222 A TEXTBOOK OF PHYSIOLOGY 



On this weights are placed until the pressure is reached at which the 

 skin is blanched and the capillaries compressed. 



In another method a small rubber bag with a hole in the centre 

 is placed on the skin. Both bag and skin are moistened with glycerine, 

 and the whole is covered with a glass plate so held as to make an air- 

 tight junction. By means of a side tube air is then blown into the 

 bag until the skin blanches, the pressure being indicated on a man- 

 ometer. These methods are inaccurate, for the horny layer of the skin 

 resists the compression. 



The web or mesentery of a frog, being laid on a glass plate, can 

 be compressed, while under the microscope, by a thin transparent 

 membrane, which forms the base of a glass capsule. The latter is filled 

 with water, and connected with a pressure-bottle and manometer. 

 By this means it was found that pressure of 100 to 150 mm. ELO 

 is sufficient to stop the circulation in the capillaries and veins of tl.o 

 frog's web; in the arteries 200 to 250 mm. H 2 0. In periods of a feu- 

 minutes the pressure may vary 20 to 30 mm. H 2 0. On producing 

 vagus inhibition of the heart by striking the abdomen, the pressure 

 sank to 0, and then rose again in the veins to 70 to 100 mm. H 2 O, 

 owing to venous congestion. Temporary anaemia of the web caused 

 dilatation of the vessels, and this produced in its turn a higher capillary 

 pressure. 



Such methods necessitate the fixation of the part, and cannot 

 be quickly performed. They obstruct the flow and therefore do not 

 give the capillary pressure under normal conditions. 



If the upper arm be constricted, so as to block the venous exits, 

 the pressure in the cutaneous veins of the arms rises fairly rapidly 

 to the static arterial pressure. It takes a long time for the capillaries 

 to all become distended with blood. The Veins fill through the 

 wider channels. If we squeeze the fist under these conditions, the 

 capillaries are emptied into the veins and momentarily blanched, and 

 we see that it is possible to have then a high pressure in both arteries 

 and veins, and a low pressure in the capillaries. The distens'on of 

 the capillaries vessels causes aching pain, and this discomfort prevents 

 our keeping our limbs motionless in a dependent posture, and causes 

 us to move the parts of the body, to fidget, and so relieve congestion. 

 In the brain, the capillary venous pressure can be estimated 

 by finding the tension which is sufficient to just compress the 

 brain as it bulges into the trephine hole. This pressure in the 

 horizontal position of the animal is usually about 10 mm. Hg. The 

 capillary pressure varies widely with changes in the general venous 

 and arterial pressures, and with the position of the animal. Thus, 

 in the brain, the pressure may fall below zero in the vertical feet-down 

 position (the fontanelle of an infant suffering from diarrhoea may 

 become depressed), and rise to almost 50 mm. Hg during the height of 

 strychnine convulsions. The intracranial pressure (cerebro-spinal 

 fluid pressure), and the cerebral venous pressure, are one and the 

 same. So, too, in the eyeball the intra-ocular pressure (aqueous fluid 

 pressure), and the pressure of the veins within the eyeball, are the 



