layer, parallel to the free surface. They lie in spaces in the inter- 

 cellular or ground substance, which are of the same shape as the cells 

 they contain, but rather larger, and which form by anastomosis a s} T stem 

 of branching canals freely communicating (fig. 36). 



To this class of cells belong the flattened tendon corpuscles which 

 are arranged in long lines or rows parallel to the fibres (fig. 42). 



These branched cells, in certain situations, contain a number of pig- 

 ment granules, giving them a dark appearance; they form one variety 

 of pigment cell. Branched pigment cells of this kind are found in the 

 outer layers of the choroid (fig. 37). In many of the lower animals, 



