It has long been recognized that an adequate the capillaries, to be visualized in the same organ. blood supply is an essential prerequisite to the In a previous paper (Millen and Woollam, 1953) a normal functioning of the neuron. The major method was described which allows such a differcontributions to the study of the vascular architec-entiation to be made. Briefly the technique was as ture of the central nervous system have naturally follows. Blue and red dispersion media were used been based on the examination of the brain. for the injection. (These media were supplied by Investigations of the arterial supply of the spinal the B.B. Chemical Co., Ltd., Ulverscroft Road, cord have been few, and have been limited to the Leicester, England, who state that the dispersions examination of human material obtained at necropsy ' were prepared by ball-milling pigments with a small (Adamkiewicz, 1882;Kadyi, 1889;Tureen, 1938-quantity of a highly active surface wetting agent.Suh and . It appeared therefore The blue pigment used was copper phthalocyanine, that a study of the arterial patterns of the spinal and the red was a red lake pigment.) When the cord based primarily on the use of an experimental red pigment was introduced into the heart or animal might serve to elucidate the principles on ascending aorta it filled the arteries and arterioles, which the blood vessels are distributed to the spinal passed on into the capillaries, and filled both the cord, at the same time providing additional informa-capillaries and veins. The introduction of the tion concerning the manner of vascularization of blue pigment immediately afterwards at the same the central nervous system in general.site pushed the red pigment on into the capillaries A major obstacle to the study of the vascular and veins, but the blue pigment did not itself enter arrangements of the central nervous system has the capillaries. By this method a picture of the been the problem of differentiating the arteries whole vascular system of the animal could be from the veins. Indeed, as recently as 1938, Campbell obtained, in which the arteries were outlined by was obliged to point out that the extensive mono-blue pigment and the veins and capillaries by red. graph of Pfeifer (1930) on the angio-architecture of This method was used for the study of the arterial the cerebral cortex suffered from the serious defect patterns in the rat's spinal cord which is the subject that the structures described as veins were in reality of the present communication. arteries and vice versa. The first experimental Materials and Methods method which allowed of the certain differentiation of the cerebral arteries and veins was that introduced The spinal cords of 16 adult rats were examined. In by Scharrer (1940). In this method the arterial each case the rat was killed with ether, and 2 ml. of syste was i .w.i...thageainssolut the red dispersion was injected into the ascending aorta ssm a injected with a geltin solutonincor through the heart at a pressure of about 80 mm. Hg.porating r...

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Specimens of material occluding ventricular catheters removed at shunt revision operations were studied by scanning electron microscopy. Immediate fixation allowed examination of human choroid plexus and ependyma which resembled living tissue.

Summary The advances in the knowledge of the histology of the central nervous system which have been made possible by improved staining techniques have complicated and confused the anatomical conception of the perivascular and perineuronal spaces. As a first step towards the resolution of this confusion, a clear picture must be formed of the elements which intervene between the tunica media of the cerebral and spinal blood vessels and the neurons. The blood vessels in the central nervous system possess little or no adventitial coat, and this element is replaced by a reticular perivascular sheath continuous with the pia‐arachnoid envelope of the brain and spinal cord. The question as to whether this reticular sheath extends to cover the capillaries remains to some extent unanswered, though the probability is that it does not do so. No clearly defined tissue layer serves to separate the outer wall of the reticular perivascular sheath from the neuron and its processes. In fixed preparations a felted network, formed by the perivascular feet of the neuroglia, which resembles a membrane, is seen, but we do not accept the view that this structure is part of the normal anatomy of the central nervous system. There remains an element on which little stress has previously been laid: this is the ground substance of the central nervous system, a tissue which is probably a muco‐polysaccharide in constitution, and which by virtue of its physical and chemical properties may well be of considerable importance in relation to the physiology of the central nervous system. The views of the earlier authorities on the anatomy of the perivascular and perineuronal spaces are difficult to reconcile with modern knowledge of the histology of the tissues concerned, and are of interest chiefly because of the influence which they have had on descriptions of these structures which are still accepted to‐day. After the first description of the perivascular space by Pestalozzi, important landmarks in the subsequent history of the spaces were: the adoption of the term ‘Virchow‐Robin space’ for the perivascular space; the description by His of a second space external to this; the discovery by Obersteiner that the perineuronal spaces were continuous with the space of His, and the confusion by later workers of the space of His with that of Virchow‐Robin, so that the perineuronal spaces were described as communicating through a perivascular canalicular system with the subarachnoid space. The most influential figure in moulding the modern conception of the perivascular and perineuronal spaces has been Weed. It appears that he accepted the notion, prevailing at the time at which his researches were carried out, of a complete canalicular system of spaces opening into the subarachnoid space; and it was unfortunate that the limitations of his Prussian blue technique were such as to lead him to support this misconception. Schaltenbrand & Bailey and Patek did much to resolve the confusion produced by the existence of the two systems of spaces, the true periva...

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