468 VETERINARY PHYSIOLOGY 



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 walls and invade the tissue spaces. This is the 

 process of diapedesis, which plays an important part in 

 inflammation. 



C. SPECIAL CHARACTERS OF THE CIRCULATION 

 IN CERTAIN SITUATIONS. 



1. Circulation Inside the Cranium (tig. 196). — Here the 

 blood circulates in a closed cavity with rigid walls, and there- 

 fore its amount can vary only at the expense of the cerebro- 

 spinal fluid (p. 511). This is small in amount, some 150 

 c.cm., and permits of only small variations in the volume of 

 blood. 



Increased arterial pressure in the body does not therefore 

 markedly increase the amount of blood in the brain, but 

 simjaly drives the blood more rapidly through it. 



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. 



The condition of the intra-cranial circulation is indicated 

 by the circulation in the fundus of the eye which 

 communicates with it, and this may be observed by means 

 of an ophthalmoscope (p. 143). 



2. Circulation in the Lungs — The action of the vaso-con- 

 strictor nerves is feeble, and adrenalin fails to cause a 

 constriction of the arterioles. The amount of blood in the 

 lungs is regulated by the blood pressure in the systemic 

 vessels, and hence the intravenous administration of adrenalin, 



