INTRACRANIAL AND INTRAOCULAR FLUIDS 



I 78l 



table 6. Distribution of Sodium in Different Species 



[After Davson (58)] 



* Between aqueous humor and plasma; f between cere- 

 brospinal fluid and plasma; J between plasma dialyzate 

 and plasma. 



main constant since the excess of chloride in the 

 aqueous humor of the horse, for example, is not by 

 any means balanced by a corresponding deficiency in 

 bicarbonate. In considering the relative concentra- 

 tions of these two ions in aqueous humor it may well 

 be that two separate factors must be borne in mind; 

 a) the variation in the tonicity of the fluids among the 

 different species and b) the variations in the buffering 

 requirements of the intraocular contents among the 

 different species. 



If we regard the cerebrospinal fluid as the typical 

 cavity-filling fluid, the absence of strong morphologi- 

 cal differences in the cerebrospinal systems of the dif- 

 ferent mammalian species would lead us to expect to 

 find, as we do indeed, only very small variations in 

 the chemical composition of this fluid among the spe- 

 cies. The shapes of the eyes of the different mammalian 

 species, on the other hand, vary greatly, and we may 

 imagine that the chemical composition of the primary 

 cavity-filling fluid has undergone modifications to 

 meet the new requirements imposed by these morpho- 

 logical variations. The aqueous humor of the small- 

 eyed rabbit and guinea pig are those that have under- 

 gone the greatest modification, having lost entirely 

 their excess of chloride and gained an excess of bicar- 

 bonate; the bicarbonate gained, however, is only 

 about 50 per cent of the chloride lost when these are 

 expressed in terms of equivalents per liter. The gain 

 in bicarbonate may be regarded as a modification to 

 meet the needs of the greater buffering requirements 

 of the small eye; as figure 1 1 shows, the lens occupies 

 an enormous percentage of the intraocular contents 



in these two eyes. We may ask, however, why the 

 concentration of chloride has fallen so much. The low 

 concentration of chloride is probably associated with 

 a low tonicity of the fluid as a whole, and it may well 

 be that in the small-eyed animals the prime physio- 

 logical problem — apart from buffering — is the main- 

 tenance of a low intraocular pressure consistent with 

 an adequate flow of fluid through the posterior and 

 anterior chambers. Other things being equal, a hypo- 

 tonic fluid would give a lower intraocular pressure 

 than an isotonic or hypertonic one. Until more is 

 known about the minute anatomy of the eyes of these 

 species, it would be unwise to speculate as to why a 

 small-eyed animal has to secrete a hypotonic aqueous 

 humor to maintain its intraocular pressure within 

 reasonable limits and large-eyed animals have to se- 

 crete a hypertonic fluid; and win, furthermore, all 

 species secrete a hypertonic cerebrospinal fluid. As 

 we shall see, the average level of the fluid pressure is 

 determined by the rate of turnover of the fluid and 

 the frictional resistance to drainage. If, in the large 

 eyes and the cerebrospinal systems of all species, the 

 frictional resistance is relatively low, while it is high 

 in small c\cs, the explanation emerges L ' 7 



INTRACRANIAL AND INTRAOCULAR FLUID PRESSURES 



General Considerations 



II .1 hypodermic needle is inserted into the anterior 

 chamber of the eye or into the lumbar sac, fluid flows 

 oui of the needle; the pressure required just to prevent 

 this outflow is the fluid pressure. The intraocular 

 pressure is of the order of 20 mm Hg, while the cere- 

 brospinal fluid pressure is considerably less, of the 

 order of 150 mm H 2 in man in the recumbent posi- 

 tion. In both systems the fluids are enclosed, together 

 with the blood in the arteries, veins and capillaries, 

 in relatively indistensible coats. The pressure at any 

 minute will be determined by a variety of fac- 

 tors which are by no means independent. The princi- 



27 It should be pointed out that the eyes of the rat, monkey 

 and man are relatively small, yet their chloride and bicarbonate 

 distribution ratios belong to those of the large-eyed animals. 

 From the point of view of the lens /eye-weight ratio, however, 

 man and the monkey are similar to the large-eyed animals. 

 The rat stands out as an interesting exception, since its lens 

 occupies a very large percentage of the eye weight, in this 

 species, however, the rate of flow of aqueous humor is very 

 rapid (62, 63), and it may be that the buffering is achieved by a 

 more rapid turnover of fluid instead of by a higher concentra- 

 tion of bicarbonate. 



