324 MECHANISMS OF THE DIGESTIVE TRACT. 



certainly and promptly if the animal be in state of digestion, or if the 

 splanchnic nerves be previously divided. 



Langley 1 has recently shown that the vagus conveys both motor and 

 inhibitory impulses to the stomach. If curari be given, stimulation of 

 the vagus causes, as a rule, contraction of the oesophagus and cardiac end 

 of the stomach. After administration of atropin, stimulation of the 

 vagus causes relaxation of the cardiac end, followed, on cessation of the 

 stimulus, by a strong contraction. At the pyloric opening, the general 

 result of vagal stimulation is contraction, though occasionally dilatation 

 of the orifice may be observed. 



According to Schiff, motor fibres also reach the stomach from the 

 sympathetic chain, by way of the splanchnics. Morat also observed 

 one case in which the rhythmical contractions of the stomach (and 

 intestine) were augmented on stimulation of the splanchnics. As a 

 rule, however, excitation of these nerves caused diminution of the tonus, 

 as well as of the rhythmic contractions of the stomach. 



The movements of the stomach, however excited, may be inhibited 

 reflexly by stimulation of the central end of the vagus, or indeed of any 

 sensory nerve. Cannon noticed that any mental disturbance (anger, 

 pain) of the cats under investigation, at once checked contractions of the 

 stomach, which had been induced by feeding the animal. 



According to Head, 2 the sensory fibres to the stomach in man pro- 

 bably run in the sixth, seventh, eighth, and ninth dorsal nerve-roots, 

 the cardiac end being especially associated with the sixth and seventh, 

 and the pyloric end with the ninth roots. 



Vomiting. Under normal circumstances, the contractions of the 

 stomach wall are directed solely to the mixing of the food with the 

 gastric juice, and to the furtherance of the passage of chyme into the 

 duodenum. If, however, irritating substances have been taken or the 

 stomach over-distended, it is important that this organ should have 

 some means of ridding itself of its contents, and this is mainly accom- 

 plished by the contraction of the abdominal muscles and diaphragm 

 pressing against it. Vomiting is usually excited by the direct contact 

 of irritants with the gastric mucous membrane, but it may be also 

 caused by reflex irritation from other parts of the body, and especially 

 of the alimentary tract, as is the case with the vomiting caused by 

 acute intestinal obstruction. It may also occur in the colic produced 

 by the passage of a renal or biliary calculus, or from inflammation of 

 the peritoneum or any of the abdominal viscera. Besides this reflex 

 causation, vomiting may be brought about directly by events occurring 

 in the central nervous system. Thus it may be excited by emotions or 

 smells, and is a very frequent symptom of diseases of the brain, such as 

 tumour and inflammation. Sea-sickness, although complex in origin, is 

 probably chiefly determined by reflex stimulation originating in the 

 semicircular canals. 



Vomiting is generally preceded by a feeling of nausea, which is 

 accompanied by copious salivation. The saliva which is swallowed 

 carries down with it a quantity of air, which distends the stomach and 

 may assist in the subsequent dilatation of the cardiac orifice. Eetching 

 movements then occur. These consist essentially in deep inspiratory 

 efforts, attended with closure of the glottis. Since no air can be drawn 



1 Journ. Physiol., Cambridge and London, 1898, vol. xxiii. p. 407. 



2 Brain, London, 1893, vol. xvi. p. 66, 



