INFLUENCE OF THE NERVOUS SYSTEM. 165 



accidentally, as by the compression of the vessel by a mus- 

 cle, by the stretching of an aponeurosis, and, most frequently, 

 simply by the application of the stethoscope. By reproduc- 

 ing these sounds, in glass tubes, Heynsius (of Utrecht) has 

 made the movements of the fluid visible, by means of colored 

 particles in suspension following the eddying and whirling, 

 which become more rapid as the sound is more decided. 



It has been also ascertained by these experiments, that 

 fluids of slight density produce sounds more readily than the 

 denser fluids. This fact explains the existence and intensity 

 of the vascular sounds in anaemia and chlorosis; in these 

 cases the quantity of blood globules is diminished, often in 

 such proportion as to lower considerably the density of the 

 blood ; we need not, with Peter, look for a spasm of the 

 arterial muscles and the contraction of the vessel, to produce 

 the phenomenon of the fluid vein. Andral's researches have 

 shown that there is always a vascular souffle when the num- 

 ber of globules descends below the proportion of 80 to 1000 

 (eight per cent), and that its intensity depends on the degree 

 of diminution of the globules. 



III. INFLUENCE OF THE NERVOUS SYSTEM ON THE CIRCU- 

 LATION. 



WE have ascertained the existence of many musculai 

 phenomena in the heart and in the vessels (arteries and 

 veins) ; this makes it probable that the contraction of these 

 muscles is influenced by the nervous system. 



The Heart. It was however long believed (as by Haller), 

 that the heart is independent of the nervous system, and 

 that the afflux of blood causes the contraction of this hollow 

 muscle, its presence directly exciting the muscular fibre of 

 the cardiac coats. Now, it has been proved beyond dispute, 

 that the movements of the heart as well as the other move- 

 ments are governed by the nervous system. The spinal 

 cord appears to be the centre of this influence, and we know 

 that any cerebro-spinal shock, or injury to the spinal axis 

 may slacken or accelerate the motion of the heart ; this 

 influence may be reflex, and a large number of peripheral 

 impressions may thus hasten or slacken the movement. 

 This is because the spinal cord and the bulb furnish nerves to 

 the heart, the effect of some of which (branches of the great 

 sympathetic) is to quicken its pulsations, while that of others 

 (the pneumo-gastric) is to retard these : thus the pneumo- 



