Medulla

“ As already stated, this section affects very greatly the circulation, but the fever is independent of such action. The upper end of the medulla oblongata is continuous with a nervous mass which joins the two brain hemispheres together, and hence is known as the pons or bridge. If, instead of cutting the spinal cord, we separate the medulla oblongata from the pons, an immediate rise of temperature occurs, and continues until death, whether the operation be performed in a cold or heated room. ”

“ In this double experiment, the nerves remain attached to the membrane separated from its medullary substance, because their nervous coat is continued with it. It is exactly as if a number of small arterial filaments went from the aorta; the parietes of this artery would be to those of these filaments, what the pia mater of the spinal marrow is to the coat of the nerves which go from it. Only the nerves are white, because their medulla fills them; whereas the canal to which they belong is transparent, because it is deprived of its own medulla. ”

“ A portion of the medulla presides over the important function of respiration, and from it arises the pneumogastric nerve, so called because its branches serve both the lungs and stomach. The feelings of hunger, thirst, and the desire for air are aroused by means of this nerve. The wounding of the gray matter of the medulla, even of a small portion of it, near the origin of the pneumogastric nerve, at once stops the action of the lungs and causes death. ”

“ The adrenal or suprarenal gland lies just above the kidney, and anatomically has two parts, an outer cortex or shell, and an inner medulla or pulp. The medulla gives off adrenalin, which regulates blood pressure in all parts of the body through action on the blood vessels. The cortex gives off an unknown secretion which seems to have a remarkable influence. When it is diseased, certain curious things happen. ”

“ While the power of motion in each part seems thus to be dependent on isolated points of the medulla oblongata and the medulla spinalis, an indirect or connecting influence is admitted to take place between them and the remaining parts of the brain; and whatever may be its nature or extent in animals, there can be no doubt of its being so infinitely greater in man as to be essentially different ”

“ We observe posteriorly the medulla oblongata, on the face of which we may observe the crossing of the fibres, and on the side of which we observe the origins of many nerves. Above the medulla we observe the pons Varolii, just above which we observe the fibres ascending to each hemisphere under the name of crus cerebri, or thigh of the cerebrum. Next we see the optic nerves crossing on the median line, the olfactory nerve, running under the front lobe, which is separated by the fissure of Sylvius from the middle lobe. ”

“ In the medulla the nerves that arise in the cerebrum cross over from one side of the body to the other on the crossed pyramidal tracts. The importance of this crossing of the nerve fibers is seen in apoplexy, when a blood-vessel is ruptured in the brain and hemorrhage causes pressure, generally on the motor tract. Paralysis of the nerves and of the muscles to which they go results. The paralysis is generally of one side of the body, the opposite side from that on which the injury occurred. ”

“ The medulla is part of the brain stem.
This was only one of the difficulties which had to be overcome. The eyes are set on the pre-fulcral lever of the head. For our safety we must be able to look in all directions—over this shoulder or that. We must also be able to turn our heads so that our ears may discover in which direction a sound is reaching us. In fashioning a fulcral joint for the head, then, two different objects had to be secured: free mobility for the head, and a safe transit for the medullary part of the brain stem. ”

“ Holding the cell bodies and fibers in place is a kind of connective tissue. The inner part (white in color) is composed largely of fibers which pass to other parts of the brain and down into the spinal cord. Under the cerebrum, and dorsal to it, lies the little brain, or cerebellum. The two sides of the cerebellum are connected by a band of nerve fibers which run around into the lower hindbrain or medulla. This band of fibers is called the pons. The medulla is, in structure, part of the spinal cord, and is made up largely of fibers running longitudinally. ”

“ The medulla is a center for the movements of chewing and swallowing. This center can be excited in a reflex manner, and by the will, but not automatically. There seems to be good evidence that the medulla is a center for combined or co-ordinated movements of the body. Wundt is of opinion that the collective motor-fibers of the body are brought into closer union with each other in this organ. His opinion rests upon the fact that, as long as the medulla is intact, sensible excitations occasion general movements of the body much more easily than when this organ is destroyed. ”

“ Another part forms the medulla oblongata, that important hind brain in which lie so many of the vital centres of nervous energy. The central cavity formed by these constrictions is of comparatively less importance, forming ultimately what is known as the mid-brain. The foremost or anterior vesicle, however, is of the very greatest importance, and its subsequent changes are more marked than either of the other two. ”

“ The white medullary substance appears to be every where uniform and similar to itself throughout the whole brain, spinal marrow, and nerves; and though the cortical substance be mixed with the medullary in the brain, and spinal marrow, and perhaps in the ganglions and plexuses, yet it does not appear that the communication of any one part of the medullary substance with every other, is cut off any where by the intervention of the cortical. There is no part of the medullary substance separated from the rest, but all make one continuous white body ”

“ The brain-centres in turn are complex, consisting of an automatic mechanism in the medulla regulating the action of all the subordinate parts below it, and of a series of cortical centres whose function it is to stimulate or inhibit the medullary mechanism. ”

“ Each division governs the vascular tone of its own side of the body,14 and lesions in its region in man produce unilateral vaso-motor symptoms.15 This centre can be excited to reflex action by strong irritation locally or through the blood, in which case a general constriction or dilatation of the vessels of the entire body will ensue. It seems probable, however, that the general centre in the medulla is made up of a number of special centres, each of which governs a definite set of organs. ”

“ Hence it appears, that as the muscles have larger fibres intermixed with a smaller quantity of nervous medulla, the organ of vision has a greater quantity of nervous medulla intermixed with smaller fibres; and it is probable that the locomotive muscles, as well as the vascular ones, of microscopic animals have much greater tenuity than these of the retina. ”

