PREHENSION OF SOLIDS AND LIQUIDS. 197 



action of the stomach, is slowly forced into the small intestine. This tube, from fifteen 

 to twenty leet in length, is covered with peritoneum and loosely bound to the spinal 

 column by the mesentery, which is formed of the two folds of the peritoneum and is 

 sufficiently long to allow of free movements of the intestines over each other and in the 

 abdominal cavity, except the first few inches, where it is pretty firmly attached to the 

 posterior abdominal wall. The small intestine commences by a dilated portion eight or 

 ten inches in length, called the duodenum. The remainder is divided into the jejunum 

 and the ileum. The former embraces the upper two-fifths of the intestine, but there is 

 no distinct line of separation between it and the ileum. The mucous membrane lining 

 the small intestine is thick, provided with an immense number of villi, and, particularly 

 in the upper portion, is thrown into transverse folds, which are called the valvulaa con- 

 niventes. The valvular conniventes disappear in the lower part of the ileum. They are 

 peculiar to the human subject. Thickly set in the upper part of the duodenum and scat- 

 tered through its lower portion and the upper part of the jejunum, are small compound 

 follicles called the glands of Brunner ; and throughout the whole of the intestine are 

 simple follicles, called the follicles of Lieberkuhn. These glandular organs secrete the 

 intestinal juice. As the food passes from the stomach into the intestine, it imbibes 

 the bile and pancreatic juice, which are poured into the duodenum, as well as the intes- 

 tinal juice. 



Between the mucous membrane of the small intestine and the peritoneum, are two 

 layers of unstriped muscular fibres, by the progressive peristaltic action of which the 

 food is passed slowly on toward the large intestine. The alimentary principles, liquefied 

 and prepared by digestion, are gradually absorbed by the blood-vessels of the intestinal 

 mucous membrane and by the lacteals. 



The indigestible residue of the food is passed by peristaltic action into the large intes- 

 tine. This portion of the alimentary canal is from four to six feet in length ; and, like 

 the small intestine, it has a peritoneal, mucous, and muscular coat. Under ordinary con- 

 ditions the large intestine is not concerned in digestion. It simply retains the residue of 

 food, with certain excrementitious substances, until its contents are expelled by the act 

 of defsecation. 



Prehension of Solids and Liquids. 



The different modes of prehension form a very interesting part of the physiology of 

 digestion in the inferior animals ; but, in the human subject, the process is so simple and 

 well known that it demands nothing more than a passing mention. The mechanism of 

 sucking in the infant and of drinking is a little more complicated. In sucking, the lips 

 are closed around the nipple, the velum pendulum palati is applied to the back of the 

 tongue so as to close the buccal cavity posteriorly, and the tongue, acting as a piston, 

 produces a tendency to a vacuum in the mouth, by which the liquids are drawn in with 

 considerable force. This may be done independently of the act of respiration, which is 

 necessarily arrested only during deglutition ; for the mere act of suction has never any 

 thing to do with the condition of the thoracic walls. The mechanism of drinking from 

 a vessel is essentially the same. The vessel is inclined so that the lips are kept covered 

 with the liquid and are closed around the edge. By a gentle, sucking action the liquid is 

 then introduced. This is the ordinary mechanism of drinking; but sometimes the head 

 is thrown back and the liquid is poured into the mouth, as in "tossing off" the contents 

 of a small vessel as a wine-glass. 



Mastication. 



In the human subject, mechanical division of food in the mouth is neither so com- 

 pletely and laboriously effected as in the herbivora, particularly the ruminants, nor is the 

 process so rapid and imperfect as in the carnivora. In order that digestion may take 

 place in a perfectly natural manner, it is necessary that the food, as it is received into 

 the stomach, should be so far comminuted and incorporated with the fluids of the mouth 



