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1. Encephalon.-Under this term are included the contents of the cranium, namely, the cerebrum or brain proper, the cerebellum or little brain, and the medulla oblongata. These parts collectively have been by some called brain.

When we look at a section of the encephalon, in its natural position, we find many distinct parts, and the appearances of numerous and separate organs. So various, indeed, are the prominences and depressions observable on the dissection of the brain, that it is generally esteemed one of the most difficult subjects of anatomy; yet, owing to the attention paid to it in all ages, it is now one of the structures best understood by the anatomist. This complicated organ presents a striking illustration of the truth, that the most accurate anatomical knowledge does not necessarily teach the function. The elevated actions, which the encephalon has to execute, have, indeed, attracted a large share of the attention of the physiologist,too often, however, without any satisfactory result; yet it may, we think, be safely asserted, that we have become better instructed regarding the uses of particular parts of the brain, within the last few years, than during the whole of the century preceding.

The encephalon being of extremely delicate organization, and its functions easily deranged, it was necessary that it should be securely lodged and protected from injuries. Accordingly, it is placed in a round, bony case; and by an admirable mechanism is defended against damage from surrounding bodies. Amongst these guardian agents or tutamina cerebri must be reckoned :the hair of the head; the skin; muscles; pericranium; bones of the skull; the diploë separating the two tables of which the bones are composed, and the dura mater.

It is not an easy matter to assign probable uses for the hair on various parts of the body. On the head, its function seems more readily appropriable. It deadens the concussion, which the brain would experience from the infliction of heavy blows, and prevents the skin of the scalp from being injured by the attrition of bodies. In military service, the former of these uses has been taken advantage of; and an arrangement, somewhat similar to that which exists naturally on the head, has been adopted with regard to the helmet. The metallic substance, of which the ancient and modern helmets are formed, is readily thrown into

Fig. 2.

[graphic]

Anterior view of the Brain

and Spinal Marrow.

1, 1. Hemispheres of the cerebrum. 2. Great middle

fissure. 3. Cerebrum. 4. nerves. 6. Corpora albi

Olfactory nerves. 5. Optic

cantia. 7. Motor oculi nerves. 8. Pons Varolii. 9.

10.

Fourth pair of nerves. oblongata. 11, 11. Medulla spinalis.

Lower portion of medulla 12, 12. Spinal nerves. 13. Cauda equina.

vibration; and this vibration being communicated to the brain might, after heavy blows, derange its functions more even than a wound inflicted by a sharp instrument. To obviate this, in some measure, the helmet has been covered with horse-hair; an arrangement which existed in the helmet worn by the Roman soldier. There can be no doubt, moreover, that being bad conductors of caloric, and forming a kind of felt which intercepts the air, the hairs may tend to preserve the head of a more uniform temperature. They are likewise covered with an oily matter, which prevents them from imbibing moisture, and causes them to dry speedily. Another use ascribed to them by M. Magendie,' is more hypothetical:-that, being bad conductors of electricity, they may put the head in a state of insulation, so that the brain may be less affected by the electric fluid!

It is unnecessary to explain in what manner the different layers of which the scalp is composed; the cellular membrane beneath; the panniculus carnosus or occipito-frontalis muscle; and the pericranium covering the bone, act the parts of tutamina. The most important of these protectors is the bony case itself. In an essay written by a distinguished physiologist,2 we have some beautiful illustrations of the wisdom of God as displayed in the mechanism of man, and of his skull in particular; and although some of his remarks may be liable to the censures that have been passed upon them by Dr. Arnott,3 most of them are admirably adapted to the contemplated object. It is impossible, indeed, for the uninitiated to rise from the perusal of his interesting essay, without being ready to exclaim with the poet, "How wonderful, how complicate is man! how passing wonder HE that made him such!" Sir Charles Bell attempts to prove, that the best illustration of the form of the head is the dome; whilst Dr. Arnott considers it to be "the arch of a cask or barrel, egg-shell, or cocoa-nut, &c., in which the tenacity of the material is many times greater than necessary to resist the influence of gravity, and comes in aid, therefore, of the curve to resist forces of other kinds approaching in all directions, as in falls, blows, unequal pressures," &c. The remarks of Dr. Arnott on this subject are just; and it is owing to this form of the cranium, that any blow received upon one part of the skull is rapidly distributed to every other; and that a heavy blow, inflicted on the forehead or vertex, may cause a fracture, not in the parts struck but in the occipital or sphenoidal bones.

