296 HANDBOOK OF PHYSIOLOGY. 



state of tonic contraction. The nervous centre which governs this con- 

 traction is probably situated in the lumbar region of the spinal cord, in- 

 asmuch as in cases of division of the cord above this region the sphincter 

 regains, after a time, to some extent the tonicity which is lost imme- 

 diately after the operation. By an effort of the will, acting through the 

 centre, the contraction may be relaxed or increased. In ordinary cases 

 the apparatus is set in action by the gradual accumulation of fasces in the 

 sigmoid flexure and rectum, pressing by the peristaltic action of these 

 parts of the large intestine against the sphincter, and causing by reflex 

 action its relaxation ; this sensory impulse acting through the brain and 

 reflexly through the spinal centre. 



The Gases contained in the Stomach and Intestines. Under 

 ordinary circumstances, the alimentary canal contains a considerable 

 quantity of gaseous matter. Any one who has had occasion, in a post- 

 mortem examination, either to lay open the intestines, or to let out the 

 gas which they contain, must have been struck by the small space after- 

 wards occupied by the bowels, and by the large degree, therefore, in 

 which the gas, which naturally distends them, contributes to fill the 

 cavity of the abdomen. Indeed, the presence of air in the intestines is 

 so constant, and, within certain limits, the amount in health so uniform, 

 that there can be no doubt that its existence here is not a mere accident, 

 but intended to serve a definite and important purpose, although, proba- 

 bly, a mechanical one. 



Sources. The sources of the gas contained in the stomach and 

 bowels may be thus enumerated i 



1. Air introduced in the act of swallowing either food or saliva; 2. 

 Gases developed by the decomposition of alimentary matter, or of the se- 

 cretions and excretions mingled with it in the stomach and intestines ; 

 3. It is probable that a certain mutual interchange occurs between the 

 gases contained in the alimentary canal, and those present in the blood 

 of these gastric and intestinal blood-vessels ; but the conditions of the 

 exchange are not known, and it is very doubtful whether anything like 

 a true and definite secretion of gas from the blood into the intestines or 

 stomach ever takes place. There can be no doubt, however, that the in- 

 testines may be the proper excretory organs for many odorous and other 

 substances, either absorbed from the air taken into the lungs in inspira- 

 tion, or absorbed in the upper part of the alimentary canal, again to be 

 excreted at a portion of the same tract lower down in either case as- 

 suming rapidly a gaseous form after their excretion, and in this way, 

 perhaps, obtaining a more ready egress from the body. It is probable 

 that, under ordinary circumstances, the gases of the stomach and intes- 

 tines are derived chiefly from the second of the sources which have been 

 enumerated. 



It is now very generally admitted that the decompositions of food in 

 the alimentary canal are partially the result of the growth of various 

 kinds of micro-organisms, some of which have been already mentioned, 

 and that these decompositions are independent of as well as distinct from 



