236 THE BODY AT WORK 



relaxed. The skin is flushed ; heat is thrown off by radiation. 

 The sweat-glands secrete water, which is evaporated by the 

 heat of the skin. Constriction and remission of constriction are 

 the processes which diminish or increase loss of heat. 



This mechanism is different in the case of glands and some 

 other structures which, when active, require an abundant supply 

 of blood. Such organs are provided with vaso-dilator in 

 addition to vaso-constrictor nerves. The most conspicuous 

 example of this is to be seen in the case of the submaxillary 

 gland. The nerve to this gland runs for some distance as an 

 isolated thread the chorda tympani. Stimulation of the 

 chorda tympani has the double effect of dilating the arteries of 

 the gland and of causing it to secrete. But the administration 

 of atropin prevents secretion. Vaso-dilation is then the only 

 visible effect. Stimulation may increase sixfold the outflow of 

 blood from the veins of the gland. It rushes through with such 

 rapidity that it retains its bright arterial hue. The gland also 

 receives a twig from the sympathetic cord in the neck, which, as 

 already stated, controls the vessels of the face. By stimulating 

 the one nerve or the other the physiologist can at will increase or 

 diminish the amount of blood flowing through the submaxillary 

 gland. Stimulating any sensory nerve causes in a reflex manner 

 an increased outflow of constrictor impulses from the centre in 

 the medulla oblongata to all parts of the body, with the excep- 

 tion of the part to which the sensory nerve appertains. Its 

 own constituency receives an increased supply of blood. It is 

 not difficult to appreciate the importance of this double action. 

 A part is injured. The restrictions placed upon its supply of 

 blood are suspended. Lest its increased consumption should 

 lead to a general fall in pressure, all other parts have their 

 supply curtailed. The effect is even more pronounced than 

 this. The whole blood-pressure is raised above its ordinary 

 level. The flow of blood to the injured part is therefore greater 

 than it would be were relaxation of its arteries the only change. 



The most important of all constrictor nerves are the 

 splanchnics which control the supply to the stomach and 

 intestines. When these nerves are cut, the digestive organs 

 become engorged to such an extent that a pronounced fall of 

 the general blood-pressure is the result. Their stimulation 

 renders the digestive organs anaemic. We have already shown 