“ The toxin acts principally on the nerve centres in the spinal medulla, to which it travels from the focus of infection by way of the nerve fibres supplying the voluntary muscles. Its first effect on the motor ganglia of the cord is to render them hypersensitive, so that they are excited by mild stimuli, which under ordinary conditions would produce no reaction. ”

“ Anatomy.#--A nerve-trunk is made up of a variable number of bundles of nerve fibres surrounded and supported by a framework of connective tissue. The nerve fibres are chiefly of the medullated type, and they run without interruption from a nerve cell or _neuron_ in the brain or spinal medulla to their peripheral terminations in muscle, skin, and secretory glands. ”

“ The brain has a central organ, which is a continuation of the spinal cord, and to which anatomists have given the name of medulla oblongata. In connexion with this there are other bodies placed in pairs. That each of these bodies has its peculiar functions there cannot be the smallest doubt; and it is, indeed, sufficiently probable that each of them is not a single organ, but a congeries of organs having distinct and separate uses.
Experimental physiology, joined with the observation of the changes produced by disease, has thrown some light on this mysterious subject. ”

“ The remaining nerves, all of which have their origin in the medulla oblongata, possess certain features in common, and as they are related to the gill-clefts in such a way that each forks over a cleft, they may be conveniently distinguished as "branchial" or "branchiomeric nerves." A typical branchial nerve consists of (1) a principal ganglion near the origin of the nerve from the brain; (2) a main trunk which gives off (3) a somatic sensory branch or dorsal nerve to the skin; (4) a palatine nerve (visceral sensory) to the oral or pharyngeal mucous membrane ”

“ In the upper pons the fibers for the face cross to the opposite side, while the rest keep on down through the medulla, and as they emerge from the medulla they too cross to the other side and keep on down in the crossed pyramidal tract. A few fibers do not cross but come down the direct pyramidal tract, which, however, disappears part way down. The crossed pyramidal tract is the true motor tract and in it the fibers are continually sending branches to the cells in the gray matter, where they connect with the anterior horn. ”

“ One of the strangest facts known about the system of inhibitory nerves is their marked tendency to leave the central nervous system at a different level to the corresponding motor nerves, as is well known in the case of the heart, where the inhibitory nerve—the vagus—arises from the medulla oblongata, while the motor nerve—the augmentor or accelerator—leaves the spinal cord in the upper thoracic region. ”

“ Running out beneath from the midbrain are the optic nerves, which cross, the left one connected with the right eye, the right one with the left eye. From each side of the medulla oblongata [Pg 269] there is given off a large nerve, the vagus, which sends branches to the lateral line organs on either side, and extends backward to the stomach and viscera. ”

“ On the closure of the medullary groove the lumen of the medullary canal is continued uninterruptedly through the brain, but dilates considerably in each of the cerebral vesicles.
The anterior lobe of the brain becomes converted into the cerebral hemispheres, the thalamencephalon, the primary optic vesicles, and the parts connected with them. The middle lobe becomes the optic lobes (corpora bigemina or corpora quadrigemina in Mammalia) and the crura cerebri; while the posterior lobe becomes converted into the cerebellum and medulla oblongata. ”

“ The next point to bear in mind is that these medullary rays (like the pith and cortex) are merely parts of the general cell-tissue, or fundamental tissue, through which the vascular bundles run upwards and downwards with a tangentially sinuous course from the leaves. The primary medullary rays, therefore, are merely spokes, as it were, joining the pith and cortex ”

“ After the removal of the brain from the skull the abnormal dimensions of the cerebrum present a remarkable contrast to the normal size of the cerebellum, pons, and medulla oblongata, which also appear flat and broad from pressure. When a horizontal section is made through the hemispheres of the cerebrum and a little above the corpus callosum, the centrum ovale appears unusually large. The cavities of the ventricles are very narrow, their walls touching one another. There is no serum in the pia mater or in the ventricles. The substance of the brain is pale, bloodless, and dry. ”

“ The medulla forms a dark line passing along the centre of the hair, and occupies about a fourth or a fifth of its thickness. The medullary line is absent in infants’ hair, and in very fine light hair; it is present in strong hair, and is not difficult 7to trace in white or grey hair. Dr. Beigel observes, that “we may drive out the air contained in the central tube—as the medullary has likewise been called—and see it re-enter again under the microscope. Turpentine penetrates all parts of the hair which are filled by air, but its action is slow and requires some time. ”

“ Caudad to the cerebrum are seen the end of the paraphysis, G.p., the optic lobes, MH, and the cerebellum, HH. From the cerebral peduncles (ventrad to the optic lobes) arises the oculomotor nerve, III, and dorsocaudad to this, between the optic lobe and the cerebellum, arises the trochlear nerve, IV. From the middle zone (in a dorsoventral direction) of the medulla, ventrad to the cerebellum, arises the very large trigeminal nerve, V; while from its usual place, on the ventral surface of the medulla, the abducens nerve, VI, takes its origin by several roots. ”

“ Objective symptoms of venous stasis—dilatation of the veins of the eye, both external and internal, œdema of the disk. Affection of the medullary centres, shown by a slowed pulse and a slight rise in temperature.
3. Stage of fully-developed compression. Widespread capillary anæmia. Medulla affected markedly. The period of vaso-motor regulation has set in with its high blood-pressure, and this, with its vagal quality, gives the pulse its bounding character. The rise in blood-pressure shows a definite rhythm. Also rhythmicities in respiration which may acquire the Cheyne-Stokes character. ”