12 Anatomy o? the; Rabrit. 



gustatory epithelium of the tongue, and the auditory epithelium of the 

 membranous lal^yrinth 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 latter consist microscopically of thin pavement-like 

 cells. Thev differ from epithelia in origin, being formed, not in con- 

 nection with originally free surfaces, but in relation to spaces of the 

 mesoderm or intermediate layer of the body. Endothelia form the 

 linings of bloodvessels 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 



;■■ X^^ ' ''\'_,.J^^:31!iMc---'i\^. 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 



,1 , cell basis, and the intercellular substance 



^jl\A y' ' 1 / |i\ , ,. or matrix. The cellular portion is formative, 



ml ''■'/*' li' ''( i A ^nd is much more conspicuous in the 





"Ml 



' ; U embryonic than in the adult condition. All 



connective tissues are products of an em- 

 I bryonic tissue, the mesenchyme (Fig. 20, 



'; ms.), which consists of branched cells con- 



'__! nected together by their outstanding pro- 



cesses. Through the activity of the cells 

 there is formed an intercellular material 

 consisting either of a homogeneous matrix, 



ui^ vf.|j,^ or more frequently a matrix containing 



formed elements of a supportive nature. 



Fio. 4. From a section of the skin 

 of the side of the body of an adult 



rabbit, x about 10; showing the a ^^ ^ ^t^ 



grouping of the hair-follicles: Co., A. ORDINARY CONNECTIVE i ISSUES. 



corium; ep., epithelium; m.c.m., 



subcutaneo^fiire^ '™^'''^ '"• In the adult Condition the ordinary 



connective tissues, with few exceptions, 

 consist of the cell basis with two kinds of fibrous elements, the white, 

 and yellow fibres (Fig. 5). White fibres are elements of great strength. 

 They are fine, unbranched fibres which do not occur independently, 

 but are joined in a parallel fashion with one another, forming 

 in this way fibre bundles of larger or smaller size. The yellow fibres are 

 of greater diameter. They branch and anastomose, but are not asso- 

 ciated to form bundles. They also differ from white fibres in being 

 highly elastic. The tissue produced in this way is known as fibrous 

 connective tissue. It occurs in several forms according to the relative 



