THE CIRCULATION OF THE BLOOD AND LYMPH 165 



horizontal axis passing under the neck, the blood-pressure 

 in the carotid artery falls greatly when the animal is made 

 to assume the vertical position with the head up, and either 

 rises a little or remains practically unchanged when the head 

 is made to hang down. So great may the fall of pressure be 

 in the former position that death may occur if it be long 

 maintained (Practical Exercises, p. 187). 



Finally, it is in virtue of the amazing power of accommoda- 

 tion possessed by the vascular system, as controlled by the 

 vaso-motor and cardiac nerves, that so long as these are 



IHWim 



< 



' ' ' ' { ' ' t [ I i i 1 i |, | | j i t I > I | f | 



FIG. 57- -BLOOD-PRESSURE TRACING FROM A DOG POISONED WITH ALCOHOL. 

 o^pressSre at m ry a C s en h tr xia e to? paralyzed ' res P irati n stopped, and the typical rise of 

 of the vaso-motor centre was near a* ES^hen at^^ anhwO^tSt 



Snirjlforv mn\/*m*a* T^K* ^., ,*:*._ _/ f 



oi oxygen thus taken in was enough 

 DSC. This was re- 



not disabled the total quantity of blood may be greatly 

 diminished or greatly increased, without endangering life, 

 or even causing more than a transient alteration in the 

 arterial pressure. It is not until at least a quarter of the 

 blood has been withdrawn that there is any notable effect 

 on the pressure, for the loss is quickly compensated by an 

 increase in the activity of the heart and a constriction of 

 the small arteries. An animal may recover after losing con- 

 siderably more than half its blood.* Conversely, the volume 



bv hi fc '!r n0t us jj ally P ssible to obtain quite two-thirds of the total blood 



rJr/^vi i *i? g a g r , om a lar S e arter y- !n seven dogs bled from the 



W, tne ratio of the weight of the blood obtained to the body weight 



P ,h 24 w f V V 7 ' i : 2 7 ' ' : 20 ' 6 ' I : l8 ' 6 ' r : l6 ' ' : '3'5- ^ the last 

 ** clotted with abnormal slowness, and the animal died in a 



