BY DR. BURDON-S ANDERSON. 253 



continued, the arteries do not remain contracted, but often 

 exhibit alternations of contraction and relaxation at irregular 

 intervals. For observing the changes of rate of movement in 

 the velocity of the blood-stream, the veins should be preferred ; 

 for in them the initial acceleration is not quite so transitory as 

 in the arteries, while the subsequent slowing is as distinct. If 

 it is desired to make a more exact observation, the method 

 devised by Dr. Riegel must be used. It consists in comparing 

 the movements of the blood corpuscles in a selected artery or 

 vein, with that of a current of water containing solid particles 

 in suspension, which passes through a horizontal glass tube 

 fixed in the eye-piece of the microscope at such a distance from 

 the eye-glass as to be distinctly seen by the observer. One end 

 of the tube communic?' with a large bottle placed on a shelf 

 at a higher level * l ., the table, containing the liquid ; the 

 other, with the ..ischarge tube of the movable warm stage 

 represented in fig. 3. By varying the height of the dropper, 

 the rate of flow through the eye-piece can be readily regulated. 

 The rate of flow is learnt by measuring the quantity of liquid 

 discharged per second, and dividing it by the product of the 

 lumen of the glass tube and the magnifying power of the 

 microscope. Thus, if the rate of discharge were a cubic centi- 

 metre in 15 seconds, i. e., 6.6' cubic millimetres per second, the 

 lumen of the tube 0.8 square mill., and the magnifying power 

 300, the velocity of the current would be 3 -^~^= 0.02775 mill. 

 The determination of the absolute velocity is of little import- 

 ance, the object being rather to appreciate, with exactitude and 

 certainty, the changes of rate which occur during the period 

 of observation, b. If, instead of the tongue, the surface of 

 the skin is excited with the brush, the appearances observed 

 are very similar. The initial acceleration of the blood-stream 

 is more easily observed by this method than by the other, c. 

 Direct Excitation of a Sensory Nerve. A frog having been 

 curarized, the integument is divided along the outer and 

 posterior aspect of the thigh in a line which corresponds in 

 direction with the slender biceps muscle, or rather with the 

 groove between the muscular mass which covers the front of 

 the femur (triceps femoris) and the bulky semi-membranosus. 

 The sciatic nerve, accompanied by the sciatic artery and vein, 

 lies immediately underneath the biceps, between it and the 

 semi-membranosus. In order to separate it from the vessels, 

 it is best to bring it into view by raising the biceps on a blunt 

 hook. Both webs having been arranged for observation under 

 the microscope, the nerve is divided a little above the knee, 

 and the central end laid on the copper points. The secondary 

 coil having been placed at a considerable distance from the 

 primary, and the eye fixed on an artery of the web of the un- 



