238 CIRCULATION OF THE BLOOD. , 



Carotid. Radial. Peripheral Arterioles. 



Fully expanded . Expanding . . Collapsed. 



Contracting . Expanded . . Expanding. 



Again expanding . Contracting . . Expanding. 



Stationary . Again expanding . Slowly contracting. 



Contracting . Contracting ' . Contracting. 



Hence, as sphygmographic tracings show to be the case, the 

 second expansion in the great arteries lasts longer than in the 

 smaller ones ; for, although it commences the sooner the nearer 

 the heart, the subsidence is simultaneous throughout the whole 

 arterial system. 



Rules for Sphygmographic Observation. 1. The 

 forearm should be supported on a table or other similar sur- 

 face, with the back of the wrist reposing on a firm, well-padded 

 cushion, of such a height that the dorsal surface of the hand 

 makes an angle of from 20 to 30 with that of the forearm. 



2. The sphygmograph must be placed on the wrist in a di- 

 rection parallel with that of the radius, in such a position 

 that the block rests upon the trapezium and scaphoid, and 

 the extremity of the spring is opposite the styloid process of 

 the radius. 



3. In beginning an observation, adjust the instrument so 

 that the pressure exerted by the spring is sufficient to flatten 

 the artery against the radius ; then weaken the spring until the 

 effects of over-compression disappear i.e., until you find that 

 the lever continues to descend until the end of diastole. Note 

 the pressure at which this result is attained, as well as that 

 which is required to flatten the artery, and take tracings at 

 each of the two pressures. 



SECTION III. PHENOMENA OF THE CIRCULATION IN THE SMALLEST 

 ARTERIES. 



The smallest arteries may be studied during life with the 

 aid of the microscope, in fish, batrachians, and mammalia. 



44. For the microscopical study of the circulation in fish, a 

 contrivance devised by Dr. Caton, of Liverpool, is used (fig. 

 216). It consists of an oblong box of gutta percha, open at 

 one end, closed at the other, and just large enough to hold the 

 body of a minnow or stickleback very loosely. This box 

 forms part of a plate of gutta percha, which is fixed on to the 

 stage of the microscope in such a position that the tail of the 

 fish contained in it covers a perforation in the plate prepared" 

 for its reception. The tail is held securely in its place by a 

 ligature, and the caudal fin which rests on a square of glass is 

 further secured by a couple of fine springs. The box itself, 

 which incloses the head and gills of the fish, contains water, 

 which is constantly renewed by means of the two tubes, of 



