1780 



II \\I>H< 11 iK ill I'm SI \ 



nkirophysiology 111 



horse, ox and guinea pi'j; (i<>;$, 1741, and negligible in 

 the cat and dog (174). The cerebrospinal fluid shows 

 a small accumulation of the vitamin (38, 172, 173). 

 The high concentration in the aqueous humor has 

 been attributed to a synthesis of the vitamin by the 

 lens (8 2, 127), but a variety of studies (35, 131, 1 43— 

 145, i();j) make it very unlikely that such a synthetic 

 activity, if it occurs at all, will contribute appreciably 

 to the maintenance of the high concentration in the 

 aqueous humor. We are faced here with a definite 

 active transport of material from the plasma to the 

 aqueous humor. 



Sodium 



The concentration of this ion in the cerebrospinal 

 or ocular fluid must exert a profound influence on the 

 dynamics of fluid exchange, since it constitutes some 

 90 per cent of the cations in the plasma and fluids 

 under consideration so that variations of small per- 

 centage magnitude will have relatively large effects on 

 the total osmolar concentrations and thereby influ- 

 ence the relative osmotic pressures. In general, it 

 would seem that the aqueous humor has a higher 

 concentration of sodium than a dialysate of plasma 

 (58, 60, 62, 63). In the cerebrospinal fluids of all the 

 species that have been examined, the concentration 

 of sodium is considerably larger than that in the aque- 

 ous humor, as table 6 shows. These facts would sug- 

 gest thai sodium is actively transported from the 

 plasma into the primary secretion — aqueous humor or 

 cerebrospinal fluid- as a result of the metabolic ac- 

 tivity of the epithelial cells of the ciliary body or of the 

 choroid plexus. Depending on the extent to which 

 other substances arc in excess or deficiency in the fluid, 

 and on the extent to which sodium is accumulated, 

 this active transport of sodium may lead to the forma- 

 tion of a fluid that is hypertonic in relation to the 

 plasma. This would certainly appear to be true of the 

 cerebrospinal fluid (68), but the degree of hyper- 

 tonicit) "l tin' aqueous humor, if it exists, is probably 

 very much smaller; in fact, it m;i\ will lie that in some 



pecies, foi example the rabbit and guinea pig, the 

 fluid is hypotonic in plasma. !B According to whether 



I '■ lii "i the osmotic pressure of tin- fluid in relat 



in plasma iln reader may !»■ referred to the following papers: 



Gilman & Yudkin (ioo),Benham<f ai 1 ;" ,Roepke& riether- 



"ii (187) .iixl Km' 1 ;■' for tin- aqueous humor; Fre- 



! Smith el ai. (94) and Blegen foi cerebrospinal fluid. 



Ih. in. mi difficult) in assessing the difference ol osmotic pres- 



ls in allow I'M .in\ .11 linn "I ih' pl.ism.i proteins Davson \- 



I'm , t 68 "' asured tin- depression "I freezing point of pi. ism. 1, 



the fluid is hyper- or hypotonic to plasma, the fluid 

 pressure will be influenced. Thus, other things being 

 equal, the secretion of a hypertonic fluid will cause 

 the fluid pressure to be high because of the influx of 

 water across the blood-aqueous humor barrier. In this 

 connection it is worth noting that a hypertonic cere- 

 brospinal fluid would draw water away from the 

 nervous tissue, if the extracellular fluid of this tissue 

 had the same osmotic pressure as that of a dialysate 

 of plasma. The possibility must not be ignored, how- 

 ever, that the extracellular fluid of the nervous tissue 

 is maintained at a different degree of tonicity from 

 that of a plasma dialysate by the secretory activity- of 

 the cells of the nervous tissue. 26 



Chloride and Bicarbonate 



In the cerebrospinal fluid the concentration of 

 chloride is some 20 per cent higher than in a dialysate 

 of plasma; only to a small extent is this excess of chlo- 

 ride balanced by a deficiency of bicarbonate. More- 

 over, the excess of negative ions caused by this pre- 

 ponderance of chloride seems not to be completely 

 balanced electrostatically by an equivalent excess of 

 the identified positive ions, although the high concen- 

 tration of sodium does contribute appreciably to this 

 balance. It is possible that the cerebrospinal fluid 

 contains some organic cation not present in the 

 plasma. In the aqueous humor the state of affairs varies 

 with the species. Large-eyed animals like the horse 

 and goat have an excess of chloride and a deficiency 1 u 

 bicarbonate as in the cerebrospinal fluids of all species; 

 in the rabbit and guinea pis' the reverse is true (62, 

 ti 3). In general, these variations in chloride and bicar- 

 bonate concentrations are not expressions of a tend- 

 ency for the total chloride-plus-bicarbonate to re- 



aqueous humor anil cerebrospinal fluid from the same animal. 

 By measuring the depression nl freezing point <>t plasma and 

 its dialysate they showed that the plasma proteins do, indeed, 

 cause a speciously high depression of freezing point . on making 

 allowance for this, it emerged that the aqueous humor of the 



1 .1I1I 111 w .1 -. sh.'lnK hv] mil In plasma, w hi li tin cerebrospinal 



lluiil was h\ pei tonic to both Ihiids. 



•'" 11 the concentration of sodium in the cerebrospinal fluid 

 was greater than that in the extracellular fluid of the nervous 

 tissue, we might expect to find significant differences in concen- 

 tration between the fluid from the \ entricles and from the spinal 

 subarachnoid space, the latter, being more stagnant, would 

 have a lower concentration than the former t"he literature does 

 not contain reliable measurements that would settle this 

 point in the monkey the author and his colleague, Dr. C. P. 

 Luck, have been unable to lind any significant differences in 

 concentration "I \a-' in the Moid from the eisterna in. iu.ua and 

 lumbar sac 48 hr. after injection ol the isotope. 