The skull does not consist of one bone, but of many. These are joined together by sutures,-so called from the bones seeming as if they were stitched together. Each bone consists likewise of two tables; an external, fibrous, and tough; and an internal, of a harder character and more brittle, hence called tabula vitrea. The two are separated from each other by a cellular or cancellated structure, called diploë. On examining the mode in which the tables form a junction with each other at the sutures, we find additional evidences of design exhibited.

'Précis Elémentaire, edit. cit. i. 177.

Sir Charles Bell, in Animal Mechanics-Library of Useful Knowledge, London, 1829. Elements of Physics, or Natural Philosophy, General and Medical, London, 1827—reprinted in this country, Philad., 1841.

The edges of the outer table are serrated, and so arranged as to be accurately dovetailed into each other; the tough fibrous texture of the external plate being well adapted for such a junction. On the other hand, the tabula vitrea, which, on account of its greater hardness, would be liable to fracture and chip off, is merely united with its fellow at the suture, by what is called harmony: the tables are merely placed in contact.

Howso

Fig. 3.

Front view of the Skull.

The precise object of the sutures is not apparent. In the mode in which ossification takes place in the bones of the skull, the radii from different ossific points must necessarily meet by the "law of conjugation," in the progress of ossification. This has, by many, been esteemed the cause of the sutures; but the explanation is insufficient. ever it may be, the kind of junction affords a beautiful example of adaptation. During the foetal state, the sutures do not exist. They are fully formed in youth; are distinct in the adult age; but in after periods of life become entirely obliterated, the bone then forming a solid spheroid. It does not seem that after the sutures are established, any displacement of the bones can take place; and observation has shown, that they do not possess much, if any, effect in putting a limit to fractures. In all cases of severe blows, the skull appears to resist as if it were constituted of one piece. But the separation of the skull into distinct bones, which have a membranous union, is of striking advantage to the foetus in parturition. It enables the bones to overlap each other; and, in this way, to occupy a much smaller space than if ossification had united them as in after life. It has, indeed, been imagined by some, that there is this advantage in the pressure made on the brain by the investing bones,-that the fœtus does not suffer from the violent efforts made to extrude the child; but, during the passage through the pelvis, is in a state of fortunate insensibility; and pressure suddenly exerted upon the brain is certainly attended with these effects,-a fact, which has to be borne in mind in the management of apoplexy, fracture of the skull, &c.

[graphic]

1. Frontal portion of the frontal bone.

ridge. 4. Optic foramen. 5. Sphenoidal

2. Nasal tuberosity. 3. Supra-orbital fissure. 6. Spheno-maxillary fissure. 7. the nasal duct. 8. Opening of the anterior Lachrymal fossa, and commencement of nares, and the vomer. 9. Infra-orbital foramen. 10. Malar bone. 11. Symphysis of the lower jaw. 12. Mental foramen. 13. Ramus of the lower jaw. 14. Parietal bone. 15. Coronal suture. 16. Temporal ala of the sphenoid bone. 19. Commencebone. 17. Squamous, suture. 18. Great of the temporal bone. 21. Mastoid proment of the temporal ridge. 20. Zygoma

cess.

The uses of the diploë, which separates the two tables of the skull, are not equivocal. Composed of a cancellated structure, it is well adapted to deaden the force of blows; and as it forms, at the same time, a bond of union and of separation, a fracture might be inflicted upon the outer table of the skull, and yet be prevented from extending to the tabula vitrea. Such cases have occurred, but they are rare.

It

will generally happen, that a blow, intended to cause serious bodily injury, will be sufficient to break through both tables, or neither.

Lastly, the dura mater, which has been reckoned as one of the tutamina cerebri, lines the skull, and constitutes a kind of internal periosteum to it. It may also be inservient to useful purposes, by deadening the vibrations, into which the head may be thrown by sudden concussions; as the vibrations of a bell are arrested by lining it with a soft material. It is chiefly, however, to protect the brain against itself, that we have the arrangement which prevails. The cerebrum, as well as the cerebellum, consists of two hemispheres; and its posterior part is situate immediately above the cerebellum. It is obvious, then, that without some protection, the hemisphere of one side would press upon its fellow, when the head is inclined to the opposite side; and that the posterior lobes of the brain would weigh upon the cerebellum in the erect attitude.

