614 NERVOUS SYSTEM 



taking care to avoid the bloodvessels. 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. 



Vasomotor Centres and Nerves. The principal or dominating vaso- 

 motor centres are situated in the bulb, one on either side, about one- 

 tenth of an inch (2.5 millimeters) from the median line. Each centre, 

 in the rabbit, is about one-eighth of an inch (3 millimeters) long and 

 about one-sixteenth of an inch (1.5 millimeters) wide. Its lower border 

 is about one-fifth of an inch (5 millimeters) above the calamus scriptorius. 

 Each side of the body has its special vasomotor centre, and very few 

 if any of the vasomotor fibres decussate. The situation of the vaso- 

 motor centres in the bulb has been determined by successive removal 

 of the nerve-centres above. If the central end of a large cerebro-spinal 

 nerve is stimulated in an animal poisoned with curare, the vasomotor 

 nerves produce contraction of the bloodvessels 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 vasomotor centres is reached. When 

 these centres are destroyed, the reflex vasomotor action is permanently 

 arrested. 



Subordinate vasomotor centres exist in the spinal cord. When the 

 vasomotor centre in the bulb is destroyed, there is a fall in the blood- 

 pressure ; but if the circulation is continued, after a time the bloodves- 

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

 action. It is probable that these spinal centres exist throughout the 

 dorsal and lumbar regions of the cord. 



All the vasomotor nerves are derived from the bulb and the spinal 

 cord. Some of the vasomotor fibres to the head pass in the trunks of 

 the motor cranial nerves, but most of them come from the anterior roots 

 of some of the spinal nerves and pass to the head by the filaments of 

 distribution of the cervical sympathetic. The vasomotor fibres pass in 

 the lateral columns of the cord, and from the cord, in the anterior roots 

 of the spinal nerves, in the dog, as far down as the second pair of 

 lumbar nerves. These fibres are medullated but are of small size. 

 They pass to the bloodvessels either through branches from the sympa- 

 thetic ganglia or through the ordinary cerebro-spinal nerves. They 

 therefore are not confined to branches of the sympathetic. 



The vasomotor nerves are capable of influencing local circulations, 



