THE RABBIT. 23 



pressures of all kinds, due to the motions of the neighbour- 

 ing organs, are readily transmitted, and hence there is very 

 great variation in the blood-pressure in veins, and at times 

 it may even be negative, i.e. in the wrong direction, pressing 

 the blood away from the heart instead of towards it. As a 

 guard against these variations of pressure -direction, many 

 veins contain valves which obstruct any flow in the wrong 

 direction, and so bring it about that of all the varying 

 external pressures acting on the blood, only those in the 

 right direction shall have any effect. It follows from all 

 this that the flow of blood in the veins is much slower than 

 that in the arteries ; and since the same amount of blood 

 (roughly) must pass along the one sot of vessels as along 

 the other in a given time, it is inevitable that the capacity 

 of a vein must be greater than that of the corresponding 

 artery. The blood-flow in a vein may be compared to 

 the slow motion of the large body of water in a mill-stream ; 

 that in an artery to the rapid flow of the small body of 

 water over the mill- wheel, 



7. The Portal System. As we trace arteries away 

 from the heart we find them branching out into smaller 

 and smaller arteries, with thinner and thinner walls, and 

 finally the smallest arteries pass into the capillary network, 

 all the layers of the walls except the endothelium dis- 

 appearing. In the same way we can trace the veins (in 

 the opposite direction to the blood-flow) and find them 

 ending similarly in the capillaries. In the other direction 

 all arteries and most veins can be traced into larger and 

 larger ones, and finally to the heart. 



But occasionally we find a system of veins which ends in 

 capillaries when traced in either direction : only one such 

 system is found hi the rabbit, and it is called the portal 

 system. This system is found in the mesentery and is 

 therefore an approximately median one. The blood from 

 the capillaries of the whole abdominal portion of the 

 alimentary canal flows along a series of veins which unite 

 into larger and larger ones, and finally into one large portal 

 vein : these smaller veins through which the blood flows 

 towards a larger vein may be called the factors of the portal 



