The cerebrospinal fluid occupies the subdural and subarachnoid spaces of the brain and cord and also the ventricular cavities of the brain.

The first step in the examination of the meninges is the removal of the brain.

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 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 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 hypoglossal nerves are exclusively motor; they supply the genio-hyoidei and the extrinsic and intrinsic muscles of the tongue except the glosso-palatini. They are usually designated as the twelfth pair of cranial nerves.

The pia mater is a delicate vascular membrane which closely invests the nervous substance.

The Sympathetic Ganglia of the Head and Their Associations with the Cranial Nerves

The sympathetic system of the head, like that of the remainder of the body described below, is arranged in the form of a continuous gangHated plexus subdivided into sub-plexuses.

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.

The spinal accessory nerve [n. accessorius] is exclusively motor. It consists of two parts, the accessory or superior, and the spinal or inferior part.

The dura mater is a tough fibrous membrane of a bluish-white colour presenting externally a rough appearance, but internally smooth and shining.

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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