192 THE VASCULAR MECHANISM. 



and 2, how the nervous system regulates the calibre of the bloodvessels. We 

 will first consider the former problem. 



The Development of the Normal Seat. 



| 140. The heart of a mammal, or of a warm-blooded animal, generally 

 ceases to beat within a few minutes after being removed from the body in 

 the ordinary way, the hearts of newly-born animals continuing, however, to 

 beat for a longer time than those of adults. Hence, though by special pre- 

 caution and by means of an artificial circulation of blood, an isolated 

 mammalian heart may be preserved in a pulsating condition for a much 

 longer time, our knowledge of the exact nature and of the causes of the 

 cardiac beat is as yet very largely based on the study of the hearts of cold- 

 blooded animals, which will continue to beat for hours, or under favorable 

 circumstances even for days, after they have been removed from the body 

 with only ordinary care. * We have reason to think that the mechanism by 

 which the beat is carried on varies in some of its secondary features in differ- 

 ent kinds of animals ; that the heart, for instance, of the eel, the snake, the 

 tortoise, and the frog, differ in some minor details of behavior, both from 

 each other and from the bird and the mammal ; but we may, at first at all 

 events, take the heart of the frog as illustrating the main and important 

 truths concerning the causes and mechanism of the beat. 



In studying closely the phenomena of the beat of the heart it becomes necessary 

 to obtain a graphic record of various movements. 



1. In the frog or other cold-blooded animal, a light lever may be placed directly 

 on the ventricle (or on an auricle, etc.), and changes of form, due either to disten- 

 tion by the influx of blood or to the systole, will cause movements of the lever, 

 which may be recorded on a travelling surface. The same methods, as we have 

 seen, may be applied to the mammalian heart. 



2. Or, as in Gaskell's method, the heart may be fixed by a clamp carefully ad- 

 justed around the auriculo- ventricular groove, while the apex of the ventricle and 

 some portion of one auricle are attached by threads to horizontal levers placed 

 respectively above and below the heart. The auricle and the ventricle each in its 

 systole pulls at the lever attached to it, and the times and extent of the contrac- 

 tions may thus be recorded. 



3. A record of endocardiac pressure may he taken in the frog or tortoise, as in 

 the mammal, by means of an appropriate manometer. And in these animals, at 

 all events, it is easy to keep up an artificial circulation. A canula is introduced 

 into the sinus venosus, and another into the ventricle through the aorta. Serum 

 or dilute blood (or any other fluid which it may be desire.d to employ) is driven by 

 moderate pressure through the former ; to the latter is attached a tube connected 

 by means of a side piece with a small mercury manometer. So long as the exit-tube 

 is open at the end fluid flows freely through the heart and apparatus. Upon closing 

 the exit-tube at its far end the force of the ventricular systole is brought to bear on 

 the manometer, the index of which registers in the usual way the movements of the 

 mercury column. Newell Martin has succeeded in applying a modification of this 

 method to the mammalian heart. 



4. The movements of the ventricle may be registered by introducing into it 

 through the auricnlo-ventricular orifice a so-called " perfusion " canula. Figs. 71 

 and 72, 1., with a double tube, one inside the other, and tying the ventricle on to 

 the canula at the auricnlo-ventricular groove, or at any level below that which may 

 be desired. The blood or other fluid is driven at an adequate pressure through the 

 tube a. enters the ventricle, and returns by the tube b. If b be connected with a 

 manometer as in method 3, the movements of the ventricle may be registered. 



5. In the apparatus of Roy, Fig. 72, II., the exit-tube is free, but the ventricle 

 (the same method may be adopted for the whole heart) is placed in an air-tight 

 chamber filled with oil, or partly with normal saline solution and partly with oil. 

 By means of the tube b the interior of the chamber a is continuous with that of a 

 small cylinder c in which a piston <7, secured by a thin flexible animal membrane 



