i 7 7o 



IIWWIODK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



11 



I 



u 



10 



* 



09 - 



06 



[Guinea-pig 



Goat 



Sheep 



Hone 



Vitreous + aqueous + lens (g.) 

 J I I L_ 



20 



40 50 60 70 80 90 



FIG. i" Variation in the chloride distribution ratio: cone, 

 in plasma cone, in aqueous humor (i.e. i/Raq) with weight of 

 the intraocular contents. [From Davson et at. (66).] 



prominent on examining anteroposterior sections of 

 the eyes of different mammalian species, as in figure 

 i i. It will he seen thai a small eye is associated with 

 a lens that is large in proportion to the total ocular 

 contents; since the lens continually produces lactic- 

 acid, the adequate buffering of the ocular contents of 

 the small eye becomes a problem which is met by 

 the secretion of increasing concentrations of bicar- 

 bonate in the aqueous humor. A second factor m.i\ 

 well be the total osmotic pressure of the aqueous 

 humor in relation to that of the plasma, this point 

 will be discussed later (p. 1780) and here il need only 

 be indicated thai the variations in the excesses of 

 chloride in the aqueous humors of the different 

 species may represent, to some extent, variations in 

 the difference of osmotic pressure between aqueous 

 humoi .Hid plasma, an important factor when the 

 intraocular pressure is considered. 



HI 1 ii Hi- Mil El 11 >, hi 1 11 111-1 EREBRi ISPINAL FLUID 

 \M> lit OOD-BRAIN BARRIERS 



Although ibis brief summary of the chemical com- 

 position of the aqueous humor and cerebrospinal fluid 

 indie. ids 1l1.1t they ate formed by ■> process ol secre- 

 tion presumably by the epithelial linings of the cili- 

 ary body and the choroid plexuses this does uoi 



mean thai their constitution al any given moment is 

 independent ol variations in the concentrations ol the 



©€)© 



Goat 12 5g Man 6-2 g Dog 4 5 g 



Horse 53 g 



© © 



Cat5 3g Monkey 3 5g Rabbit 2 2g Guinea-pig Rat0095g 



54g 



fig. it. Outlines of meridional sections of eyes of various 

 mammalian species, drawn to scale; note the differing propor- 

 tions of the globe contents occupied by the lens. [From Davson 

 & Luck (6a).] 



constituents of the blood plasma. To understand the 

 true relationships between the blood and the two fluids 

 under consideration, a thorough knowledge of the pos- 

 sibilities of exchange between them and the blood 

 plasma is necessary, in other words, a knowledge of 

 the nature of the blood-fluid barriers. The early wink 

 on this subject was largely confined to a study of the 

 ability, or otherwise, of dye-stuffs to pass from the 

 blood into the fluid; at best, the results of such studies 

 were equivocal, and generally they were misleading, 

 so that nothing will be lost if in the present discussion 

 we confine ourselves to the consideration of more re- 

 cenl studies in which substances of well-defined chem- 

 ical constitution have been employed. The technique 

 for the quantitative study of the blood-fluid barriers 

 consists generally of maintaining a definite concen- 

 tration in the plasma of the substance to be examined 

 and determining, alter an appropriate interval, the 

 concentration in the fluid. By a suitable mathematical 

 analysis a parameter, /., may usually be computed, 



indicating the rate at which the fluid comes into equi- 

 librium with the plasma so far as this particuar sub- 

 Stance is concerned. Thus, a high value of/, indicates 

 that the various membranes, or barriers, separating 

 the plasma from the fluid are easily permeated by the 

 substance so that the attainment of equilibrium is 

 rapid; a low value ol /. indicates the reverse. 



/■' {-Aqueous Fluid Barriei 



Some of the results of quantitative studies of this 



barrier are shown in table ; ll will be clear dial die 



