18 ANATOMY OF THE RABBIT 



Sensory Epithelia 



There is a close association between the epithelia of the surface 

 of the body and the nervous tissues. In the adult we may dis- 

 tinguish as sensory epithelia special aggregations of cells lying in 

 either a deep or a superficial position, associated more or less 

 closely with the central nervous system, and functioning for the 

 reception of stimuli. That is, appropriate agencies set up in them 

 states of excitation which are then transmitted along sensory 

 nerves. 



They comprise the olfactory epithelium of the nasal cavity, 

 some of the cells of which are true nerve cells, the gustatory 

 epithelium of the tongue, and the auditory epithelium of the mem- 

 branous labyrinth of the ear. The retina — the nervous portion of 

 the eye — is a modified portion of the central nervous system. 



As linings of surfaces, the ordinary epithelia may be distinguished 

 from certain special coverings of internal spaces, the endothelia 

 and mesothelia. The two latter consist microscopically of thin 

 pavement-like cells. They differ from epithelia in origin, being 

 formed, not in connection with originally free surfaces, but in 

 relation to spaces of the mesoderm or intermediate layer of the 

 body. Endothelia form the linings of blood-vessels and lymph 

 canals, while mesothelia are the chief layers of the smooth, moist 

 serous membranes which line the peritoneal, pleural and pericardial 

 cavities. 



2. Connective Tissues 



The connective tissues form the supporting elements of the 

 body. As ordinary connective tissues they serve to connect organs 

 or parts of organs, and as skeletal tissues they provide the rigid 

 framework or skeleton from which all soft parts of the body are 

 suspended. They are distinguished by the presence of two main 

 components — the cell basis, and the intercellular substance or 

 matrix. The cellular portion is formative, and is much more 

 conspicuous in the embryonic than in the adult condition. All 

 connective tissues are products of an embryonic tissue, the mesen- 

 chyme (Fig. 22, ms.), which consists of branched cells connected 

 by their outstanding processes and typically suspended in a rela- 

 tively large amount of tissue fluid. Through the activity of the 



