AND BLOOD PRESSURE 139 



If the arteries dilate in the brain or eyeball, room must be 

 made by increased outflow of cerebro-spinal fluid or aqueous or 

 by narrowing of the veins, or by both means. Probably each 

 pulsatile expansion of the arteries helps to drive a little of the 

 cerebro-spinal fluid or aqueous into the veins, and thus the effective 

 circulation of these tissue lymphs is maintained. Each pulsatile 

 expansion of the arteries also causes a pulsatile expulsion of venous 

 blood into the venous sinuses and thus helps the circulation. A 

 rise of arterial pressure accompanied by expansion of the arteries 

 of the brain or eye, cannot compress the veins so as to produce 

 venous congestion or stasis, for the pressure transmitted through 

 the walls of the arteries must always be less than the pressure 

 transmitted directly through the blood within the vessels. The 

 veins are narrowed to compensate for the expansion of the arteries 

 until the whole system becomes more like a rigid system, and the 

 velocity of flow is increased, less of the head of pressure being spent 

 in overcoming the resistance in the arterioles, more going to the 

 kinetic energy of flow. Experimental observations show that the 

 outflow is always increased by a rise of arterial pressure. 



In pathological states the intra-ocular or intra-cranial pressures 

 are often increased. Such increase is entirely circulatory in origin. 

 In inflammatory states the toxins produced by bacteria on the 

 one hand dilate the blood-vessels, and on the other hand alter the 

 metabolism of the tissues, and increasing those cell products which 

 are crystalloidal in character, thus raise the osmotic pressure. The 

 tissues swell, and the blood pressure rises, keeping pace with the 

 pressures of the tissue lymph. The swelling must be at the ex- 

 pense of the blood supply of other surrounding parts, where there 

 is no toxin to excite and no vaso-dilatation or cedema. The 

 anaemia so produced may in its turn damage the tissue metabolism 

 of these parts, and set up congestion and oadema therein, and thus 

 a vicious circle may be established. Relief is obtained by opening 

 or softening the confining capsule, and so allowing free flow of 

 blood and free exudation of lymph through the inflamed part. 

 The lymph brings with it neutralises of the toxins, the phagocytes 

 and their opsonins, the chemical repairers of disordered cell meta- 

 bolism. If the products of tissue damage are successfully removed 

 by the increased flow in the surrounding dilated vessels the whole 

 mischief subsides, and the killed tissues are removed and replaced 

 by growth of scar tissue. During inflammation of such an organ as 



