« PreviousContinue »
tissue in the skin does not disprove the statement, as the spinal nerves are compound, and filaments from the anterior roots may supply the tissue. The sympathetic may also supply the contractile tissue of the skin, hut as the phenomenon of cutis anserina is perceived over the extremities where that nervous system is scantily, if at all, distributed, it is more probable that its source is the spinal nerves. A few filaments may be traced running from the posterior root to the anterior, and the sensibility they confer threw some doubt on the statement, that the latter was exclusively motor; but these fibres may be regarded as merely " nervi nervorum." They plainly come from the posterior root, as all sensibility is lost by its division, and irritation of the central portion of the divided anterior root will not produce pain, though irritation of the detached half will, as it is carried back by the posterior root. The two roots unite after the posterior has formed the ganglion, and then two branches, of which the anterior is the larger, separate. There are 31 pairs of spinal nerves—or, adding the spinal accessory, as Willis did, 32, arranged as 8 cervical, 12 dorsal, 5 lumbar, and 6 sacral, the anterior branches of which mingle together, and form the cervical, brachial, lumbar, and sacral plexuses. The only posterior branches which form a plexus are those of the first four cervical. The general arrangement of the cerebro-spinal axis and the spinal nerves will be seen in the preceding figure.
The Medulla Oblongata is an oval mass continued from the cord into the cranium; like it, it is divided into halves by a posterior fissure—at the bottom of which, however, no grey commissure exists—and an anterior, across which are found many fibres decussating to the opposite side. Each half is marked by grooves into 4 bodies, termed, from before backwards, the pyramid, olivary, restiform, and posterior pyramid. Solly and Rolando have described arciform fibres, which curve under the olivary body—as it is but half as long as the other bodies—and uniting the pyramid and restiform, pass back to the cerebellum; and they may explain the crossed character of lesions of that organ. The pyramid consists of fibres, both from the antero-lateral column of its own side and of the opposite, which, we have stated, decussate across the anterior fissure. These fibres can be traced upwards through the deeper fibres of pons, lower layer of crus cerebri, and thus into those ganglionic masses, the optic thalamus, corpus striatum, and finally into the hemisphere, as is represented in this dia
The Fibres of the Ansero- Lateral Tract traced through the pyramid and olivary body, deep part of pons, crura cerebri, optic thalamus, and corpus striatum into the hemispheres, and those of liie posterior tract through the restiform body into the lateral lobe of the cerebellum, after Gray.
gram of the physiological method of dissecting the cerebro-spinal axis from below upwards. Their course very clearly accounts for cross-paralysis in cerebral lesions, the limbs of the opposite half of the body being paralyzed. The olivary body exists only in man and quadrumana, but in some other animals the great size of the pyramid may conceal it. It contains a grey ganglionic mass* the corpus dentatum, which seems connected with
the functions of the tongue, as both glossopharyngeal and lingual nerves have some filaments arising from it. The fibres of the olivary body extend from the anterolateral column, through the pons and crus, into the optic thalamus and quadrigeminal bodies.
The restiform body, so named from its twisted appearance is mainly continued from the posterior column. As it ascends it diverges from its fellow, and joining the lateral lobe of the cerebellum, has been termed " processus e cerebello ad medullam oblongatam." It is said to have a small grey mass at the origin of the pneumogastric. The posterior pyramid seems merely a portion of the restiform marked off by a very superficial groove; but unlike it, Solly says, it passes to the opposite side of the pons. A grey mass on the back of the medulla, just below the fourth ventricle, is regarded as the auditory ganglion. Some fibres were described by Cruveilhier as passing antero-posteriorly, like a septum, in the middle of the medulla. There are many transverse fibres also mainly connecting the olivary bodies, which, from this fact and their abundant vesicular matter, must be regarded as the seat of all the independent functions the medulla possesses. The 8th, 9th, and facial portion of 7th, are most intimately connected with the medulla, which, through them, controls the great acts of respiration and deglutition. In animals which breathe without a diaphragm, the spinal cord, and in frogs the cerebrum also, may be sliced away without interfering with respiration; but destruction of the medulla at once kills by npnoea, unless they are etherized; and irritation of this centre produces spasm of the respiratory and pharyngeal muscles. It is therefore the seat of such reflex acts, as vomiting, coughing, &c., which we have before considered. In foetuses where the cerebral lobes are suppressed, in the kangaroo before they are developed, and in puppies from which they have been removed, the power of sucking exists, dependent, as Grainger remarks, on the medulla. Van der Kolk regarded the olivary bodies as ganglia of speech through the lingual nerves. The Cerebellum, or lesser brain, is about one-sixth the size of the cerebrum, and consists of 3 lobes, of which the middle is found in all vertebrates save the amphioxus; and the lateral lobes are added in birds and mammals—never, however, except in man, attaining an equal size with the most essential part. Like most other ganglionic masses, its surface is arranged in convolutions, which are narrower, straighter, and deeper than those of the cerebrum. The sulci are either primary, dividing the lobes into about 5 lobules, or secondary, which cut these again into lamellae. The middle lobe is curved forwards, and ends above and below in a vermiform process. The central stem of each lateral lobe is white and branched, like a tree, the "arbor vitae," in the inner third of which there is a grey mass with an irregular edge, the " corpus dentatum," which is very similar to that found in the olivary body. The cerebellum is connected to the mesocephale by the " processus e cerebello ad testes," whose fibres extend as far as the optic thalami, and the "processus e cerebello ad pontem," or crus, whose fibres form the superficial part of the pons, which is therefore a transverse commissure of the cerebellum: the "processus e cerebello ad medullam oblongatam," or ,restiform body, joins it to the medulla. The grey matter or cortical substance has large vesicles with the caudae projecting towards the surface, which fact is evidence against the continuity of these processes and nervetubes. Between the cerebellum, mesocephale, and medulla lies the 4th ventricle, on the floor of which is seen a longitudinal groove, the " calamus scriptorius," continued up from the posterior fissure of the cord. The auditory nerve takes root at its posterior or plume-like extremity; and Dr. Alcock, late Professor of Physiology, Queen's College, Cork, traced the trifacial nerve to the anterior extremity. It was here that Flourens thought
he discovered a " nceud vital," or vital point, as injury of it often causes instantaneous death; but it has been removed in frogs without fatal effects following for some days.
The experiments of Magendie and Flourens have clearly shown that the function of the cerebellum has more connexion with motion than any other nervous endowment. In pigeons, Flourens sliced it away, and no pain was produced; but the animal performed rapid, irregular, and unharmonious movements, which could not be co-ordinated into their usual acts of standing, walking, or flying. Common and special sensation, and probably volition, were retained. The condition has been aptly compared to that of a drunken man. Section on one side only gave rise to rapid gyrations of the body from that side, even to the number of 60 per minute, and lasting for some days; but Broum-Sequard has shown that a much less important injury, as division of the auditory nerve, will produce this effect. Serres reports that rotations occurred in a man in whose crus cerebelli an apoplectic clot was found; and they have been observed in a case in which, after death, an exostosis was discovered pressing on the same point. Unsteady gait has been found a pretty constant symptom of disease of of the cerebellum. Flourens then regards the cerebellum as the co-ordinator of movements.
Foville believes it is rather the organ of muscular sense, which its close connexion with the posterior column of the cord and the quadrigeminal bodies renders very probable; but it is not inconsistent to suppose that it fulfils both these offices.
Oall started the notion that the cerebellum was the seat of the amative and propagative instincts. It was merely one of those unfounded guesses which have been often substituted for rational opinions founded on laborious research. It is contradicted by the numerous connexions of the cerebellum, whereas it would be alone