106 THE BODY AT WORK 



that for a time the lungs may be independent of a fresh supply 

 of air. The glottis is closed, the diaphragm fixed. Contraction 

 of the abdominal wall presses the stomach against the dia- 

 phragm ; its cardiac sphincter relaxes, and its contents are 

 squirted into the oesophagus, which undergoes a forcible 

 retrogressive peristalsis. 



It is interesting to note the difference between carnivora and 

 herbivora in regard to vomiting. Carnivora swallow fur and 

 other indigestible materials, as well as many unwholesome 

 things which they need to be able to return. A dog can, 

 apparently, vomit at will. Never, while in a state of nature, 

 do herbivora need to return the contents of the stomach. No 

 provision is made for vomiting. A heifer which has strayed 

 into a dewy clover-field is not unlikely to die from the effects 

 of distension of its paunch, if relief be not given by opening 

 it with a knife. In a horse the cardiac sphincter is strong, 

 the pyloric weak. Pressure on the stomach tends to drive its 

 contents through the pyloric valve into the duodenum, not 

 backwards into the oesophagus. The stomach is not so placed 

 as to allow of its being compressed between the wall of the 

 abdomen and the diaphragm. Horses cannot vomit. It is a 

 mistake to suppose that they suffer from sea-sickness. In 

 rough weather they sweat, their limbs tremble, they go off 

 their feed ; but these symptoms are probably due to the 

 fatigue which results from excessive anxiety to maintain their 

 balance, and to fear. We can never know their feelings, but 

 there is no reason for supposing that they experience the 

 sensation of nausea. 



Vomiting is a frequent symptom of cerebral disturbance. The 

 fluctuations of pressure which the brain experiences as it rocks 

 about on its " water-bed '' within the skull is the cause of sea- 

 sickness. Yet the motion of a ship may produce violent 

 headache without nausea, the brain only, not the stomach, 

 appearing to be troubled by the motion. Not that headache 

 is a pain " inside the head." Nor is it properly described as 

 a pain in the scalp, although the messages which are felt in 

 consciousness as headache originate in the endings of the 

 nerves of the skin which covers the skull. The excessive 

 sensitiveness of these nerves is due to vaso-motor conditions, 

 usually the dilation, occasionally the constriction, of the 



