l6o SCHEMATIC REPRODUCTION OF THE CIRCULATION. 



Physical Explanation. The conditions detailed may be illustrated by means 

 of simple physical experiments. If a rigid tube be connected with the nozzle of 

 a syringe, every movement of the piston will be followed by an intermittent 

 expulsion of water, which will correspond in time exactly to the movement of 

 the piston. The effect of the intermittent injection of fluid into an elastic system 

 of tubes is best exemplified in a fire-hose. Here the air contained in the air- 

 chamber which is under elastic tension takes the place of the elasticity of the 

 tubes themselves in the circulatory apparatus. With slow intermittent strokes 

 of the pump, the stream of water is interrupted; but if the movements of the 

 pump are more frequent, the compressed air in the air-chamber effects a continuous 

 outflow, although a distinct acceleration of the stream is seen in correspondence 

 with each stroke of the pump. 



Landois was able without difficulty to demonstrate that the particles of water 

 in an elastic tube are set in motion during the passage of the current by every 

 pulsatile wave, in correspondence with the picture presented by the sphygmo- 

 graphic tracing, by introducing in the course of a long elastic tube, in which both 

 a continuous and an undulatory movement could be produced by intermittent 

 pumping, a short glass tube containing a thread passing through an opening in 

 the side and floating to and fro in the stream. Immediately in front of the 

 thread a sphygmograph was connected with the tube. Each pulse-beat caused a 

 synchronous movement of the sphygmograph and of the thread, each upward 

 stroke of the writing lever corresponding to a more marked oscillation of the 

 thread toward the periphery (acceleration) , while each downward stroke was 

 marked by a slight diminution in the oscillatory movement (retardation) . 



In the capillary vessels the pulsatory acceleration of the current 

 ceases with the disappearance of the pulse-wave. The two movements 

 are gradually extinguished by the marked resistance encountered by the 

 blood in the capillary system. It is only when the capillary vessels 

 are greatly dilated and the pressure in the arterial system increases that 

 both pulse and pulsatory acceleration of the current are sometimes 

 communicated to the initial portions of the veins through the capillaries. 

 Such conditions are observed in the vessels of the salivary glands after 

 stimulation of the facial nerve, which dilates the vascular channels. 

 After constriction of the finger with an elastic band, which impedes the 

 return flow of venous blood, and causes an increase in the arterial pres- 

 sure, with dilatation of the capillaries of the finger, the swollen skin is 

 seen to become intermittently more deeply red isochronously with the 

 well-known throbbing sensation. This is the capillary pulse. 



Pathological. The capillary pulse is found sometimes when the action of the 

 left ventricle is greatly increased, for example in cases of aortic insufficiency and 

 of exophthalmic goiter, and often in cases of jaundice. 



SCHEMATIC REPRODUCTION OF THE CIRCULATION. 



The arrangement of the circulation as described permits a reproduction by 

 physical means, of the most essential conditions, in the so-called model of the circu- 

 lation. Weber's model will be briefly described here. The arterial system and 

 the somewhat larger venous system are represented by portions of animal intestine 

 (Fig. 64). 



The system of capillaries between the two is formed by a glass tube of sufficient 

 size, the lumen of which, however, is occupied by a piece of sponge. A short 

 section of intestine into each extremity of which a piece of glass tube is tied 

 represents the heart. The glass tube directed toward the arterial trunk is 

 provided with the necessary valves, which are reproduced by having a piece 

 of small intestine project beyond the edges of the glass tube and securing its free 

 margins with three threads. Through this piece of intestine water can enter 

 only in the direction from the glass tube toward the free intestine, but not 

 in the opposite direction, as the free edges would then come together and close 

 the lumen. From the venous side a similar valve, mounted on the extremity 

 of a separate piece of tube, is inserted into the glass tube directed toward 



