These

Fig. 13.

the medulla spinalis in the cavity of the cranium. It is likewise termed mésocéphale, from its being continuous with the spinal marrow in one direction, and sending towards the brain strong prolongations-crura cerebri; and to the cerebellum similar prolongations-crura cerebelli; so that it appears to be the bond of union between these various parts. In its lower portion, it seems to be merely a continuation of the medulla spinalis, except that it is more expanded superiorly where it joins the pons Varolii. This portion of the medulla oblongata is called, by some, tail of the medulla oblongata; by others, the rachidian bulb; and, by others again, it is regarded as the medulla oblongata. Its lower surface rests on the basilary gutter of the occipital bone, and exhibits a groove which divides the spinal cord into two portions. On each side of this furrow are two oblong eminences, the innermost of which is called corpus pyramidale, the outermost, corpus olivare, which arise from the anterior column of the medulla spinalis, or are a continuation and subdivision of this column. oval bodies are surrounded by a superficial groove, which, in some instances, is partially interrupted by some arciform fibres, which cross it at its lower part. At the lower third of the medulla oblongata, fibres of the anterior pyramids decussate, and form an anatomical demarcation between the medulla oblongata and the spinal cord. The decussation takes place by from three to five bundles of fibres from each pyramidal body. This decussation, as will be seen, hereafter, is interesting in regard to the cross effect induced by certain diseases of the brain. the posterior surface of the medulla oblongata, the posterior fasciculi separate to form the fourth ventricle: at the sides of this ventricle are the corpora restiformia, or inferior peduncles of the cerebellum, -so called because they seem to aid in the formation of that part of the encephalon; and on the inner side of each corpus restiforme is the small body-the posterior pyramid. Again, in addition to the corpora pyramidalia and olivaria-which derive their origin from, or are continuous with, the anterior fasciculi of the spinal cord, and are destined, according to some, to form the brain-and the corpora restiformia, which are continuations of the posterior fasciculi, and are destined to form the cerebellum, there exists, according to some anatomists, other fasciculi in the rachidian bulb. All these are interesting points of anatomy, but are not of so much importance physiologically; notwithstanding even the views promulgated by Sir Charles Bell. He considers that a column exists between the corpora olivaria and corpora restiformia,

On

Anterior View of the Medulla Oblongata, showing the decussation of the Pyramids, and of the upper part of the Spinal Cord. (After Mayo.) p. Anterior pyramids. 7. Restiform bodies. 0.

Olivary bodies. d. Decussating fibres. al. Anthe spinal cord. c. Anthe floor of which forms

tero-lateral column of

terior fissure of the cord,

The Nervous System of the Human Body: from Transactions of the Royal Society from 1821 to 1829, London, 1830. Reprinted in this country, Washington, 1833.

B

A

с

D

E

F

Fig. 14.

a

α

a

p

a

Transverse Sections of the Spinal Cord.

pyramids. B. At middle

which extends below through the whole spine, but above does not proceed farther than the point where the rachidian bulb joins the tuber annulare; and that this column gives origin to a particular order of nerves-the respiratory. The corpora olivaria, and the posterior corpora pyramidalia, are regarded by Mr. Solly' as ganglia;-the former of the function of respiration, the latter of the sense of hearing.

The anterior and upper half of the medulla oblongata bears the names pons Varolii, tuber annulare, and nodus cerebri; and to this are attached, superiorly, the corpora or tubercula quadrigemina. In the very centre of the pons, the crura cerebri bury themselves; and by many, they are considered to decussate; by others, to be prolongations of the anterior column of the spinal marrow. Sir C. Bell thinks, that the pons Varolii stands in the same relation to the lateral portions of the cerebellum, that the corpus callosum does to the cerebrum;-that it is the great commissure of the cerebellum, uniting its lateral parts, and associating the two organs.

The medulla oblongata consists chiefly of the centres of the nerves of respiration and deglutition, which, as elsewhere shown, are strictly reflex in their action.

