74 A MANUAL OF VETERINARY PHYSIOLOGY 



The influence of the nervous system on blood-pressure will be 

 studied presently. 



Circulation in the Living Tissues. — The circulation in the living 

 animal may be readily seen in the web of a frog's foot, or in the 

 mesentery of a mammal, and in this way we learn exactly how 

 the corpuscles behave within the vessels. 



In all capillary vessels of small size the corpuscles pass through 

 singly, sometimes revolving in the plasma, traversing certain 

 sections very rapidly, others very slowly. In the vessels larger 

 than the capillaries, such as the commencement of the small 

 veins, the stream of blood behaves somewhat differently ; in these 

 the centre of the vessel is occupied by a rapidly moving column 

 of red cells, the axial stream, whilst between them and the coats 

 of the vessel is a clear layer or zone, the inert layer, in which may 

 be seen the white corpuscles strolling lazily along the sides, 

 occasionally stopping, then moving forward once more. This 

 difference in the behaviour of the corpuscles is due to the physical 

 fact that the friction against the sides of the vessel is greater 

 than in the centre, and the red corpuscles being heavier than the 

 plasma, are drawn into the rapid part of the current. This also 

 explains why the lighter leucocytes hug the wall of the vessel, 

 through which, as previously pointed out, they may pass in 

 order to gain the tissues without. 



Under the influence of inflammation the slowly moving leuco- 

 cytes attach themselves to the wall of the capillaries and venules, 

 and pass into the tissues in large numbers. Small numbers of 

 red corpuscles may also pass out. This process is known as 

 diapedesis. Inflammatory changes are essentially due to the 

 cell wall, and not the blood, and this is proved by the fact that 

 an artificial corpuscular fluid introduced into an inflamed area 

 behaves exactly as does the blood. 



The Pulse. — When the left ventricle contracts it drives a new 

 supply of blood into an already full aorta, and room for it has 

 to be found. This can be effected in one of two ways, either 

 by displacing an equal volume of fluid already in the system, or 

 by temporarily making the artery larger. The latter process 

 is followed as being the most economical in energy. The aorta 

 distends to receive the additional blood, followed by an elastic 

 recoil of its walls which drives more blood along another segment 

 of the vessel ; this accordingly distends and then recoils, and 

 so the process is repeated as a wave throughout the arterial 

 system. This distension and elastic recoil is the pulse. 



Each expansion of the arterial wall coincides with a contraction 

 of the ventricle, and so each beat or throb of the pulse corre- 

 sponds to a contraction of the heart. The rhythmical force of the 

 heart is stored up in the arteries as elastic recoil ; this inter- 



