BY DR. BURDON-SANDERSON. 231 



resembles that in which the heart discharges its blood into the 

 arteries. Several instruments of this kind have been con- 

 trived, from the simple schema of E. H. Weber, to the com- 

 plicated " artificial heart" of Marey. 



It may be stated generally that those forms of schema are 

 most instructive which are of the simplest construction ; and 

 inasmuch as the object in view is not to illustrate but to 

 explain, it is of no importance whatever that the schema 

 should have any outward resemblance to the organs of circu- 

 lation for which it stands. What is essential in a schema is, 

 that as regards the quantity of liquid discharged at each 

 stroke of the pump, the period occupied in the discharge, the 

 distribution in time of the pressure exercised on the mass of 

 liquid expelled, and the resistance opposed to the terminal 

 outflow of liquid from the elastic tube, the representation 

 should resemble, as closely as possible, the thing represented. 

 To the student, it is far from an advantage that the resem- 

 blance should extend beyond this to the details of external 

 form and arrangement; for his attention is thereby apt to be 

 drawn off from the essential conditions of the act, to the 

 accessory peculiarities of the machine which produces it. Two 

 kinds of schema may be usefully employed for the study of 

 the phenomena of the pulse, which differ from each other in 

 the construction of the pump which does the work of the 

 heart. The first is represented in fig. 211. Here the pump 

 consists of a glass tube (A), closed at the upper end, and 

 connected below by two branches on one side with a cistern, 

 at a level of some eight or ten feet above the table ; on the 

 other, with the experimental tube which represents the artery. 

 These communications are controlled by valves, placed at the 

 opposite ends of a horizontal lever (E, D) of such construction 

 that the same act which closes the one must necessarily open 

 the other ; so that, as regards their actions, one represents the 

 semilunar, the other the auriculo-ventricular valves of the 

 heart. By means of a spring (shown in the figure to the right 

 of D), when the apparatus is not working, i. e., during the 

 period corresponding to diastole, the former is kept closed, 

 the latter open. Under these circumstances, the water rises 

 in the tube, compressing the column of air which it contains 

 in a proportion which is determined by Marriotte's law. If, 

 as in the present instance, the pressure is about one-third of 

 an atmosphere, the volume of the inclosed air is diminished in 

 the proportion of 2 : 3, and so on. When, by depressing the 

 opposite end of the lever, the aortic valve is opened, and the 

 mitral closed, the compressed air suddenly expands, and forces 

 the water which the tube contains into the aorta. We shall 

 see, when we come to consider the modes of contraction of 

 the heart, that the above is as close an imitation as could be 



