is supposed to be the point at which excitement to motion commences and sensation terminates, and to possess the power of originating motion in itself. Mr. Mayo infers this, from the fact that the brain proper, the optic tubercles, and even the cerebellum may be removed by successive portions, and still the animal live, and exhibit feeling and instinct; but if the medulla oblongata be removed, all consciousness is immediately destroyed. Other physiologists say, that it is the hemispheres of the cerebrum that take cognizance of sensation, and the source from whence volition proceeds; and that though they may be removed, layer after layer, without pain, yet by so doing, the senses are destroyed, memory and intelligence abolished, and the animal reduced to an automaton, destitute of design, apprehension, and judgment. As all the nerves which supply the larynx, lungs, heart, stomach, and external muscles of respiration originate from it, its removal must instantly destroy life.

4. The spinal cord passess down through the foramen magnum, or great hole of the skull, and the canal of the vertebra, till it reaches the second vertebra of the loins, where it generally terminates in a tapering point Besides the bony case in which it is lodged, the cord is enclosed by a prolongation of the dura mater, which, however, does not adhere immediately to the bone, as it does in the skull. It also is supplied with extensions of the arachnoid and pia mater; between which, there is usually a quantity of serous fluid, varying from two to six ounces, which seems to impart a certain degree of support by its pressure and obviates the effects of concussion.

5. The general form of the spinal cord is cylindrical, though somewhat flattened, from its transverse diameter being the greatest. Its thickness, however, is not uniform, as it bulges out at the lower part of the neck, where the nerves go off to the arms, and also at the lower part of the back, from whence spring the nerves of the legs. It is composed of grey and white, or cineritious and medullary mat

ter, like the brain; the situation of the two is, however, reversed; the medullary matter being disposed in a thin layer upon the surface, while the grey portions occupies the centre. We, however, find the same arrangement in the medulla oblongata, and the central parts of the cerebrum and cerebellum. The chord, as I have already mention. ed, is divided into two lateral halves by a groove in front and a similar groove behind. Besides this, there are on each side two lateral grooves faintly marked, from which the anterior and posterior nerves have their origin, so that in fact, the cord consists of four columns; from the two anterior of which, the nerves of motion spring, and from the two posterior those of sensation.

6. There are thirty pairs of nerves that spring from the spinal marrow, viz., eight from the region of the neck, twelve from that of the back, and five from the pelvis. By examin. ing the above cut, it will be seen that they arise by two roots, one from the anterior, and one from the posterior column. The fibres of the posterior swell out into a ganglion before they unite with the anterior.

7. A few years ago, Sir Charles Bell discovered, that by opening the spinal canal in a living animal, and dividing the posterior roots of the nerves, the parts to which they are distributed are deprived of feeling. The limb may be pricked or lacerated in any way, without the animal manifesting the least feeling, or indication of suffering, while at the same time the power of motion remains. He also found that when the posterior roots were divided, sensation was destroyed, but motion remained. One of Majendie's experiments, is interesting. Availing himself of the fact, that the introduction of nux vomica into the system, produces violent spasms, tremors, and rigidity of the muscles: he administered it to an animal, after having severed the anterior roots of the spinal The consequence was, that while all the muscles, whose nerves remained entire, were thrown into a state of violent spasm; those supplied with the nerves, whose roots had been divided, remained unaffected.

nerves.

8. The spinal cord then serves to connect the brain, which is the common centre of sensation and voluntary motion, with all the sensible parts of the body, and with the muscles, the instruments of motion. We may view it in this sense, as forming a channel of communication for a bundle of nerves running from the brain to every part of the system to carry messages from the will, and to transmit intelligence back again, as if all the roads and high-ways in the country should terminate in one grand post-road, connecting them with the seat of government.

9. To illustrate, suppose I seize a hot iron in my hand, not knowing it to be heated; the painful impression is immediately transmitted along the sensiferous nerves to the spinal cord, and through that to the brain. The brain takes cognizance of it, and a mental determination is immediately formed to drop the iron. The will sends back its command through the anterior column of the spinal cord to the motific nerves, distributed to the muscles, which go to the hand. The muscles instantly obey; the iron drops, and the whole

process occupies but an instant. Thus we have two channels of nervous influence; the one transmitting intelligence to the mind, the other conveying the purposes of the will to the muscles necessary to carry them into effect.

10. That such is the chief function of the spinal marrow, is proved by the well-known fact, that if it is divided in any part of its course, that portion of the body, as well as the limbs, which are situated below the seat of the injury, will be paralyzed; that is, all sense and motion will be lost, while the parts above the injury will be unaffected. If the injury occur very high up in the neck instant death will ensue, because the nerves, which go the diaphragm and are necessary to carry on breathing, are separated from their connection with the brain. These leave the spine as high as the third vertebra of the neck.

11. I have stated that it has been proved, that the principle of motion resides to some extent in the spinal marrow itself. This is shown by the following experiment. If after the spinal cord be divided in the neck, and then lower down in the back, we irritate any of the muscles which are supplied with nerves from this isolated segment, motion will be produced. If the foot, or the hand be pricked, it will immediately contract; and this contraction is produced by the irritation being transmitted to that portion of the spinal marrow, from whence the nerves of the part arise, and whose connection with the brain has been severed, and then an impulse is sent back again from this portion to the muscles of the hand or foot which causes them to contract. Still, all voluntary motion, as well as sensation, depend on the brain.

12. If the medulla oblongata is injured, breathing immediately ceases. If the spinal marrow is severed opposite the second bone of the neck, death also speedily follows; because the nerves of respiration are cut off from their connection with the part above. If the spinal cord be divided, as low as the fifth cervical bone, then life will not be immediately destroyed; but the breathing will be difficult; because the dia.

phragm is paralyzed, and death sooner or later follows from suffocation. If it be divided about the first dorsal vertebra, then life may be maintained for a considerable time, although the ribs cannot be elevated, as the intercostal muscles are rendered paralytic. Breathing is, however, kept up by the action of the diaphragm. I have seen a man whose spine was dislocated in this region, live seven or eight weeks. All sensation and motion were lost in the parts below the seat of the injury, but his reason and senses were perfect.

13. As the heart, lungs, larynx, and many of the most important organs of the body are supplied with nervous influ ence by the eighth pair of nerves, or par vagum, why is it that a division of the spinal marrow causes death? This question is easily answered, by remembering that one of the (functions of the par vagum is, to convey to the brain the sense of the want of air, or of respiration, and that this stimulus reacts upon those parts of the spinal cord which give rise to the respiratory nerves of the chest, Now if this communication be cut off, the influence of the brain, or the me. dulla oblongata, cannot be transmitted so as to excite those muscles which are employed in breathing.

14. That this is the true answer to the above question is also shown by dividing the par vagum in the neck. This causes palsy of the lungs, and also of the muscles which open the larynx; in consequence of which, the top of the windpipe is immediately closed, and death follows from suffocation. Besides this, it also prevents transmitting to the medulla oblongata, the sense of the want of respiration, and thus prevents also the reaction of this part upon the spinal marrow.

15. Does the spinal marrow exert any influence upon the circulation of the blood? (It is ascertained that the action of the heart is nearly independent of the spinal marrow Its nervous influence is derived from the par vagum and the great sympathetic nerve, the former of which has but little connection with the spinal cord. The whole spinal marrow has been removed, and still the heart has continued to act.