CHAP, ii.] THE BRAIN. 1127 



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 sub- 

 arachnoid space ; but the fluid in the subarachnoid space has other 

 sources besides. 



The roof of the fourth ventricle is, as we have said ( 601) 

 reduced to a single layer of non-nervous columnar epithelium, 

 which appears as a mere lining to the pia mater 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 cavities, 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 ( 602) that a large fold of the pia mater, 

 carrying in with it the thin non-nervous epithelium which alone 

 represents at the place the original wall of the medullary canal, 

 is thrust inward at the transverse fissure of the brain, beneath the 

 fornix, to form the velum interpositum, thus supplying a roof to 

 the third ventricle, and that it thence projects into each lateral 

 ventricle as the choroid plexus of each side, reaching from the 

 foramen of Monro in front along the edge of the fornix to the tip 

 of the descending horn. The velum being a fold of the pia mater 

 consists theoretically of two layers, and between the upper dorsal 

 layer and the lower ventral layer, lies a thin bed of connective 

 tissue carrying arteries forwards from the hind edge of the corpus 

 callosum, and similarly carrying veins backwards; these vessels 

 supply the choroid plexus with an abundant supply of blood. In 

 the choroid plexus, the folded pia mater is developed into a 

 number of villus-like processes, the primary processes bearing 

 secondary ones. Each process consists, like a villus, of a basis 

 of connective tissue, in which the blood vessels end in close set 

 capillary loops, covered with an epithelium. The epithelium, 

 though continuous with the rest of the epithelium lining the 

 lateral ventricle, and thus as we have said shutting off the lateral 

 from the third ventricle (except at the foramen of Monro), and 



