INTRACRANIAL AND INTRAOCULAR FLUIDS 



'763 



they have become, in effect, the choroidal epithel- 

 ium- On cross-section, therefore, the choroid plexus 

 appears as a fold of epithelium with a core of highly 

 vascularized connective tissue (fig. 3), the epithelium 

 being modified ependyma and the connective tissue, 

 modified pia. The choroid plexuses were early con- 

 sidered to be the regions where the cerebrospinal 

 fluid was elaborated, and this opinion has prevailed 

 to this day, although views still differ as to the exist- 

 ence and the importance of other loci of formation. 

 Early evidence adduced in favor of the choroid 

 plexuses as the sites of formation of the fluid, namely 

 the effects of intravenous injections of drugs such as 

 pilocarpine and of extracts of various glands includ- 

 ing the choroid plexuses themselves (73, 157, 171, 

 217, 237) on the histological appearance of the 

 choroidal epithelium, or on the apparent rate of 

 formation of the fluid, is probably invalid [sec, for 

 example, Becht (22)], and it is very questionable 

 whether the epithelial cells do really contain secretory 

 granules (128). The main reason for attributing in 

 the choroid plexuses the role of formation of the 

 fluid rests on the classical studies of Dandy & Black- 

 fan (56) and Frazier & Peet (91 1 who produced an 

 experimental hydrocephalus in dogs by plugging the 

 aqueduct of Sylvius. If the choroid plexus of one 

 lateral ventricle was removed, and both foramina of 

 Monro blocked, one ventricle dilated while tin- other 

 collapsed (55). 



If the choroid plexuses are the sites of formation of 

 the fluid, and if the latter is being formed continu- 

 ously—as the pathological phenomenon of hydro- 

 cephalus would suggest — we must seek a region in 

 which the fluid is returned to the blood. Key & 

 Retzius (130), by injecting colored gelatin solutions 

 into the subarachnoid space ol cadavers, showed that 

 the Pacchionian bodies large wart-like outgrowths 

 of the arachnoid protruding into the lumina of the 

 dural sinuses — were sites at which the colored ma- 

 terial escaped into the vascular system; and more 

 recent work of Weed (233, 235) and of Scholz & 

 Ralston (202), in which the Prussian blue reagents 

 were shown to pass out of the subarachnoid space 

 into the dural sinuses, has confirmed this view, with 

 the difference, however, that the drainage occurs 

 principally by way of microscopic arachnoid villi 

 which are delicate web-like structures continuing the 

 arachnoid membrane into the dura in immediate 



2 Strictly speaking, the choroid plexus, as defined by the 

 histologist, consists only of the highly vascularized pia; this, and 

 the closely apposed choroidal epithelium, or lamina epithelialis, 

 make up the tela choroidea. 



relationship with a sinus (fig. 4). The villus is capped 

 with a mesothelial covering of arachnoid cells so 

 that the cerebrospinal fluid is separated from the 

 blood in the sinus by this layer and also by the endo- 

 thelium of the sinus itself. The fluid must therefore 

 filter through these layers. 3 



So far, then, we may picture the cerebrospinal 

 mechanics as the continuous formation of fluid, de- 

 rived from the blood in the choroid plexuses and 

 associated with a flow out of the ventricles into the 

 subarachnoid space where it passes over the surface 

 of the brain to pass back into the blood in the dural 

 sinuses. 1 It is possible that this picture is too simple; 

 for example, it has been argued that a significant 

 pathway for drainage from the subarachnoid space 

 is by way of the cranial and spinal nerve sheaths 

 [see, for example, Bricrlev and Field (40, 41 )] whence 

 the fluid finds its way into the lymphatics of the 

 epidural tissue (the meninges and brain having no 

 Lymphatic system 1. While it is certainly true that 

 colored solutions pass fairly rapidly from the cranial 

 subarachnoid space along the sheaths of the optic, 

 olfactory and acoustic nerves, the evidence for a flow 

 along the other cranial, and the spinal, nerves is not 

 ver\ striking. Moreover, the studies of Courtice & 

 Simmonds (51), who actually collected lymph after 

 subarachnoid injections, have shown that drainage 

 by this roundabout route is at most only subsidiary, 

 the main escape being ,1 direct one into the blood. A 

 passage of fluid down the so-called Virchow-Robin 

 perivascular spaces'' has also been postulated by 



; The Pacchionian bodies are absent in the newborn human 

 and in all the lower animals. It «.is for this reason that Key & 

 Rrt/ius' demonstration of a flow from the subarachnoid to the 

 dural sinuses u .is largely ignored. Clark (46) has described the 

 detailed histology <>l these bodies; they are essentially enlarge- 

 ments of the arachnoid villi, and they occur in all human 

 brains and are not to be regarded as pathological. 



' The subject of the general direction of flow has been in- 

 vestigated repeatedly bv injecting colored particulate matter 

 into the ventricles or subarachnoid space and subsequently 

 inspecting the surface of the brain. [See, for example, Weed 

 (233I, Riser (184), Bedford 128), Solomon el at. (210) and Sachs 

 el 11I. 1194). It is generally agreed that flow from the cisterna 

 magna is mainly by way of the basal cisterna — cisterna basalis, 

 cisterna ambiens, etc. Thence it flows upward over the cerebral 

 and cerebellar convexities. Mixing with the fluid in the spinal 

 subarachnoid space is slow and probably dependent on reflex 

 movements consequent on sudden changes in posture, coughing 

 and so on. 



5 The evidence bearing on the existence of the various 

 spaces — Virchow-Robin, His, Held, etc. — has been well sum- 

 marized by Woollam & Millen 12531. Other useful papers on 

 this subject are those of Patek (169) and Woollam & Millen 

 (256). 



