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 dura mater
The dura mater is a tough fibrous membrane of a bluish-white colour presenting externally a rough appearance, but internally smooth and shining.
The cerebrospinal fluid occupies the subdural and subarachnoid spaces of the brain and cord and also the ventricular cavities of the brain.
The pia mater
The pia mater is a delicate vascular membrane which closely invests the nervous substance.
Structure of a Pacchionian body
A Pacchionian body consists of a central core of subarachnoid tissue which is joined to the general subarachnoid tissue by a comparatively narrow stalk.
The arachnoid is a thin delicate membrane, which presents a well-defined limiting surface towards the dura mater, but on its deep or pia-matral surface passes insensibly into the subarachnoid tissue.
Dissection of the meninges
The first step in the examination of the meninges is the removal of the brain.
The spinal cord
The spinal cord [medulla spinalis] is the lower (caudal) and most attenuated portion of the central nervous system. It is approximately cylindrical in form and terminates conically. Its average length in the adult is 45 cm. (18 in.) in the male and 42 cm. in the female. It weighs from 26 to 28 grams or about 2 per cent, of the entire cerebro-spinal axis.
After birth it grows more rapidly and for a longer period than the encephalon, increasing in weight more than sevenfold, while the brain increases less than half that amount. Its specific gravity is given as 1.038.
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