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 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.
The vagus or pneumogastric nerves are the longest of the cranial nerves, and they are remarkable for their almost vertical course, their asymmetry, and their extensive distribution, for, in addition to supplying the lung and stomach, as the name ' pneumo-gastric ' indicates, each nerve gives branches to the external ear, the pharynx, the larynx, the trachea, the oesophagus, the heart, and the abdominal viscera. They are commonly referred to as the tenth pair of cranial nerves.
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