The hemispheres are separated from each other by the falx cerebri,

Fig. 4.

The

in the upper margin of which is the superior longitudinal sinus. falx passes between the hemispheres. The tentorium cerebello superextensum-a prolongation of the dura mater-passes horizontally forwards so as to support the posterior lobes of the brain, and prevent them from pressing injuriously on the cerebellum. A process of the dura mater passes also between the hemispheres of the cerebellum. Independently of the protection afforded to the encephaÎon, the dura mater lodges the great sinuses into which the veins discharge their blood. These different sinuses empty themselves into the torcular Herophili or confluence of the sinuses; and ultimately proceed to conFalx Cerebri and Sinuses of upper and back stitute the lateral sinuses, which pass

[graphic]

part of Skull.

1, 2, 3. Section of the bones of the cranium,

through the temporal bone, and form

showing the attachment of the falx major. 4. the internal jugular veins.

Anterior portion of superior longitudinal sinus.

5. Middle portion. 6. Inferior portion; the outer

The tutamina are not confined to

table of the cranium removed. 7. Commence- the contents of the cranium. The ment of the inferior longitudinal sinus. 8. Its

quartus or rectus. 10. Vena Galeni. 11. One

13. Sinus of the falx cerebelli. 14. Internal

marrow.

Falx cerebri.

termination in the straight sinus. 9. Sinus spine appears to be, if possible, still of the lateral sinuses. 12. Torcular Herophili, better protected. In the skull, we jugular vein. 15. Dura mater of the spinal see a firm, bony case; in the spine, 16. Tentorium cerebelli. 17, 17. a structure admitting considerable motion of the parts, without risk of pressure to the marrow. Accordingly, the spine consists of numerous distinct bones or vertebræ, with fibro-cartilaginous-technically called intervertebral-substances placed between each, so that, although the extent of motion between any two of these bones may be small, when all are concerned, it is considerable. The great use of this interver

tebral substance is to prevent the jar, that would necessarily be communicated to the delicate parts within the cavities of the spine and cranium, were the spine composed entirely of one bone. In falls from a height upon the feet or breech, these elastic cushions are forcibly compressed; but they immediately return to their former condition, and deaden the force of the shock. In this they are aided by the curvatures of the spine, which give it the shape of the Italic S, and enable it to resist in the same manner as a steel spring-any force acting upon it in a longitudinal direction. So well is the medulla spinalis protected by the strong bony processes jutting out in various directions from the spine, that it is extremely rare to meet with lesions of the marrow; and it is comparatively of late years that any ex professo treatises have appeared on the subject.

Besides the protection afforded by the bony structure to the delicate medulla, M. Magendie' has pointed out another, which he was the first to detect. The canal, formed by the dura mater around the spinal cord, is much larger than is necessary to contain that organ; but, during life, the whole of the intermediate space is filled with a serous fluid, which strongly distends the membrane, so that it will frequently spirt out to a distance of several inches, when a puncture is made in the membrane. To this fluid he has given the epithet cephalo-spinal; and he conceives, that it may act as one of the tutamina of the marrowwhich is, as it were, suspended in the fluidand exert upon it the pressure necessary for the healthy performance of its functions.

Beneath the dura mater is a very delicate membrane, the arachnoid, belonging to the class of serous membranes. It surrounds the encephalon in every part; but is best seen at the base of the brain.

Fig. 5.

Its chief use is to secrete a thin fluid, to Lateral View of the Spinal Column.

Précis, &c., édit. cit. i. 181. For an elaborate description of the fluid, see Magendie, Recherches Physiologiques, &c., sur le Liquide céphalo-rachidien, Paris, 1842; and Dr. Todd, Cyclop. of Anat. and Physiol., part xxv. p. 639, Lond., 1844.

1. Atlas. 2. Dentata. 3. Seventh cervical vertebra. 4. Twelfth dorsal vertebra. 5. Fifth lumbar vertebra. 6. First piece of sacrum. 7. Last spinous process. 10, 10. Intervertepiece of sacrum. 8. Coccyx. 9. A bral foramina.

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[graphic]
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