DISTRIBUTION OF BLOOD IN THE BODY 239 



the vasomotor center located in the medulla, which constitutes the chief center 

 of the vasoconstrictor nerves. The centers distributed in the spinal cord 

 represent centers of the second order, and the peripheral ganglia or the muscu- 

 lature of the vessels, as the case may be, represent centers of the third order. 

 The last named can exert a powerful influence even when they are isolated 

 from the others. As a rule, the musculature of the vessels is in a state of 

 tonic contraction because of impulses passing out over this chainlike series 

 of centers. The contraction is more or less weakened by stimulation of the 

 vasodilator nerves, since they exert an inhibitory influence on the peripheral 

 mechanism. 



The parts of the brain above the medulla, especially the motor zone of the 

 cerebrum, influence the blood vessels. With regard to this influence, I believe 

 with Fr. Franck that it is to be conceived in the same way as that produced by 

 the same parts upon the cardiac nerves i. e., that the vasomotor center of the 

 medulla is set in action by these parts of the brain in exactly the same way as 

 it is stimulated by the other parts of the body through their afferent nerves. 

 And just as we have seen that in muscular activity the acceleration of the heart 

 is conditioned by this influence of the cerebrum upon the medulla, we may infer 

 from the facts which we already have that the change in vascular tonus taking 

 place in muscular work are produced by a similar influence. 



13. GENERAL CONSIDERATIONS ON THE DISTRIBUTION OF 



BLOOD IN THE BODY 



The distribution of blood in the body depends partly upon mechanical 

 conditions, but chiefly upon the vasomotor nerves. 



A. MECHANICAL IMFLUENCES 



To these belong first the caliber of the afferent arteries; the greater the 

 caliber, the greater, other things being equal, must be the supply of blood to 

 the part. Moreover the attitude of the body plays a prominent part especially 

 in the flow of blood in the veins. In the upright position, for example, the veins 

 of the lower extremities are dilated because of the hydrostatic pressure of the 

 blood column, and they contain a large quantity of blood. In changing to the 

 horizontal position the rich supply of blood to the lower extremities decreases. 

 The quantity which is shifted in this way amounts in a grown man to about 

 100 g. (Mosso). 



The influence of the respiratory movements on the circulation has already 

 been discussed. With a positive pressure in the thoracic cavity, for example, in 

 severe muscular effort, when the lungs are inflated and the glottis is closed, the 

 return flow of blood to the heart is hindered and the quantity in the extremities 

 increases. 



If with the body in a vertical position, the head being upward, the splanch- 

 nics be cut so as to paralyze the great vascular region of the abdominal viscera, 

 and the respiration be then interrupted, the circulation stops at once. The 

 dilated vessels of the abdomen contain so much blood that the heart no longer 

 receives a sufficient supply. In such cases, however, the circulation can be 

 restored to some ex-tent by respiratory movements, the blood being drawn by the 

 aspiration of the thorax from the vensB cavse to the heart (Hill and Barnard). 



