CHAPTER IV. 

 SPLANCHNOLOGY. 



This section of the subject treats of the viscera, or organs which 

 occupy chiefly the cavities of the body, and compose respectively 

 the digestive, respiratory, urinary, and genital systems, all of 

 which communicate with the outside world by apertures on the 

 surface of the body. We will describe these systems in the order 

 here given, first noting that in all of them there is a certain 

 similarity in structure, in so far that each consists of a tube oi 

 canal, lined by mucous membrane, and provided with various 

 glands and other accessory organs. 



Before treating of these systems, it will be advisable to alludt 

 briefly to certain tissues and structures which are closely associ- 

 ated with them. These are epithelium, mucous and serous 

 membranes, and glands. Involuntary muscular tissue which 

 occurs extensively in these organs has been alluded to already. 

 (See p. 157.) 



EPITHELIUM. 

 This is a tissue cousistiug chiefly of cells, which, in one form 

 or other, covers all the free surfaces of 

 the body — i. e., the skin and mucous 

 membranes ; one variety of it also covers 

 the inner surfaces of closed and air-tight 

 membranous sacs, the serous and syno- 

 vial membranes, in which case it is dis- 

 tinctively known as endotkelium. The 

 following varieties of epithelium are de- 

 scribed : — squamous, columnar, sphe- 

 roidal, and ciliated. 



Squamous, tesselated, pavemental, or 



scaly epithelium consists of flattened scaly cells, usually 



arranged in superimposed layers. In shape irregularly oval, 



these cells vary somewhat, those on the surface being more 



270 



Fio. 88: 



Detached epithelium cells 

 from the mucous membrane 

 of the mouth, a. Cell wall ; 

 h. Nucleus ; c, Nucleolus. 



