INTRACRANIAL AND INTRAOCULAR FLUIDS 



'783 



sodium and chloride ions must mean that water will 

 pass rapidly out of the aqueous humor into the blood. 

 Such effects have, indeed, been described (72, 121, 

 123); but the effects of hypertonic solutions are often 

 so long-lasting that it seems quite certain that any 

 osmotically induced changes are complicated by alter- 

 ations in vascular tone that may lead to a prolonged 

 fall in intraocular pressure (177)."' 



Nervous Influences 



It is generally agreed that stimulation of the periph- 

 eral end of the cut cervical sympathetic nerve leads to 

 a fall in the intraocular pressure (64, 114, 248); but 

 in some animals, notably the cat and dog, the fall may 

 be masked by the contraction of the smooth muscle of 

 the orbit (114, 122, 248). Section of the sympathetic 

 nerve has variable results; frequently it has no effect 

 at all on the intraocular pressure, while on other occa- 

 sions it may result in a considerable rise in the intra- 

 ocular pressure (64). The effects of the sympathetic 

 nerve are undoubtedly due to the constriction of the 

 small vessels in the eye, causing a reduction in the 

 capillary and venous pressures. Attempts to demon- 

 strate a parasympathetic control over the intraocular 

 pressure have generally failed. Stimulation of certain 

 parts of the diencephalon would seem to lead to spe- 

 cific influences on the intraocular pressure (199, 2-><h. 



Cerebrospinal Fluid Pressure 



In experimental animals this is measured by a can- 

 nula inserted into the cisterna magna. In man, the 

 cannula is inserted into the lumbar sac with the sub- 

 ject usually in the lateral recumbent position. The 

 pressures so recorded are considerably less than the 

 intraocular pressure, being of the order of 150 mm 

 saline, i.e. 1 1 mm Hg. [See Dixon & Halliburton (74), 

 Becht (22), Bedford (26) and Goldensohn et al. (102) 

 for experimental animals, and Masserman (152, 153) 

 and Merritt & Fremont-Smith (158) for man.] The 

 pressure exhibits cardiac and respiratory rhythms 

 which were found by the usual methods of measure- 

 ment to be of the order of only a few millimeters of 

 saline in excursion. O'Connell (167) considered that 

 these figures were grossly in error, owing to the damp- 



'" Injections of colloidal material, such as gum acacia or 

 gelatin, are unlikely to have any significant effect on the rela- 

 tive osmotic pressures of plasma and aqueous humor; they may 

 reduce the intraocular pressure, however. So far as gum acacia 

 is concerned, this would appear to be due to the toxic action of 

 this substance (15). 



ing that takes place during recording; and recently 

 Goldensohn et al. (102) and Bering (33, 34) have 

 shown that O'Connell was correct, the cardiac pulse, 

 measured with a manometer with a minimum amount 

 of inertia, being of the order of 50 mm saline (fig. 12). 

 Removal of the choroid plexuses (32, 34) reduced very 

 considerably the pulsations recorded from the ven- 

 tricles so that it would seem that it is in this region of 

 the vascular system that the arterial pulse is actually 

 transmitted to the cerebrospinal fluid. As figure 12 

 shows, simultaneous recording from the ventricle, 

 cisterna magna and lumbar sac indicates a progressive 

 damping; in the cerebrospinal system, the pulse pres- 

 sures being 62, 49 and 29 mm saline in the respective 

 locations. 3 ' 



The effects of the vascular pressures on the cerebro- 

 spinal fluid pressure have been studied by Becht (22), 

 Weed & Flexner (259) and Bedford (26, 27). In gen- 

 eral, the venous pressure, measured in the sagittal 

 sinus or torcular, seems to dominate the picture, some 

 60 per cent of any change in this pressure being trans- 

 mitted to the cerebrospinal fluid. That the whole of 

 the change in venous pressure is not transmitted is 

 probably due parti) to the circumstance that the 

 elastic tension developed in the veins lakes up some of 

 the change, and partly to the circumstance that the 

 venous pressure actually measured is that in the dural 

 sinuses which is not necessarily the same as that in the 

 cerebrospinal veins; it is these last that will transmit 

 changes of pressure directly, while changes in the 

 dural sinus pressure will influence the fluid pressure 

 only by virtue of their effects on drainage. The effects 

 of jugular occlusion which raises the cerebral venous 

 pressure, and thus the fluid pressure, are not perma- 

 nent (26), the fluid pressure returning to normal 

 within 30 min., while the venous pressure remains 

 high. Release of the jugular now causes a fall in fluid 

 pressure below normal. The cerebrospinal system thus 

 adapts itself to a raised intracranial venous pressure. 

 It is unlikely that the adjustment consists in a more 

 rapid drainage of fluid since, for this to occur, the 

 drop in pressure between the cerebrospinal fluid and 



31 Carmichael el al. (42) have analyzed the respiratory varia- 

 tion in some detail, the rise occurs during expiration and was 

 earlier considered to be the result of the increased pressure in 

 the great veins transmitted centrifugally. These authors showed 

 this to be incorrect, the events in the cycle being more closely 

 related with the fluctuations in the arterial pressure. Presum- 

 ably the respiratory variations in arterial pressure are trans- 

 mitted, by way of the capillaries and veins, to the cerebrospinal 

 fluid; it is very questionable whether the changes in arterial 

 pressure are directly transmitted, owing to the small distensi- 

 bility of the arterial wall. 



