GENERAL HISTOLOGY. 83 



continuity. In both the peripheral end of the cell bears appen- 

 dages for sense perception; auditory and tactile hairs, stronger 

 processes in the case of olfactory and taste cells, conspicuous rods 

 in visual cells. Almost without exception the sensory cells are 

 part of the skin (ectoderm), or at least arise from it in develop- 

 ment. This is true for sense organs like the eye and ear of verte- 

 brates, which are separated from the skin by thick intermediate 

 tissue, for in these the sensory epithelium (retina, crista acustica) 

 is derived from the ectoderm. 



Supporting Cells. In the region of the sensory epithelium 

 and between the sensory cells are found still other epithelial cells, 

 which are not connected with nerves, but have accessory functions: 

 they serve as supports for the sensory cells; in the eyes they con- 

 tain pigment; in the auditory organs they often bear the otoliths, 

 etc. They have the general name of supporting or sustentative 

 cells. 



2. Connective Tissues. 



Contrast of Epithelium with Connective Tissue. From a his- 

 tological point of view there can be found no greater difference 

 than exists between epithelium and connective tissue; the former 

 belongs to the surface, the latter to the interior of the body; in 

 the former the cells play the chief role, in the latter, on the con- 

 trary, their importance is subordinate to the plasmic products, the 

 ' intercellular substances ' which chiefly determine the character 

 of the various kinds of connective tissue. 



In spite of this contrast the connective tissues are genetically 

 connected with epithelium. In embryos which at first consist 

 only of epithelia the connexion can be directly seen. The 

 epithelia secrete a gelatinous substance from their deeper surfaces 

 into which separate cells migrate. Thus arises the embryonic 

 connective tissue, the mesenchyme (fig. 107). 



Function of Connective Tissue. The primary function of con- 

 nective tissue is to fill the spaces between the various organs in 

 the interior of the body, thus connecting not only the single parts 

 of the organs, but also the various organs themselves. In conse- 

 quence of this the connective tissues contribute to the firmness of 

 body, and are frequently employed in building up a skeleton. To 

 accomplish this, substances which are usually firmer than proto- 

 plasm are formed on the surface of the cells, and, since they lie 

 between the cells, these are called intercellular substances. In 



