486 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



much less constriction. The superficial veins of the rabbit's abdomen are 

 constricted by the stimulation of the cervical spinal cord at the second ver- 

 tebra. 



The observations of Bernard and his contemporaries led to a very great 

 number of researches on the general properties and the distribution of the 

 vaso-motor nerves, in the course of which a variety of ingenious methods of 

 observation have been devised. 



Methods of Observation. One fruitful method of research has been 

 already incidentally mentioned, namely, the direct inspection of the vessel, or 

 region, the vaso-motor nerves of which are being studied. 



A second method consists in accurately measuring the outflow from the 

 vein. If the blood-vessels of the area drained by the vein are constricted by 

 the stimulation of a vaso-motor nerve, the quantity escaping from the vein in 

 a given period previous to constriction will be greater than that escaping in an 

 equal period during constriction. This well-known method is especially avail- 

 able where an artificial circulation is kept up through the organ studied, as 

 the blood drained from the vein does not then weaken the animal and thus 

 disturb the accuracy of the observations. 1 



A third method is founded on the principle in hydraulics that the lateral 

 pressure at any point in a tube through which a liquid flows depends, other 

 things being equal, on the resistance to be overcome below the point at which 

 the pressure is measured. In the animal body the resistance to be overcome 

 by the blood-stream varies with the state of contraction of the smaller vessels, 

 and thus the variations in the lateral pressure of a given artery may, under 

 certain restrictions, be used to determine variations in the size of the smaller 

 vessels distal to the artery. The restrictions are, that the variations in the 

 lateral pressure in the artery are indicative of changes in the size of the distal 

 vessels only when the general blood-pressure remains unaltered, or alters in a 

 direction opposite to the change in the artery investigated. 2 An example will 

 make this plain. Dastre and Morat, 3 in order to demonstrate the presence of 

 vaso-motor fibres for the hind limb in the sciatic nerve, connected a manometer 

 with the central end of the left femoral artery, and a second manometer with 

 the peripheral end of the right femoral artery, distal to the origin of the pro- 

 funda femoris. The anastomoses between the principal branches of the fem- 

 oral artery are so numerous and so large that the circulation in the limb can 

 be maintained by the profunda femoris alone. Dastre and Morat could there- 

 fore compare the general blood-pressure with the blood-pressure in the right 

 hind limb. On stimulating the peripheral end of the right sciatic nerve, the 

 blood-pressure rose in the arteries of the limb, but remained stationary in the 

 arteries of the trunk, connected with the first manometer through the central 

 end of the left femoral artery. The rise of blood-pressure in the operated 

 limb, while the blood-pressure in the rest of the body remained unchanged, 

 proved that the vessels in the operated limb were constricted. 



1 Cavazzani and Manca, 1895, p. 33. 2 Hiirthle, 1889, p. 563. 



3 Dastre and Morat, 1883, p. 556. 



