24 ANATOMY OF THE RABBIT. 
There is a close association between the epithelia of the surface 
of the body and the nervous tissues, arising from the circumstance 
that they are derivatives of a common embryonic layer, the ecto- 
derm. In the adult we may distinguish as sensory epithelia 
special aggregations of cells lying either in a deep or superficial 
position, and associated more or less closely with the central 
nervous system. They comprise the olfactory epithelium of the 
nasal cavity, some of the cells of which are true nerve cells, the 
gustatory epibhelumo of the tongue, and the auditory epithelium 
of the membranous 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 dis- 
tinguished from certain special 
coverings of internal spaces, the 
endothelia and mesothelia. The 
latter consist microscopically of 
—ts, thin pavement-like cells. They 
differ from epithelia in origin, being 
formed, not in connection with 
originally free surfaces, but in 
a ase AE niet eS relation to spaces of the mesoderm 
of the side of the body of an adult rabbit. or intermediate layer of the body. 
X about 10; showing the grouping of the a . riches 
hair-follicles: co., corium; ep., epithelium; Endothelia form the linings of 
m.c.m., cut aneus maximus muscle; (ESA 
subcutaneous tissue. 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 together, 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 
