394 GENERAL CONSIDERATIONS ON THE DIGESTIVE APPARATUS. 



Fig. 213. 



CONICAL VILLI ON MUCOUS MEM- 

 BRANE OF SMALL INTESTINE 

 (MAGNIFIED 19 DIAMETERS). 



a, Zone of follicles surrounding b. 

 solitary gland ; 6, apertures of 

 simple follicles. 



Fig. 214. 



more particularly the vascular and absorbent organs. The ^M_/?^7/fl? are found in 

 the vicinity of the natural apertures, and are rich in nerves ; they are more 

 especially the organs of sensibility. The follicles, 

 lined by one or other form of cell, are exclusively 

 organs of secretion. 



b. The second membrane met with in the walls 

 of the hollow organs is of a muscular nature. 



The mvsndar membrane is formed of unstriped 

 fibres, the slow contraction of which is involun- 

 tary. In certain organs — those adjoining the 

 natural apertures — the unstriped fibres are re- 

 placed by striped, which are under the influence 

 of the will, or have the same physiological and 

 reflex properties as the smooth fibres — as in the 

 oesophagus. 



{Unstrijied or smooth bands of muscles are 

 composed of long fusiform cells with staff -shaped, 

 elongated nuclei, the cells varying from ttVs" ^^ 

 -^ of an inch in length, and from s^^s ^^ ttW 

 of an inch in breadth.) 



c. When the organs are lodged in one of the 

 great splanchnic cavities, such as the chest or 

 abdomen, they have a third membrane — a serous 

 layer, which lines the cavity, and is reflected 

 around the viscera contained in it, so as to 

 envelop them more or less completely. This 

 layer has, therefore, an adherent face, applied 

 either against the walls of the cavity or the 

 external surface of the splanchnic organs ; and 

 a free face, always in contact with itself. 



A sei-ous membrane is composed of two layers : 

 a deep, connective portion, analogous to the 

 mucous corium ; and a superficial, which is only 

 tesselated epithelium. The free surface of this 

 epithelium is perfectly smooth, and lubricated by 

 a limpid serosity to facilitate the gliding of the 

 parts the membrane covers. (The epithelium is 

 a simple tesselated layer of flattened and poly- 

 gonal nucleated cells, about y^Vo- of an inch in 

 diameter.) 



2. The SOLID ORGANS are either contained 

 in the splanchnic cavities or situated outside 

 them, in the midst of connective tissue, which, 

 in becoming more dense around them, often 

 forms a fibrous covering. 



Like the hollow organs, they are single — 

 spleen, liver ; or in pairs — kidneys ; and sym- 

 metrical or asymmetrical in shape. They are 

 retained in their situation by their vessels and 

 nerves, by adherence to the neighbouring organs, or by particular serous bands. 



FUSIFORM CELLS OF SMOOTH 

 MUSCULAR FIBRE. 



Two cells in their natural state, 

 one showing the staff-shaped nu- 

 cleus ; 6, a cell with its nucleus, 

 c, brought distinctly into view by 

 acetic acid. 