2. The spinal marrow extends, in the vertebral canal from the foramen magnum of the occipital bone above to the first or second lumbar vertebra, where it terminates in the cauda equina. It is chiefly composed of medullary matter, but not entirely so. Within, the cineritious substance is ranged A. Immediately below irregularly, but has a crucial form when a section the decussation of the is made. The marginal illustrations exhibit sections of cervical bulb. c. Mid- of the spinal cord of man at different points; and and lumbar bulbs. D. the proportion of gray and white matter at each. inch lower. F. Very From the calamus scriptorius in the fourth ventricle, near the lower end. a. and the rima formed by the corpora pyramidalia beterior surface. The fore, two fissures extend downwards, which divide the anterior and poste- the spinal marrow into lateral portions. The two rior roots of the nerves lateral portions are divided into an anterior and a posterior, so that the cord has four distinct portions. By some, indeed, it is conceived to consist of three columns-an anterior, posterior, and a middle or lateral.

way between cervical

Lumbar buib. E. An

Anterior surface. p. Pos

points of emergence of

are also seen.

The vertebral canal is lined by a strong ligamentous sheath, running down its whole length. The dura mater likewise envelopes the medulla at the occipital foramen, being firmly united to the ligaments; but farther down it constitutes a separate tube. The tunica arachnoidea from

The Human Brain, its Configuration, Structure, Development, and Physiology, &c., p. 147, London, 1836. See, on this subject, Dr. John Reid, on the Anatomy of the Medulla Oblongata, in Edinb. Med. and Surg. Journ., Jan., 1841, p. 12.

the brain adheres loosely to the cord, having the cephalo-spinal fluid within it; and the pia mater closely embraces it.

Fig. 15.

3. Nerves. The nerves are cords of the same nervous substance as that which composes the encephalon and spinal marrow; extending from these parts, and distributed to the various organs of the body, many of them interlacing in their course, and forming plexuses: others having knots or ganglions, and almost all vanishing in the parts to which they are distributed. The generality of English anatomists reckon thirty-nine pairs of nerves; the French, with more propriety, forty-two. Of these, nine, according to the Englishtwelve, according to the French-draw their origin from, or are connected with, the encephalon; and are hence

called encephalic nerves; Shows the under Surface or Base of the Encephalon freed from

its Membranes.

A, anterior, B, middle, and c, posterior lobe of cerebrum.-a. The fore part of the great longitudinal fissure. b. Notch between hemispheres of the cerebellum. c. Optic commissure.

and thirty from the medulla spinalis ; and hence termed spinal. Left peduncle of cerebrum. e. Posterior perforated space. The encephalic nerves to i. Interpeduncular space. ff. Convolution of Sylvian emerge from the cra- fissure. i. Infundibulum.7. Right middle crus or peduncle nium by means of fora- albicantia. 9. Pons Varolii, continuous at each side with middle mina at its base. They crura of cerebellum. p. Anterior perforated space. g. Horizontal fissure of cerebellum. 7. Tuber cinereum. s, s'. Sylvian are proceeding from fissure. t. Left peduncle or crus of cerebrum. u, u. Optic tracts. v. Medulla oblongata. x. Marginal convolution of the before to behind-the longitudinal fissure.-1 to 9 indicate the several pairs of cerebral nerves, numbered according to the usual notation, viz., 1. Olfacfirst pair or olfactory, tory nerve. 2. Optic. 3. Motor nerve of eye. 4. Pathetic. 5. distributed to the organ. Glosso-pharyngeal, S'. Vagus, and 8". Spinal accessory Trifacial. 6. Abducent nerve of eye. 7. Auditory, and 7. Faof smell; the second pair nerve.

fissure. h. Termination of gyrus fornicatus behind the Sylvian of cerebellum. m, m. Hemispheres of cerebellum. n. Corpora

or optic, the expansion

of which forms the retina; the third pair, motores oculi or common oculo-muscular, which send filaments to most of the muscles of the eye; the fourth pair, trochleares, pathetici or internal oculo-muscular, distributed to the greater oblique muscle of the eye; the fifth pair, trifacial, trigemini, or symmetrical nerve of the head, (Bell,) which send their branches to the eye, nose, and tongue; the sixth pair, abducentes or external oculo-muscular, which are distributed to the abductor or rectus externus oculi; the facial nerve, portio dura of the seventh pair,

nervus communicans faciei or respiratory nerve of the face, distributed to the muscles of the face; the acoustic nerve, auditory nerve or portio mollis of the seventh pair, which passes to the organ of hearing; the eighth pair, pneumogastric, par vagum or middle sympathetic, which is dispersed particularly on the larynx, lungs, heart, and stomach; the glosso-pharyngeal, often considered as part of the last, and whose name indicates its distribution to the tongue and pharynx; the great hypoglossal, ninth pair or lingual nerve distributed to the tongue; and the spinal accessory of Willis, which arises from the spinal cord in the cervical region; ascends into the cranium, and issues by one of the foramina to be distributed to the muscles of the neck.. All these proceed, perhaps, from the medulla oblongata;-the brain and cerebellum not furnishing one.

