464 MATER i A MEDIC A AND THERAPEUTICS. 



The surface of the "blood-stream is broken only in the arteries 

 by the wave raised by each fresh discharge from the heart, and 

 this wave is called the pulse. 



The heart performs its work by virtue of being a nervo- 

 muscular organ, freely supplied with blood by the coronaries. 

 The muscular tissue is normally stimulated to contract by the 

 intra-cardiac ganglia, which, whilst automatic in action, are 

 excited by impressions coming from the inner surface of the 

 heart chiefly impressions of pressure or resistance ; and the 

 vigour of systole is in direct proportion to this pressure, which 

 in turn is referable, partly to the auricular charge, and partly to 

 the resistance ahead. The movements of the heart are regu- 

 lated by the cardiac centre in the medulla, which is that part 

 of the nervous system where afferent impressions are first 

 received, and then reflected as motor impulses to the heart, 

 either by the vagus or by the sympathetic, the terminations of 

 which are connected with the cardiac ganglia. An impression 

 made upon the terminations of the vagus diminishes the frequency 

 of the nervous discharges from the ganglia, that is, inhibits the 

 contractions of the heart ; an impression made on the termina- 

 tions of the sympathetic accelerates them. With regard to the 

 heart- or pulse-rate, it is highly important to observe that the 

 length of systole varies very little : whatever the work done or to 

 be done, the ventricle takes ^" to contract. The part of the car- 

 diac revolution that varies in length is the diastole, which is some- 

 times long, giving an infrequent pulse-rate, say 50, sometimes 

 short, giving a frequent pulse-rate, say 100. Now, during diastole 

 the nervo-muscular apparatus rests and is nourished, and the 

 ventricles are filled from the auricles and veins. An infrequent 

 pulse is thus (to a certain extent) an indication that the heart is 

 being rested and filling well, whilst the force of the systole is 

 not weakened, probably the reverse, by these two effects. 

 Agencies which thus affect the rate of the heart through the 

 terminations of the vagus and sympathetic, either reach them 

 through the coronary blood, such as drugs, or are transmitted 

 from the central nervous system through the nerve-trunks. 

 Central impulses affecting the force of the heart probably 

 reach it through the same channels. 



The cardiac centre in the medulla is the centre of an area of 

 impressionable matter, which is as extensive as the nervous 

 system itself. Into this centre there pour constant streams of 

 impressions from the vessels, abdominal viscera, skin, muscles, 

 central nervous system (including the seat of mind), from the 

 lungs, and indeed from every organ, including the heart itself ; 

 and thence the resulting impulses descend through the vagus 

 and sympathetic to the heart, which is thus subject to every 





