ABDOMINAL CAVITY ORGANS OF DIGESTION. 75 



The abdominal cavity is lined with a serous membrane 

 called the peritoneum. Like all the membranes of this 

 nature, it is formed of cellular or laminated tissue and 

 elastic fibres; its free surface is covered with an epithelium, 

 a sort of epidermis, which resists the continual friction result- 

 ing from the movements of the organs; and lastly, like all 

 its congeners, it is a sac without an opening, folded on itself, 

 and consequently with double walls. The space between 

 these walls is empty, their corresponding surfaces rub freely 

 against each other, and are moistened by a fluid analogous 

 to the serum of the blood, a secretion peculiar to these 

 membranes, and from which they derive their name. The 

 internal wall of the sac covers all the organs which it con- 

 tains, and the external wall is attached throughout its whole 

 extent to the cavity which it lines. We shall, by-and-by, 

 have occasion to return to the disposition of the peritoneum. 



The digestive apparatus is one of the most complex and 

 extensive in the organism; it is accessible to our investiga- 

 tions in all its parts, and we are able to follow the working 

 of the functions which devolve upon it. We can observe 

 the metamorphosis which the food undergoes; we can repro- 

 duce in our laboratories a part of these transformations; a 

 step farther, and, as Fontenelle has said, we should surprise 

 nature in the very act; but this impossible step is the immense 

 distance which separates inert matter from organized sub- 

 stance, physical and chemical phenomena from the vital 

 functions. 



The organs of digestion are the mouth, the pharynx, the 

 oesophagus, the stomach, the liver, and the pancreas. The 

 spleen and kidneys are appendages of the digestive apparatus, 

 but belong rather to the circulatory or excretory. 



The mouth forms the entrance to the digestive apparatus; 

 it contains the organ of taste, and serves in eating and in 

 articulating sounds. Bounded above by the palatine arch, 

 below by a muscular wall and by the tongue, on the sides 

 and in front by the cheeks and the lips, the mouth presents 

 in front the opening of the lips, behind, the isthmus of the 

 throat by which it communicates with the pharynx and over 

 which the soft palate falls. 