The spinal nerves are thirty in number on each side. They make their exit by the intervertebral foramina, and are divided into eight cervical, twelve dorsal, five lumbar, and five or six sacral.

The encephalic nerves are irregular in their formation, and, with the exception of the fifth pair, originate from one root. Each of the spinal nerves arises from two fasciculi, the one anterior, and the other posterior: these roots are separated from each other by the ligamentum denticulare; but they unite beyond this ligament, and near the intervertebral foramen present one of those knots, known under the name of ganglions or ganglia, in the formation of which the posterior root is alone concerned.

When the nerves have made their exit from the cranium and spine,

Fig. 16.

Terminal nerves, on the sac of the second molar tooth of the lower jaw, in the sheep; showing the arrangement in loops. (After Valentin.)

they proceed to the organs to which they have to be distributed; ramifying more and more, until they are ultimately lost sight of, even when vision is aided by a powerful microscope. It is not positively decided, whether the nervous fibres have any distinct terminations either in the nervous centres, or in the organs to which they are distributed. In the gray matter of the brain of the vertebrata, they would appear to form a kind of plexus of loops; and the ultimate fibres do not seem to anastomose. The following has been described as the mode in which the nervous fibres are generally distributed to the peripheral organs. The trunks subdivide into small fasciculi, each of which consists of from two to six fibres, and these form plexuses, whose arrangement bears a general resemblance to that of the elements of the tissue in which they are placed. The primitive fibres then separate; and each, after passing over several elementary parts of the contain

ing tissue, or after forming a single narrow loop, as in the sensory papillæ, returns to the same or to an adjoining plexus, and pursues its way to the nervous centre from which it set out. According to this view, there is no more a termination of nerves, than there is of bloodvessels. Both form circles. More recent observations seem, however, to have demonstrated, that in different situations the loop-like appearance is fallacious; and that the ultimate fibres divide into fibrils, the terminations of which are lost in the tissues.

Fig. 17.

A

Fig. 18.

B

Investigations, again, by Henle and Kölliker1 show, that some of the peripheral nervous fibrils terminate in small bodies, seated especially in the nerves of the fingers and toes, which, from their having been discovered, in 1830, by Pacini of Padua, have been called Pacinian corpuscles; but of whose uses little can be said. They have not been observed on any motor nerves, so that they would not seem to have anything to do with motion. They exist in many nerves of the sympathetic class, and are not present on many sensitive nerves; so that, it has been properly inferred, they are probably not connected with acuteness of sensation.

Pacinian Corpuscles.

Of the encephalic nerves, the olfactory, auditory, and acoustic the Pacinian corpuscles.

B. Unusual form, from the mesentery of the cat;

-nerves of special sensibility-showing two included in a common envelope:clearly pass on to their destination, a, b are the two nerve-tubes belonging to them. without communicating with any other nerve. The spinal nerves, at their exit from the intervertebral foramina, divide into two branches, an anterior and a posterior, one being sent to each aspect of the body. The anterior branches of the four superior cervical pairs form the cervical plexus, from which all the nerves of the neck arise; the last four cervical pairs and the first dorsal form the brachial plexus, whence proceed the nerves of the upper extremities; whilst the branches of the five lumbar nerves, and the five sacral form the lumbar and sciatic plexuses; the former of which gives rise to the nerves distributed to the parts within the pelvis; the second to those of the lower limbs. The anterior branches, moreover, at a little distance from the exit of the nerve from the vertebral canal, communicate with an important and unique portion of the nervous system, the great sympathetic.

Each nerve consists of numerous fasciculi surrounded by areolar

1 Ueber die Pacinischen Körperchen an den Nerven des Menschen und der Saugethiere, Zürich, 1844; reviewed in Brit. and For. Med. Rev., January, 1845, p. 78; and Todd and Bowman, Physiological Anat. and Physiology of Man, i. 395, London, 1845, or Amer. edit.; and W. Bowman, Cyclopædia of Anat. and Physiol., by Dr. Todd, pt. xxvii. p. 876, Lond., Mar., 1846.