THE LYMPHATIC ARRANGEMENTS OF THE BRAIN. 729 



ing nerves and bloodvessels, especially venous sinuses, between the pia mater 

 a lid dura mater, and over the surface of the brain by villus-like projections 

 of the arachnoid called Pacchionian glands, some of which pierce the venous 

 sinuses of the dura mater. It is lined throughout, both on its dural and on 

 its arachnoid wall, by an epithelium of flat epithelioid cells, and may be 

 compared to a serous cavity, such as that of the peritoneum. Like the serous 

 cavities it contains normally a small quantity only of fluid, and its size is 

 potential rather than actual. 



The subarachnoid space on the other hand is, especially in certain regions, 

 such as the dorsal portions of the vertebral canal and the base of the brain, 

 much broken up by bridles of connective tissue passing from it to the pia 

 mater, as well as by a network of sponge-like arrangement of bundles of 

 connective tissue lying immediately beneath itself, and giving it, when viewed 

 from below, a honeycomb or fenestrated appearance. The under surface of 

 the membrane itself, as well as all the trabeculse of the sponge-work and the 

 bridles, are covered with an epithelium of flat epithelioid cells, which is con- 

 tinued also over the pia mater and the ligamentum denticulatum, and lines 

 the tubular sheath-like prolongations of the space along the issuing nerve- 

 roots. The subarachnoid space, therefore, like the subdural space, may be 

 regarded as a serous or large lymphatic space, but it is an actual not a mere 

 potential space ; it always contains an appreciable quantity of fluid, which, 

 however, is not ordinary lymph, but is furnished in a particular way and 

 deserves special study. To understand the nature and origin of this cerebro- 

 spinal fluid, as it is called, we must turn to some special arrangements of the 

 pia mater. 



606. The pia mater proper, consisting of interwoven bundles of con- 

 nective tissue with some elastic fibres and a considerable number of 

 connective-tissue corpuscles, serves, as we have said, as the bearer of blood- 

 vessels to the nervous structures which it invests. The small arteries, as 

 they pass into the nervous substance by way of the septa, are surrounded by 

 perivascular lymphatic canals, with which spaces in the neuroglial ground- 

 work, both of the brain and spinal cord, especially spaces surrounding the 

 larger nerve-cells, are continuous. As is the case with other tissues, so with 

 the central nervous system, the several elements of the tissue are bathed 

 with lymph derived from the blood ; and this, oozing through the spaces into 

 the perivascular canals and the other lymphatic vessels of the pia mater, 

 makes its way into the subarachnoid space ; but the fluid in the subarachnoid 

 space has other sources besides. 



The roof of the fourth ventricle is, as we have said ( 514), reduced to a 

 single layer of non-nervous columnar epithelium, which appears as a mere 

 lining to the pia rnater overlying it. In the hinder part of the ventricle this 

 roof is perforated by a distinct narrow oval orifice, the foramen of Majendie. 

 By this orifice, which passes right through both the pia mater and the 

 underlying layer of epithelium, the cavity of the fourth ventricle, and so the 

 whole series of cavities derived from the original medullary canal, the lateral 

 and third ventricles, the aqueduct, and the central canal of the spinal cord, 

 are made continuous with the subarachnoid space. There are also other less 

 conspicuous communications between the subarachnoid space and the fourth 

 ventricle. Hence the cerebro-spinal fluid is made common to all these cavi- 

 ties, and is furnished not only by the pia mater investing the outside of the 

 brain and spinal cord, but also, and indeed probably to a larger extent, by 

 the epithelium lining the several cavities of the cerebro-spinal axis, especially 

 perhaps by those portions of that epithelium which coat the processes of pia 

 mater projecting into those cavities at certain places. 



We saw previously ( 515) that a large fold of the pia mater, carrying in 



