Medullary ray

“ 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 ”

“ The rapid withdrawal of cerebro-spinal fluid has been [228] followed by disastrous results, the medullary stem being suddenly forced downwards into the foramen magnum, with corking up of that aperture, and complete upset of the medullary centres. Such a disaster is the more likely to occur when the tumour is subtentorial in position. Looking at the question from the broadest point of view, it would appear that the performance of lumbar puncture in cases of suspected brain tumour is fraught with considerable peril.
X-ray examination. X-ray examination should always be carried out. ”

“ Fig. 7 shows such a nerve-trunk with side branches; neither in the main stem nor in the offshoots are there any medullary nerves discoverable.
If this observation is correct, we see that what has to be traced further, is not the more easily recognized medullary fibre, but exceedingly fine fibrils carrying in certain intervals nuclei, the only distinguishing feature. It will be admitted also (first) that it must be a matter of considerable difficulty to follow such indifferent looking structures, as delicate fibres. ”

“ 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 ”

“ In the Teleostei a very shallow depression along the axis of the keel is the only indication of the medullary groove of other forms.
In Amphioxus (fig. 190) , the Tunicata, Petromyzon (?) , Elasmobranchii (fig. 182) , the Urodela and Mammalia (fig. 187) , the epiblast of the medullary plate is only formed of a single row of cells at the time when the formation of the central nervous system commences; but, except in Amphioxus and the Tunicata, [Pg 303] it becomes several cells deep before the completion of the process. ”

“ The whole central marrow is merely a simple cylindrical cord which runs the length of the body, and ends equally simply at both extremities—a plain medullary tube. All that we can discover is a small vesicular bulb at the foremost part of the tube, a degenerate rudiment of a primitive brain. We meet the same simple medullary tube in the first structure of the ascidia larva, in the same characteristic position, above the chorda. On closer examination we find here also a small vesicular swelling at the fore end of the tube, the first trace of a differentiation of it into brain and spinal cord. ”

“ 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. ”

“ The nervous system consists of the brain and the spinal cord, and of the cranial and spinal nerves. The rudiment of the future brain and spinal cord first appears in the embryos of some Cyclostomes (e.g. Bdellostoma) , of Elasmobranchs, and of Chondrostei (e.g. Acipenser) , and of Neoceratodus among the Dipnoi, in the form of a tubular medullary canal pinched off from the epiblast of the dorsal surface of the body. ”

“ Behind the optic vesicle are seen two visceral arches.
Anura. The pyriform medullary plate, already described, is the first external indication of the embryo. This plate appears about the stage represented in longitudinal section in fig. 71 B. The feature most conspicuous in it at first is the axial groove. It soon becomes more prominent (fig. 77 A) , and ends behind at the blastopore (bl) , the lips of which are continuous with the two medullary folds. As the sides of this plate bend upwards to form the closed medullary canal, the embryo elongates itself and assumes a somewhat oval form. ”

“ It is for this reason that the layers of the Vertebrate embryo are commonly represented as being quite isolated from one another, and that the medullary canal is nearly always represented as being completely isolated at certain stages from the surrounding tissues. In reality the layers are all connected together by this delicate tissue—in a sparse form, it is true—which not only extends between them, but also in a denser and more distinct form pervades them. ”

“ The ectoderm (ec) is much thinner than in the preceding figure and the medullary groove (mg) is much more shallow. The notochord (nt) [249] is of about the same diameter as before, but is here quite distinct from the entoderm (en) as well as from the mesoderm (mes) .
Figure 7f is seven sections posterior to Figure 7e. The medullary groove has disappeared and the medullary folds have flattened to form what might be called a medullary plate (at the end of the reference line ec) , which continues to the anterior border of the blastopore. ”

“ There is always a grey axis cylinder (a.c.) , which may (in medullated nerves) , or may not (in non-medullated or grey nerves) have a medullary sheath (s.S.) interrupted at intervals by the nodes of Ranvier (n.R.) . Nuclei (n.) at intervals under the sheath indicate the cells from which nerve fibres are derived by a process of elongation. The nerves of invertebrata, where they possess nerves, are mostly grey, and so are those of the sympathetic system of vertebrata, to be presently described, g.c., g.c. are ganglion cells ”

