532 THE MECHANICAL PROCESSES OF DIGESTION [CH. XXXV. 



But almost complete security is afforded against the passage of the 

 contents of the large into the small intestine by the ileocaecal valve. 



Proceeding from above downwards, the muscular fibres of the 

 large intestine become, on the whole, stronger in direct proportion 

 to the greater strength required for the onward moving of the faeces, 

 which are gradually, owing to the absorption of water, becoming 

 firmer. The greatest strength is in the rectum, at the termination of 

 which the circular unstriped muscular fibres form a strong band 

 called the internal sphincter ; while an external sphincter muscle 

 with striped fibres is placed rather lower down, and more externally, 

 and holds the orifice closed by a constant slight tonic contraction. 



Nervous mechanism. Experimental irritation of the brain or 

 cord produces no evident or constant effect on the movements of the 

 intestines during life ; yet in consequence of certain mental conditions 

 the movements are accelerated or retarded ; and in paraplegia the 

 intestines appear after a time much weakened in their power, and 

 costiveness, with a tympanitic condition, ensues. 



As in the case of the oesophagus and stomach, the peristaltic 

 movements of the intestines may be directly set up in the muscular 

 fibres by the presence of food or chyme acting as the stimulus. Few 

 or no movements occur when the intestines are empty. 



The small intestines are connected with the central nervous 

 system by the vagi and by the splanchnic nerves. The fibres which 

 leave the medulla in the vagal rootlets are fine medullated ones : they 

 arborise around cells in the ganglion trunci, whence non -medullated 

 fibres continue the impulse to the intestinal walls ; they pass through 

 the solar plexus, but are not connected with nerve-cells in that plexus. 

 In animals stimulation of the pneumogastric nerves induces peri- 

 staltic movements of the intestines. If the intestines are contracting 

 peristaltically before the stimulus is applied, the movements are 

 inhibited for a brief period, after which they are greatly augmented. 

 The sympathetic fibres leave the cord as fine medullated fibres by 

 the anterior roots from the sixth thoracic to the first lumbar, pass 

 through the lateral chain, but do not reach their cell-stations until 

 they arrive at the superior mesenteric ganglia : thence they pass as 

 non-medullated fibres to the muscular coats. Stimulation of these 

 fibres causes inhibition of any peristaltic movements that may be 

 present. These nerves also contain vaso-motor fibres, and section of 

 these leads to vaso-dilatation and a great increase of very watery 

 succ us entericus. 



Peristalsis in the small intestine can be excited artificially even 

 when all nerves running to it from the central nervous system have 

 been cut through. After pinching any particular spot a wave of 

 inhibition travels downwards, and a wave of contraction upwards. 

 (Starling.) 



