642 NERVOUS SYSTEM. 



intestines, following extirpation of the cceliac plexus. By comparative ex- 

 periments it was shown that this did not result from the peritonitis pro- 

 duced by the operation. 



As regards secretion, the influence of the sympathetic is very marked. 

 When the sympathetic filaments distributed to a gland are divided, the sup- 

 ply of blood is much increased and an abundant flow of the secretion follows 

 (Bernard). Peyrani has shown that the sympathetic has an influence upon 

 the secretion of urine. When the nerves in the neck are stimulated, the 

 quantity of urine and of urea is increased, and this increase is greater with 

 the induced than with the constant current. When the sympathetic is 

 divided, the quantity of urine and of urea sinks to the minimum. 



Moreau published in 1870 a series of observations on the influence of the 

 sympathetic nerves upon the secretion of liquid by the intestinal canal, which 

 are important as affording a possible explanation of the sudden occurrence 

 of watery diarrhoea. In these experiments, the abdomen was opened in a 

 fasting animal, and three loops of intestine, each loop four to eight inches 

 (100 to 200 mm.) long, were isolated by ligatures. All of the nerves passing 

 to the middle loop were divided, taking care to avoid the blood-vessels. 

 The intestine was then replaced, and the wound in the abdomen was closed 

 with sutures. The next day the animal was killed. The two loops with 

 the nerves intact were found empty, as is normal in fasting animals, and 

 the mucous membrane was dry ; but the loop with the nerves divided was 

 found filled with a clear, alkaline liquid, colorless or slightly opaline, which 

 precipitated a few flocculi of organic matter on boiling. 



Vaso-Motor Centres and Nerves. The principal or dominating vaso- 

 motor centres are situated in the medulla oblongata, one on either side, 

 about one-tenth of an inch (2*5 mm.) from the median line. Each centre, in 

 the rabbit, is about one-eighth of an inch (3 mm.) long and about one-six- 

 teenth of an inch (1-5 mm.) wide. Its lower border is about one-fifth of an 

 inch (5 mm.) above the calamus scriptorius. Each side of the body has its 

 special vaso-motor centre, and. very .few if any of the vaso-motor fibres decus- 

 sate. The situation of the vaso-motor centres in the medulla has been de- 

 termined by successive removal of the nerve-centres above. If the central 

 end of a large cerebro-spinal nerve be stimulated in an animal poisoned with 

 curare, the vaso-motor nerves produce contraction of the blood-vessels, by 

 reflex action, and there is a rise in the blood -pressure. The action is not 

 interfered with by removal of the encephalic ganglia from above downward, 

 until the part of the medulla containing the vaso-motor centres is reached. 

 When these centres are removed, the reflex vaso-motor action is permanently 

 arrested. 



Subordinate vaso-motor centres exist in the spinal cord. When the 

 vaso-motor centre in the medulla is destroyed, there is a fall in the blood- 

 pressure ; but if the circulation be continued, after a time the blood-vessels 

 regain their " tone " and the pressure may then be affected by reflex action. 

 It is probable that these spinal centres exist throughout the dorsal region 

 and in the upper part of the lumbar region of the cord. 



