THE MECHANICS OF DIGESTION 1015 



larger ones. This rhythmic play continues for some time at the rate 

 of 20 to 30 in a minute (cat) until the chyme has been thoroughly mixed 

 with the intestinal secretions, ^ and naturally, the cessation of these 

 alternate constrictions and' relaxations must leave the now rather 

 liquid material again reunited into a single mass. A regular peristaltic 

 wave then sweeps it onward into a lower segment of the small intestine, 

 where the pendular movements are repeated with the same result. 

 While this mechanical and chemical reduction of the food is continued 

 far into the ileum, the material already reduced is absorbed; in fact, 

 absorption begins very soon after the entrance of the chyme into the 

 duodenum and reaches its height in the jejunum and upper ileum. In 

 the lower ileum, on the other hand, most of the assimilable material 

 has already been removed, but naturally, much depends upon the 

 character of the ingesta, and the tonicity of the intestinal musculature. 



It should also be noted that the peristalsis and pendular motion 

 undoubtedly facilitate absorption in a mechanical way, because they 

 tend to increase the flow of the lymph and blood. Secondly, they tend 

 to bring the individual villi into a more intimate relation with the 

 intestinal contents. As far as the regular peristaltic wave is concerned, 

 it should be mentioned that it occurs in two forms, namely, as a slowly 

 advancing contraction (2 to 3 cm. per sec.) which again disappears at 

 a distance of about 5 cm. from its place of origin, and as a more rapid 

 contraction which may cover a distance of 10 to 15 cm. and more. 

 The former, therefore, remains more localized and serves to disseminate 

 the material so that it may be acted upon later on by the pendular 

 motions. The latter, on the other hand, serves to remove the com- 

 pletely digested material into more distant segments situated nearer 

 the ileocecal valve. No definite statements can be made regarding the 

 degree of pressure which may be developed by these waves, but since 

 the fecal material is in a liquid state, it may be surmised that the energy 

 required to move it is very slight. This deduction is upheld by the 

 experiments of Cash^ which show that a weight of 5 to 8 gm. applied 

 to the surface of the intestines, sufB.ces to block the progress of the 

 feces. 



Antiperistaltic movements occur in the small intestine only under 

 abnormal conditions, such as may arise in consequence of obstructions 

 by foreign bodies and tumors, or as a result of an invagination or 

 kinking of the entire gut. If the lesion is a high one, the fecal mate- 

 rial is often forced into the stomach, whence it is expelled by the proc- 

 ess of vomiting. 



The Nervous Control of the Intestinal Movements. — While these 

 peristaltic movements may be evoked almost anywhere along the intes- 

 tine, they begin as a rule high up in the duodenum, and hence, it would 

 not be incorrect to speak of a *' pace-maker" of peristalsis. In all 

 these instances, the stimulations are local in their character and may 



1 Magnus, Pfliiger's Archiv, cxi, 1906, 152. 



2 Proc. R. Soc, London, 1887. 



