XXXV111 



Animals. 



In the young vertebrate embryo the first direction of the 

 blood-flow is from the food-containing structures (umbilical 

 vesicle, food-yelk) towards the body. It is due to the demand 



Vein. To return blood 

 to heart. 



Lymphatic vessel. To 

 return lymph to 

 blood. 



Lymphatic capillaries. 



Heart. Contractile, To 

 the blood. 



propc 



Large Artery. Elastic. 

 To equalize blood - 

 flow, and diminish 

 shock (i. e. sudden 

 increase of pressure) 

 to smaller vessels. 



Small Arteries. Elastic 

 and Contractile. To 

 regulate the amount 

 of blood flowing 

 through a given 

 vessel. 



Capillaries. Thin-walled. To allow of 



passage of nutriment and gases. 



open into tissue-spaces, to take up fluid 

 exuded by blood-capillaries, and 

 waste products of the tissues. 



DIAGRAM OP CIRCULATION. 



tor formative material by the embryo, and it.s absorption from 

 the food-yelk. The heart as a blood-propelling structure is a 

 secondary cause of the circulation. 



In the adult the causes of the circulation are : the contraction 

 of the Heart, and the demand for fresh material by the tissues : 

 these forces bring the blood to the tissues. The return of the 

 blood to the Heart is the result of the passage of the blood from 

 the capillaries (through whose walls the nutrient fluid and gases 

 pass) into the veins ; intermittent pressure by the tissues on the 

 veins squeezing the' blood towards the Heart, as valves whose 

 edges are directed towards that organ prevent a backward flow : 

 and the suction action of the dilating heart and chest. 



