290 VETERINARY PHYSIOLOGY 



This is well seen in any small vessel placed under the micro- 

 scope, and in such situations it will be found that, while the 

 erythrocytes are chiefly carried in the axial stream, the leuco- 

 cytes are more confined to the peripheral stream, where they 

 may be observed to roll along the vessel wall with a tendency 

 to adhere to it. 



When from any cause the flow through the capillaries is 

 brought to a standstill, the leucocytes creep out through the 

 vessel wall and invade the tissue spaces. This is the process 

 of diapedesis, which plays an important part in inflammation. 



SPECIAL CHARACTERS OF THE CIRCULATION IN CERTAIN 

 SITUATIONS 



1. Circulation Inside the Cranium (fig. 133). Here the blood 

 circulates in a closed cavity with rigid walls, and therefore its 

 amount can vary only at the expense of the cerebro-spinal fluid. 

 This is small in amount, and permits of very small variations 

 in the volume of blood. Increased arterial pressure in the 

 body does not therefore increase the amount of blood in the 

 brain, but simply drives the blood more rapidly through the 

 organ. There seems to be no regulating nervous mechanism 

 connected with the arterioles of the brain, and the cerebral 

 pressure simply follows the changes in the general arterial 

 pressure. The splanchnic area is the great regulator of the 

 supply of blood to the brain. Since the cerebral arteries are 

 supported and prevented from distending by the solid wall of 

 the skull, the arterial pulse tends to be propagated into the veins. 

 In these veins the respiratory pulse also is very well marked. 



2. Circulation in the Lungs. Vaso-motor nerves seem to be 

 absent, and hence drugs like adrenalin fail to cause a constric- 

 tion of the arterioles. The amount of blood in the lungs is 

 regulated by the blood pressure in the systemic vessels. 



3. Circulation in the Heart Wall. A peripheral vaso-motor 

 mechanism is not present in the arterioles of the coronary 

 vessels (see also p. 245). 



4. Circulation in the Spleen. Here the blood has to flow 

 through a labyrinth of spaces in the spleen pulp, and it is 

 driven on by the alternate contraction and relaxation of the 

 non-striped muscles in the capsule and trabeculse (see p. 221). 