“ Its shape is like an heraldic lozenge. Its floor is formed by the grey matter of the posterior surfaces of the medulla oblongata and pons, already described (see figs. 3 and 6) ; its roof partly by the inferior vermis of the cerebellum, the nodule of which projects into its cavity, and partly by a thin layer, called valve of Vieussens, or superior medullary velum; its lower lateral boundaries by the divergent clavae and restiform bodies; its upper lateral boundaries by the superior peduncles of the cerebellum. ”

“ When entire Star-fish are mutilated by a section of each nerve-trunk half-way up each ray, and the animal is then placed upon its back, the tetanic contraction of the muscles in the rays 298 before mentioned as occurring under this form of section in detached rays, has the effect, when now occurring in all the rays, of elevating the disc from the floor of the tank. This opisthotonous-like spasm is not, however, permanent; and the distal ends of the rays forming adhesions to the floor of the tank, thy animal eventually rights itself, though much more slowly than unmutilated specimens. ”

“ The muscle-cell consists of two portions, a contractile part which lies next the sub-cuticle, and which usually, to some extent, wraps round the second or medullary half. The latter consists of a fibrillar spongioplasm, in the meshes of which lies a clear structureless hyaloplasm. The nucleus always lies in the medullary half. ”

“ By a simple modification, the open pit becomes a solid ectodermal ingrowth, just as in Teleostean fishes the hollow medullary tube, or the auditory pit of other vertebrate embryos, is formed at first as a solid cord of cells, which acquires a cavity secondarily. Moreover, the entocodon, however developed, gives rise at first to a closed cavity, representing a closing over of the umbrella, temporary in the bud destined to be a free medusa, but usually permanent in the sessile gonophore. ”

“ The inner shell of bone, as may be seen in the above sketch, perishes more extensively than the outer; and this arises probably from inflammation of the medullary membrane, in consequence of exposure, or, perhaps, from its being sometimes injured by the operator or assistants seizing the bone rudely to steady the stump, in order to facilitate the ligature of the vessels. In experiments on animals, the disturbance and injury of the medullary membrane is followed by internal necrosis, thickening of the outer living shell, and effusion betwixt the periosteum and bone. ”

“ On à priori grounds we should certainly expect the rods and cones to have resulted from the differentiation of a layer of cells external to the conducting nervous structures. The position of the rods and cones posterior to these suggests therefore that some peculiar infolding has occurred, and may be used as an argument to prove that the medullary groove is no mere embryonic structure, but the embryonic repetition of an ancestral change. ”

“ They also have respiratory organs and a nervous system, but, judging from the manner they endure vivisection, their nervous system must be of a very low order, for if they are broken in pieces the missing parts will soon grow again—in fact, they do not seem to be disturbed in the least no more than if it were a cast-off garment, and evidently go about as happy with the remaining rays as with the complete body, and, what is still more strange, the broken ray will grow a complete set of arms and a new body. ”

“ On the other hand, whenever a spasmodic contraction is started in the nervo-muscular sheet by stimulating any of the marginal ganglia, it will pass all through that sheet, from end to end of the ribbon, in the form of a spasmodic or violent contractile wave. Hence the muscular fibres of this medusa are capable of liberating this energy in either of two very different ways; and whenever some of them liberate their energy in one of these two ways, they determine that all the other fibres in the nervo-muscular sheet shall do the same. ”

“ The medullary canal serves not only to lodge and defend the medullary organ, but also to give more resistance to the bone; for we know, that of two cylinders formed of an equal quantity of matter, one of which is hollow, and147 consequently has a greater diameter than the other which is full, the first will resist more than the second, because we can bend and break it with less facility. ”

“ It is a fact that a frog, having simply the cord and medulla, will react not only in the manner already described, but also by movements of the entire body, away from the scarce of trouble; the animal may even utter a cry as if in pain, yet he may be "merely a non-sentient, non-intelligent reflex mechanism." We know that the medulla is the last portion of the nervous system to come under the influence of anæsthetics. Persons submitting to severe surgical operations frequently cry out violently, and as if in intense pain ”

“ The medulla oblongata consists, like the spinal cord, of columns of white fibers and masses of gray matter, but differently arranged. The gray matter is broken up into masses which serve as centers of origin for various nerves. The functions of the medulla oblongata are closely connected with the vital processes. It is a great nerve tract for transmitting sensory and motor impressions, and also the seat of a number of centers for reflex actions of the highest importance to life. ”

“ 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. ”