A text-book of pathology in relation to mental diseases.

  • Robertson, W. Ford (William Ford), 1867-
Date:
1900
Catalogue details

Licence: In copyright

Credit: A text-book of pathology in relation to mental diseases. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    161/466 (page 119)
    Some of the above structural features are diagramatically repre- sented in plate xi. There is a fact regarding the structure of the arterioles of the pia-arachnoid which, I think, helps us to understand the true consti- tution of the membrane. It is that, in comparison with the arterioles in other situations, their adventitia is extremely thin and incon- spicuous. Indeed, in many instances it is practically invisible, the vessel appearing as if bounded by its muscular coat. Beyond this there are seen the lymph-spaces and trabeculae which form the essential structure of the membrane. This trabecular tissue practi- cally forms for these vessels a common adventitia. From a con- sideration of these features of structure, Dr Middlemass and I have advocated the view that the whole extra-vascular structure of the pia-arachnoid may be looked upon as the conjoined and hyper- trophied adventitial coats of the pial vessels, the lymphatic spaces of which have undergone a special development so as to form a spongy lymph-sac. In further support of this view, there is the fact that the trabecular tissue is directly continuous with the relatively very thick adventitial coats of the cortical arterioles. The main object of this special development of the lymphatic spaces of the adventitia of the pial arterioles is probably to give to the brain the protective advantages afforded by its envelopment in a thin water-cushion. The arachnoid cisterns and the so-called sub-arachnoid spaces of the cord are merely the result of special local enlargement of numerous spaces, and do not represent any essential structural difference. The arachnoid in these situations is supposed to be distinguished from the subjacent pia in being non-vascular. Even if this supposition were accurate, which, strictly speaking, it is not, the argument based upon it is quite fallacious, for as far as nutrition by blood-vessels is concerned, the whole pia-arachnoid is practically non-vascular. The vessels it contains are almost entirely arterioles and venules, going to and coming from the nervous tissues. Capillaries, as has been said, are virtually absent. The membrane is nourished not by capillaries of its own, but by the lymph which transudes from those within the brain, and which is conveyed to it by way of the adventitial lymph-channels of the larger intra-cerebral vessels. These facts regarding the microscopic anatomy of the pia- arachnoid lead to the conclusion that it can only be correctly looked upon as consisting of one membrane. If this view is accepted, it is obvious that there is much in the present termin- ology that should be altered. The best name to apply to the whole membrane, since it is the only one that can be employed without risk of confusion, is probably that used above, viz., “ pia-