168 THE BLOOD AND ITS CIRCULATION. 



system; and Ludwig and Cyon proved, by new experiments, 

 that this influence on the peripheral circulating system is 

 principally exercised in vascularizing the abdominal viscera, 

 and is conveyed to them by the medium of the splanchnic 

 nerves: by dividing these nerves we obtain effects similar to 

 those which result from section of the cord between the oc- 

 cipital bone and the atlas. 



The influence of the cord on the pressure of the blood (we 

 are not now speaking of the number of pulsations) is as 

 stated by Ludwig; but Cyon has also demonstrated that this 

 influence which is the result of a peripheral vaso-motor 

 modification (see further on, vaso-rnotors), is by nature 

 reflex, and may therefore be caused by excitation of a sen- 

 sory nerve, beginning in the heart itself: if, after cutting 

 off this nerve, which is a branch of the pneumo-gastric, its 

 peripheral end be excited, no effect is produced ; but excita- 

 tion of the central end is painful, and causes, when the 

 manometer is applied to the carotid artery, considerable 

 diminution of pressure from a reflex influence bearing espe- 

 cially on the abdominal vascular system (splanchnic nerves), 

 resulting in paralysis and dilatation : in short, the depressing 

 nerve of the circulation (of Cyon), represents the centripetal 

 course of a paralyzing reflex action, producing depletion of 

 the heart, and, consequently, diminution in the pressure of 

 the blood in general. 



Under the influence of these reflex actions, which may 

 also have their starting point in the brain (emotional influ- 

 ences, palpitations, syncope, owing to mental causes), the 

 pulsations of the heart offer the greatest possible variety in 

 number and rhythm, especially in cases of disease. In the 

 normal condition, the average number of pulsations is 72 a 

 minute. This is the average in the adult stage, its minimum 

 being found at the period when growth ceases, and the epi- 

 physes are found united ; statistics seem to show that the 

 heart in old age beats faster on the average than in the adult. 



In the pathological condition, the changes in the pulsation 

 of the heart, ascertained by the throbbing of the pulse, sup- 

 ply us with valuable information on the subject of the 

 innervation of this organ, but the quickness of the pulse 

 yields no indication as to the state of the circulation, prop- 

 erly so called. If we go back to our study of the mechanism 

 of this phenomenon, we shall understand how the pulse may 

 be quick without the circulation being active ; if, for instance, 

 the heart at each contraction sends out less than the usual 



