THE CIRCULATION 235 



forwarded to the heart. As in the case of the arteries, the 

 portal system of vessels is controlled by the nervous system. 

 When the spinal cord is divided they also dilate. The whole 

 vascular system becoming more capacious, blood-pressure falls 

 in veins as well as in arteries. 



When the digestive organs are active, other parts of the body 

 are kept short of blood. It chanced to the writer, in his student 

 days, to spend the early summer in Paris, with a big healthy 

 Yorkshireman as companion. We dined together each night 

 at one of the restaurants of the Palais Royal a prix fixe. After 

 dinner, with British regularity, my friend called for the Times. 

 Then followed a short period of placid reading, interrupted by 

 the remark : " How cold it is !" Half an hour later, giving 

 himself a shake : " Suppose we go and dine somewhere else ?" 

 His well-ordered digestive organs had made short work of the 

 two-franc dinner. They had been ably supported by the vaso- 

 motor system of nerves which provided them with the bulk 

 of the blood, while limbs and skin ran short. 



Vaso-constrictor nerves leave the spinal cord by the roots 

 (called " rami communicantes ") of sympathetic ganglia. 

 Beyond the ganglia they apply themselves to the large arteries 

 whose course they follow. The constrictor nerves for the face 

 and neck leave the spinal cord within the chest by the roots of 

 the first four thoracic nerves. They do not at once apply them- 

 selves to the great artery of the head. Until the upper part 

 of the neck is reached, they traverse the ganglionated sympa- 

 thetic cord, which lies behind the carotid artery and internal 

 jugular vein. If in a rabbit this cord be cut, the vessels of its 

 ear dilate, as evidenced by the rosy blush which is observed 

 when a light is held behind it. If the upper part of the sympa- 

 thetic cord be stimulated, the ear grows pale. The redness of 

 the ear remains for many days after section of the nerve ; but 

 gradually the engorgement diminishes, and the vessels acquire 

 the power of automatically regulating the flow. 



The classical experiment with the rabbit's ear suffices to 

 show the relation of bloodvessels and nerves which holds good 

 for all areas of the skin. The condition of the skin is the chief 

 factor in regulating the temperature of the body. In a cold 

 atmosphere its vessels are severely constricted to limit loss of 

 heat. When one passes into a warm room the constriction is 



