i 7 6 4 



HAMlHiiiik (ll I'HVSIOl.OOV 



NEUROPHYSIOLOGY III 





TISSUE 



pig. j Choroid plexus. [From Clara (45).] 



Mutt (itjj); the large vessels passing from the pia- 

 arachnoid into the nervous tissue carry with them a 

 sheath of pia-arachnoid and there is no doubt that 

 cerebrospinal fluid may, under appropriate condi- 

 tions, pass from the subarachnoid space proper along 

 the perivascular sheaths which mav be regarded as 

 prolongations of the subarachnoid space. If these 

 sheaths were to acl .is channels carrying cerebro- 

 spinal fluid away, they would have to continue as 

 far as the capillaries, and the pressure in these capil- 

 laries would have to be low enough to permit the 

 absorption of the fluid. Alternatively, it has been 

 suggested that .11 least .1 pari of the cerebrospinal 

 fluid is formed l>\ the capillaries of the nervous tissue 

 whence it passes to the surface of the brain up the 

 Virchow-Robin spaces (233). These propositions 

 need nol concern us seriousl) hen-, .n best the) ma) 

 be regarded as subsidiary mechanisms, either of 

 formation or of drainage, and the evidence adduced 

 in their favor, based .is ii is largely on the fate of in- 

 d partii ulate matter, is equh ocal, to say the 



The pi. 1, it will be recalled, is .1 connective tissue 

 membrane, hind with mesothelium, thai invests 



the mi 1 the brain and cord, the greal majority 



of the large arteries and veins supplying the nervous 



tissue run in the pia before plunging into the par- 

 enchyma. It might be considered that in this tissue 

 there would be considerable possibilities of exchanges 

 between the blood and the cerebrospinal fluid. Such 

 exchanges, to be of any quantitative significance, 

 would have to take place across a capillary bed, yet 

 it is by no means certain that the pia has such a 

 capillary bed, 6 so that this region of exchange be- 

 tween blood and fluid is of questionable significance. 

 As we shall see, however, physiological experiments 

 indicate quite unequivocally that exchanges of 

 material between blood and the cerebrospinal fluid, 

 apart from those in the choroid plexuses, do take 

 place and may be of greater significance than those 

 taking place in the plexuses. We must assume, there- 

 fore, either that these exchanges occur across the 

 pial vessels or, more probably, from the capillaries 

 of the nervous tissue and thence by diffusion around 

 and through the cells of the parenchyma, up to the 

 subarachnoid space. 



0( ulai System 



The aqueous humor occupies the anterior and 

 posterior chambers of the eve (fig. 5); by the pos- 

 terior chamber is meant the very small space bounded 

 by the posterior surface of the iris, the lens and the 

 vitreous body. The remainder of the cavity of the 

 globe is occupied by the last two mentioned struc- 

 tures, the vitreous body being, as mentioned earlier, 

 a dilute gel built up on a scaffolding of a collagen- 

 like protein and the mucopolysaccharide, hyaluronic 

 acid; since its water content is some 98 per cent or 

 more, it may be considered as a liquid from the point 

 of view of the diffusion of dissolved material within 

 it while its rigidit) probably precludes the existence 

 of ,mv significant flow of fluid through it. Function- 

 ally, tin- eve in. iv be regarded as being made up of 

 three coats: .m outer protective one, consisting of the 

 sclera and transparent cornea; a middle vascular 

 layer, made up of the choroid, posteriorly, which is 

 continued forward .is the ciliary bodv and the his, 

 and an innermost Liver, the retina, continued for- 

 ward as the (ili.nv epithelium and the posterior 

 epithelium of the iris. Figure <> illustrates the general 

 scheme of vascularization of the human eyeball. 



8 Weed stated in inn .it Ins papers th.it lie had QCVCr seen a 



capillary in the pia. < >n the other hand, I'm Ins & ( lobb (88) in 

 their description ol the pial vessels .is seen through .1 cranial 

 window certainly mention capillaries. Schaltenbrand & 

 Putnam (196) observed clouds of fluorescein leaving the pial 

 sels after an intravenous injection "1 tln^ dyesruff. 



