322 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



observers the movement is blocked at the ileo-csecal valve, and the peristaltic 

 movements of the large intestine form an independent group similar in all 

 their general characters to those of the small intestine, but weaker and slower. 



Mechanism of the Peristaltic Movement. The means by which the peri- 

 staltic movement makes its orderly forward progression have not been satis- 

 factorily determined. The simplest explanation would be to assume that an 

 impulse is conveyed directly from cell to cell in the circular muscular coat, so 

 that a contraction started at any point would spread by direct conduction of 

 the contraction change. This theory, however, does not explain satisfactorily 

 the normal conduction of the wave of contraction always in one direction, nor 

 the fact that a reversed piece of intestine continues to send its waves in what 

 was for it the normal direction. It is possible, therefore, that the co-ordination 

 of the movement may be effected through the local nerve-ganglia, but our 

 knowledge of the mechanism and physiology of these peripheral nerve-plexuses 

 is as yet too incomplete to be applied satisfactorily to the explanation of the 

 movements in question. 



Pendular Movements. In addition to the peristaltic wave a second kind 

 of movement may be observed in the exposed intestines of a living animal. 

 This movement is characterized by a gentle swinging to and fro of the different 

 loops, whence its name of pendular movement. The oscillations occur at 

 regular intervals, and are usually ascribed to rhythmic contractions of the 

 longitudinal muscles. Mall, 1 however, believes that the main feature of this 

 movement is a rhythmic contraction of the circular muscles, involving a part 

 or all of the intestines. He prefers to speak of the movements as rhythmic 

 instead of pendular contractions, and points out that owing to the arrangement 

 of the blood-vessels in the coats of the intestine the rhythmic contractions should 

 act as a pump to expel the blood from the submucous venous plexus into the 

 radicles of the superior mesenteric vein, and thus materially aid in keeping up 

 the circulation through the intestine and in maintaining a good pressure in the 

 portal vein, in much the same way as happens in the case of the spleen (see p. 

 272). How far these rhythmic or pendular contractions occur under perfectly 

 normal conditions has not been determined. 



Extrinsic Nerves of the Intestines. As in the case of the stomach, the 

 small intestine and the greater part of the large intestine receive viscero-motor 

 nerve- fibres from the vagi and the sympathetic chain. The former, according 

 to most observers, when artificially stimulated cause movements of the intestine, 

 and are therefore regarded as the motor fibres. It seems probable, however, 

 that the vagi carry or may carry in some animals inhibitory fibres as well, and that 

 the motor effects usually obtained upon stimulation are due to the fact that in these 

 nerves the motor fibres predominate. The fibres received from the sympathetic 

 chain, on the other hand, give mainly an inhibitory effect when stimulated, 

 although some motor fibres apparently may take this path. Bechterew and 

 Mislawski 2 state that the sympathetic fibres for the small intestine emerge from 



1 The Johns Hopkins Hospital Reports, vol. i. p. 37. 



2 Du Bois-Reymond's Archivfur Physiologic, 1889, Suppl. Bd. 



