THE REGULATION OF THE CIRCULATION 267 



we cannot ordinarily assume that no other kinds of 

 fibers are present. It will make for clearness if a classic 

 experiment is now described. 



More than sixty years ago the eminent French 

 physiologist, Bernard, severed a certain nerve in the neck 

 of a rabbit and witnessed the curious effect manifested in 

 the ear of the animal. If the ear of a white rabbit is held 

 up to the light the red network of the blood-vessels will 

 be conspicuous. Bernard found that a general enlarge- 

 ment of the vessels in the ear followed the cutting of 

 the nerve. It is now known that the nerve operated 

 on, the cervical sympathetic, contains fibers which in- 

 fluence the muscle cells in the walls of these vessels. 



What interpretation is to be placed upon Bernard's 

 experiment? We believe that the enlargement of the 

 vessels indicates that they were previously held in a 

 state of partial contraction maintained by the constant 

 arrival of impulses from the central nervous system. 

 With the cutting of the vasomotor fibers the vessels 

 were paralyzed, their tone was lost. Very soon after 

 the original observation the complementary discovery 

 was made that electric stimulation of the cut nerve would 

 restore the tone of these vessels and, in fact, reinforce 

 it beyond the normal state, causing the ear to blanch. 

 The fibers whose existence is thus demonstrated are 

 logically called vasoconstrictors. The term implies that 

 they convey impulses which make the vessels contract. 

 Such contraction naturally reduces the local blood- 

 supply. 



It has become evident since the time of Bernard that 

 vasoconstrictor fibers have a most extensive distribution 

 in the animal body. If a nerve is chosen at random 

 and stimulated, it is usual to see signs of a lessened 

 blood-supply in the region influenced by the nerve. 

 This is most strikingly true in the case of a pair of 

 nerves, the splanchnics, which take their rise from 

 certain ganglia at the back of the thorax and pass down 

 through the diaphragm to be distributed in the ab- 



