The lymphatics of the brain and spinal cord are peculiar, inasmuch as they open into the subarachnoid space, and are only indirectly connected with the general lymphatic and venous systems. The communications with the venous system are effected by the Pacchio- nian bodies. The lymphatics of the peripheral nerves are in the form of tubular spaces placed between the lamellae of the perinem-al sheaths. These tubular channels open into the subdural and subarachnoid spaces.
The subdural space is a very narrow interval between the dura mater and arachnoid. It normally contains only sufficient fluid to moisten its sm-faces. It is in commimication with the lymphatics of the neck and also of the back and loins. It also commimicates with the perineural spaces around the nerves, and with the lymph spaces which smTound the olfactory, optic, and auditory nerves. It sends prolongations around the Pacchionian bodies. It does not communicate with the subarachnoid space.
The subarachnoid space contains the greater part of the cerebro-spinal fluid, the fluid occupying the meshes of the subarachnoid tissue. The lymphatics of the brain and cord and also the perineural spaces of the nerves open into this space. It is also in commimication with the ventricular system of the brain by means of the foramen of Majendie and the foramina of Key and Eetzius. Slit-like commmiications between the subarachnoid space and the descending horn of the lateral ventricle have also been described (Merkel and Mierzejewsky). It also commmiicates with the perilymph spaces of the internal ear and with the lymphatics of the mucous membrane of the nose. It sends prolongations aroimd the optic and auditory nerves. The lymphatic vessels of the brain and cord surroimd the arteries, and are hence caUed perivascular lymphatics. As each artery dips into the nervous substance it carries with it a tubular process of the pia mater. This tubular process is lined by endothelial cells and a similar layer covers the coat of the artery. In this manner the lymph-space is bounded. The perivascular lymphatic follows the artery as far as its capillary ramifications.
There is a lymph-space between the two layers of the spinal pia mater. Lymph-spaces between the outer and middle coats of the cerebral arteries (Virchow-Eobin space) and others around the individual nerve-cells of the brain have been described, but these are xeiy general!}' believed to be artificial, being due to shrinking from the action of reagents. According to Obersteiner, however, the existence of the pericellular spaces ' is proved by the presence within them of lymphatic cells.' The so-called epicerebral and epispinal spaces, situated between the deep surface of the pia mater and the nervous substance, are also artificial.
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