plies the cranial bones with blood in early life, but is less closely attached about the spinal cord, gives off processes which divide and support the parts of the brain, sheathes every nerve as it emerges, and lastly forms that freely communicating system of veins we call the sinuses. 2. The arachnoid is a continuous serous sheet lining the dura mater-save where the pituitary body intervenes the outside of the brain and spinal cord, and in fœtal life passes through the canal of Bichat to line the ventricles. This communication is subsequently obliterated in the same way as the tunica vaginalis is shut off from the peritoneum. Sharpey believes the

arachnoid is inflected through the fissures, and lines the subarachnoid spaces. The membrane pours out a halitus which facilitates the motions of the brain. Rainey states that it is most abundantly supplied by sympathetic filaments-but if so, it must widely differ from other serous sacs. 3. The pia mater, an areolar-vascular web which dips into all the sulci and conducts bloodvessels into the substances of the brain. On the spinal cord it is far less vascular, and acts more as a tight fibrous sheath which ends below in a thread, the filum terminale of Macartney tying it down to sacrum. gether with the arachnoid, the spinal pia mater forms two bands between the anterior and posterior roots of the nerves, with a straight edge attached to the side of the cord, and a serrated edge, consisting of about 22 teeth, attached to the intervertebral foramina. They are termed ligamenta dentata, and are represented at page 272. They steady the cord.

To

The Subarachnoid Fluid, first noticed by Cotunnius, is far more copious than that in the arachnoid sac, as it usually exceeds two ounces. It is alkaline and slightly albuminous, containing no materials out of which the nervous matter could be laid down, as once supposed. It is abundant according as the amount of blood or size of nervous centre is diminished, thus keeping up a com

pensatory pressure. It affords an equal fluid-] -pressure to the brain and delicate nerve-filaments, especially at the base of the brain. If removed by puncture of the arachnoid, it is rapidly re-secreted by the pia mater; and

4.

1. Posterior cerebral lobes. 2. Cerebellum. 3. Pneumogastric lobules. Anterior cerebellar lobe. 5. Trifacial nerves. 6. Abducens nerves. 7. Facial nerves. 8. Auditory nerves. 9. Oculo-motor. 10. Crura cerebri.

11. Optic commissures and nerves. 12. Pisiform bodies. 13. Olfactory roots. 14. Anterior lobes. 15. Middle lobes.

thus pints of it are occasionally poured through the ear in fractures of the base of the skull-a symptom, as Colles remarked, almost surely fatal.

The Circulation of the cerebro-spinal axis is peculiar : 1. For the quantity of blood (which even Hippocrates remarked, and it suggested to him that the brain was a secreting gland). Haller estimated it at one-fifth of the entire. The generation of nerve-force requires a free circulation of blood, and thus it is that the benumbing

influence of cold dulls cerebro-spinal action. 2. The blood is made to ascend against gravity through very tortuous arteries, which then break up into the circle of Willis at the base of the brain. Some special arrangements in lower animals are described at page 142. 3. The vessels cannot dilate, as they pass through bony canals or fibrous sheaths. 4. The arteries do not penetrate the nervous substance, but ramify freely on its surface. The cerebral arteries are figured on the previous page. 5. The capillaries and smaller arteries have much more muscular coats than those of other situations, and thus the amount of blood can be easily arranged.

Kellie first started the plausible notion that as atmospheric pressure did not bear upon the cerebral circulation, it could never vary. He bled sheep to death, and hung others with the head downwards, and stated that the amount of blood in the cranium was equal; whereas other parts were blanched in the one instance, or gorged in the other. When, however, he trephined the skull, the pressure of the air made the condition of the brain like other parts, and it varied according to the mode of death. Dr. Burrows disproved both these results; and the free communication between the veins of the scalp-those in the diploe, which Breschet described, and the sinuses should render the cranial vessels subject to the same changes as those of other parts.

The subarachnoid fluid is also of the highest value in preserving an equable pressure on the brain, as it is vicarious in amount to the blood with the skull. There is good reason, as explained at page 243, to regard the thyroid as an organ for regulating the supply of blood to the brain. Coma, convulsions, or delirium, result from want of pressure on the cerebral mass, as well as from undue pressure upon it. This is shown in Sir A. Cooper's experiments of ligaturing the carotids; by the effects of hæmorrhage; when pericarditis suddenly obstructs the

heart's action; and in the hydrocephaloid disease of children, which Gooch and Marshall Hall so graphically

1. The optic nerves. 2. Internal carotid. 3. Circular sinus of Ridley. 4. Cavernous sinus. 5. Carotid in the cavernous sinus. 6. Infundibulum. 7. Transverse basilar. 8. Internal auditory meatus. 9. Superior petrosal sinus. 10. Inferior petrosal sinus. 11. Occipital sinus. 12. Torcular Herophili 13. Lateral sinus. 14. Jugular foramen. 15. Foramen magnum. 16. Superior longitudinal sinus.

described and distinguished from hydrocephalus, more especially as regards treatment. As the cerebral circu

lation does not materially differ from that of other places, the effect of bleeding will be to diminish its quantity of blood, and, moreover, by weakening the heart's force it will decrease the pressure upon the brain, and powerfully

3. Falx cerebri. 4. Infe6. Venae magna Galeni. Jugular vein.

Sinuses of Skull. 1. Superior longitudinal. 2. Veins of dura mater. rior longitudinal sinus. 5. Straight sinus. 7. Torcular Herophili. 8. Lateral sinus. 9. interfere with its action. The veins which return the blood of the cerebro-spinal axis are termed sinuses, and are represented in the accompanying transverse and vertical sections. They are destitute of valves, and have no elastic coat, as it might influence an injurious backward