1782 



HANDBOOK OF I'll YSIOI c 11 . Y 



NEUROPHYSIOLOGY III 



pal factors arc the arterial and venous pressures, the 

 rate of secretion and the (fictional resistance to drain- 

 age of the fluid. In die cerebrospinal system, more- 

 over, gravitational factors have a marked significance. 

 So far as the vascular system is concerned, we may ex- 

 pect fluctuations in the arterial pressure to have the 

 smallest effects per se because the low distensibility of 

 the arterial muscular coat reduces the amount of 

 pressure that may be transmitted. Changes in the 

 venous and capillary pressures, on the other hand, 

 will have strong effects as regards both short-term and 

 long-term influences. A rise in venous pressure causes 

 the veins 10 dilate and thus transmit their increased 

 pressure to the fluid; the high distensibility of the coats 

 of the veins means, moreover, that most of the rise in 

 pressure will be transmitted in this way. If the in- 

 creased fluid pressures caused in this way led to a 

 more rapid elimination of fluid, the effect of a rise in 

 venous pressure would be transitory, the dilatation of 

 the veins being compensated by a loss of fluid. How- 

 ever, a raised venous pressure might easily result in a 

 diminished outflow of fluid since the latter has to drain 

 into the venous system. In consequence, changes in 

 venous pressure may be reflected in corresponding 

 changes in fluid pressure that last as long as the 

 changed venous pressure is maintained. 



Intraocular Pressure 



In experimental animals this is usually measured 

 by a compensated manometer connected with a needle 

 inserted in the anterior chamber.'-" The value so 

 measured is usually given as about 25 mm Hg (150); 

 but the trauma associated with insertion of the needle, 

 and the general anesthesia employed, probably lead 

 to erroneous results and it is doubtful whether abso- 

 lute values of the intraocular pressure determined 

 manometrically have much significance. For studies 

 on man some form of 'tonometer' is employed; thus 

 the impression tonometer of Schiotz measures, essen- 

 tiallv, the extent to which a weighted plunger indents 

 the cornea. By adequate calibration (95, 96) a reason- 

 able estimate of the true intraocular pressure may be 

 made; this would appear to be on the average some 

 18 to ig mm Hg (96, IOI, 2l6, 244). As recorded 



m. netrically, the intraocular pressure shows pulses 



thai are synchronous with the cardiac ,\\u\ respiratory 



Foi ' mi metric iik-iIickIs employed, the reader 



may consul! the classic papa of Wessely (^48); more recent 

 application! are those "I Davson >v Purvii (67) employing an 



■ ■I d manometer, and of Guerry (1 18) employing an el« tro- 



manometei 



cycles, the cardiac variation being of the order of 1 

 mm Hg and the respiratory variation rather more 

 (248). Artificially induced variations in the general 

 arterial pressure influence the intraocular pressure 

 in the same sense, usually, but on a much smaller 

 scale. To what extent they are directly transmitted 

 effects from the intraocular arteries, and to what ex- 

 tent they are secondary consequences of changed 

 venous and capillary pressures, is diflicult to say. The 

 fact that intravenous epinephrine may actually lower 

 the intraocular pressure (248) in spite of an enormous 

 rise in systemic arterial pressure emphasizes the im- 

 portance of local, as opposed to general, vascular 

 pressures. Thus the strong constriction of the arterioles 

 lowers the venous pressure within and outside the eye 

 so that the transmitted venous pressure is less and the 

 ease of drainage is increased. Locally applied epineph- 

 rine may certainly lower the intraocular pressures, as 

 Colic el a/. (50) have shown, and it increases the visible 

 drainage of aqueous humor in the aqueous veins (5, 6, 

 [36). The action of amyl nitrite provides another 

 example of the importance of local vascular pressures. 

 On inhalation, this drug lowers the systemic arterial 

 pressure; its vasodilator action, however, causes an 

 increased venous pressure within and outside the eye, 

 slowing drainage and increasing the transmitted ve- 

 nous pressure. The effect of the drug is consequently to 

 raise the intraocular pressure (249). 



Of some interest is the action of nitrogen mustard, 

 applied locally. This drug causes a marked dilatation 

 of the intraocular vessels; the blood-aqueous barrier is 

 broken down and the intraocular pressure rises to 

 enormous values (60 to 70 mm Hg). Here it is likely 

 that the extreme vasodilatation is the most important 

 factor, although the rapid exudation of fluid from the 

 vessels of the iris and ciliary body is doubtless a con- 

 tributory factor.' ' 



There have been various studies on the modification 

 of the intraocular pressure by osmotic means, theoret- 

 ically, if the blood is suddenly made hypertonic, for 

 example by the injection of 20 per cent sodium chlo- 

 ride, the fact that the blood-aqueous barrier is highrj 

 permeable to water but only slowly permeable to 



■'' There .ire \ .irious drills that may be classic! with nitrogen 

 mustard in this particular respect, raising the intraocular pres- 

 sure and breaking down the blood-. u|ueous barrier, pilocarpine, 

 physostigmine and diethylfluorophosphate m.is be cited; excel- 

 lent histological and slit-lamp microseopie.il studies of their 

 effects have been described by Larsson (139), um Sallmann 

 & Dillon is 15 . Scholz (aoo), Cristini 153) and Poos (176, 

 178). Subconjunctival inject ion of hypertonic salts has essentially 



sllllll.ll .ll.i Is 1 .148). 



