124 PRACTICAL PHYSIOLOGY 



the peripheral resistance be constant, then as the energy of the heart 

 increases or decreases both the pressure and the velocity in the aorta 

 together become greater or less. By compensatory changes taking 

 place in the heart and the resistance, the velocity may remain constant 

 while the pressure varies, or the pressure may remain constant while 

 the velocity varies. 



The average velocity at any part of the vascular system is inversely 

 proportional to the total cross section at that part. If the total cross 

 section of any one part of the circuit be dilated the velocity becomes 

 slower there, while it proportionately increases in the other parts. 

 This must be so if the blood continues to circulate round the whole 

 system in the same time. Vaso-dilatation in one part is normally 

 compensated for by constriction in other parts. 



Velocity in the Capillaries. Pith the cerebrum of a frog and plug 

 the hole. Lightly curarise the frog, and spread the web over the hole 

 in the web-board. Examine the circulation under the microscope, 

 and with the aid of an ocular micrometer and a clock beating ^ seconds 

 measure the time it takes for a red corpuscle to move through T J<y mm. 

 Note in an arteriole that the red corpuscles move the fastest in the 

 axial stream, while the white corpuscles roll slowly along the margin. 



Place on the web a drop of hot water (50-60 C.). The flow at first 

 is accelerated owing to vaso-dilatation, but soon slackens as the red 

 corpuscles clump together owing to the escape of the plasma through 

 the damaged capillary walls. 



CHAPTER XXXII. 

 CIRCULATION CONTINUED. THE INFLUENCE OF GRAVITY. 



Artificial Schema. Influence of Gravity. The schema is set up as 

 figured (Fig. 102). The lower distensible rubber bag B represents the 

 splanchnic vein and capillaries. The upper one A those of the head and 

 neck. The skeletal muscles and vaso-motor mechanism are supposed 

 to be paralysed, and therefore the bags are unsupported. Work the 

 Higginson syringe. In the horizontal posture the circulation continues 

 through both bags. In the vertical posture the lower bag fills and 

 becomes distended, owing to the weight of the fluid, the heart no 

 longer fills, and the upper bag is empty and collapsed. Compress the 

 lower bag with your hand, the heart and upper bag fill. This represents 

 the normal action of the skeletal muscles and the vaso-motor 

 mechanism. 



