70 A Maimed of Vetcriiinr;/ Physiology. 



The Capillary circulation is readily observed microsco- 

 pically in the web of the frog's foot. The velocity of 

 blood through the capillaries depends upon their size ; in 

 some vessels the red cells may be seen rushing along the 

 centre of the tube, whilst the white ones course leisurely 

 along the side, exhibiting a tendency to adhere together, a 

 phenomenon which is not observed in the red cells of the 

 circulating blood : where the capillary vessels are very small 

 the cells only pass through one abreast, and the rate of the 

 current is reduced. 



The walls of the capillaries are elastic, and by this means 

 they readily distend or recoil, and by so doing accommodate 

 themselves to the increase or decrease in the blood supply 

 to the part. 



The Venous Circulation. — The contraction of the left ven- 

 tricle is sufficient to drive the blood all over the body, but 

 in the veins this force is assisted by the muscles compress- 

 ing the vessels, by the presence of valves which prevent 

 regurgitation, (especially in the veins of the limbs where 

 the fluid has to ascend against gravity), and, lastly, the 

 circulation is assisted in the larger veins by the process of 

 inspiration and the dilatation of the right auricle, both of 

 which have an aspirating effect on the blood in the larger 

 veins. The sucking action of the left auricle assists also 

 in drawing the blood in the pulmonary veins towards the 

 heart. 



A r eins are normally pulseless, but an exception must be 

 made to this statement in the case of the lower extremity 

 of the jugulars, just where they enter the chest. It is quite 

 common in the horse to observe pulsations in these vessels 

 for an inch or so up them, due no doubt to the action of 

 the tricuspid valve, it is, however, distinctly abnormal 

 for these venous pulsations to extend any distance up the 

 neck, for then it indicates insufficient closure of the tri- 

 cuspid, due to a debilitated state of the system or actual 

 disease of the valve itself, which latter in the horse is a 

 very rare condition. 



There are certain veins where no valves exist — for 



