Brain tissue

“ Now of all the tissues the nervous tissue is that which takes longest to repair when injured; and it seems a not improbable inference that it is a tissue which is slower in its histological development than others. If this be so, we may see why, in the embryos of the higher vertebrates, the central nervous system quickly grows large in comparison to the other systems—why by pre-adaptation the brain of a chick develops in advance of other organs so much more than the brain of a fish. ”

“ Lee, the big thing missing in that summation is the seemingly impossible task of synthesizing nerve tissue, especially that of the cerebral cortex. It's been approximated, at any rate closely enough to give us good enough results to allow an artificial tissue to respond to brain signals about ninety-eight per cent as well as the original would. ”

“ Nerve cells and often large areas of gray matter are replaced by connective tissue (resembling scar tissue) , which grows and increases in what would otherwise be vacant spaces. All areas which contain this connective tissue, this filling which has no function, of course, cease to join with other parts of the brain in concerted action, and so the power of the brain is diminished, and certain of its activities are restricted or abolished. ”

“ The brain and cord are made up of blood-vessels, nerve cells, nerve fibres, and, holding them all together, connective tissue. The cells are very small, being microscopic in size; there are an immense number of them, [Pg 3] and they make up most of the gray matter or outside of the brain, but in the spinal cord the gray matter is in the centre. The fibres that go to make up the nerves begin and spring from the cells, and they also unite them together. ”

“ The gray nerve-tissue exists in the form of aggregations of minute cells in various parts of the nervous system. In man, by far the largest collections are found in the brain, and especially on the outside of it, covering it as the rind covers an orange, and hence called the cortex, or the cortical substance. Besides this large mass, spread out to the thickness of the twelfth of an inch or more over the exterior of the brain, there are masses of gray nerve-tissue in other parts, varying in size from that of a walnut to that of a small pea. ”

“ He then cuts off very thin slices of the tissue one after another, and, by employing different chemical reagents for which the various minute elements of the brain have varying susceptibilities, he obtains transparent colored sections of the nervous matter, which throw into strong relief the distinction between cells and fibers, besides exhibiting clearly the nature and direction of their intricate ramifications. ”

“ The brain is the largest and most complex mass of nerve tissue in the body, made up of an enormous collection of gray cells and nerve fibers. This organ consists of a vast number of distinct ganglia, or separate masses of nerve matter, each capable of performing separate functions, but united through the cerebral action into a harmonious whole.
Fig. 114.—The Upper Surface of the Cerebrum. (Showing its division into two hemispheres, and also the convolutions)
The average weight of the adult human brain is about 50 ounces for men and 45 ounces for women. ”

“ 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. ”

“ In other words, there is every reason to believe that numerous "centres," in this restricted sense, exist in the brain that have as yet eluded the investigator. Indeed, the current conception regards the entire cerebral cortex as chiefly composed of centres of ultimate co-ordination of impressions, which in their cruder form are received by more primitive nervous tissues—the basal ganglia, the cerebellum and medulla, and the spinal cord.
This, of course, is equivalent to postulating the cerebral cortex as the exclusive seat of higher intellection. ”

“ The brain is that portion of the cerebrospinal axis within the cranium, which may be divided into four parts—the medulla oblongata, the cerebellum, the pons Varolii, and the cerebrum—and it is covered by three membranes, called the meninges. The outer of these membranes, the dura mater, is a thick, white, fibrous membrane which lines the cavity of the cranium, forming the internal periosteum of the bones; it is continuous with the spinal cord to the extremity of the canal. The second, the arachnoid, is a delicate serous membrane, and loosely envelops the brain and spinal cord ”

“ The appearances left in the body are increased density and firmness of the whole brain or a part of it,—flattening of the convolutions on their outer surface, so that their grooves are almost obliterated and the investing membrane uncommonly dry,—unusual emptiness of the blood-vessels of the brain and its membranes,—and a protrusion of the brain upwards on removal of the skull-cap, as if the organ were too large for its containing cavity. ”

“ The air from the lungs is forced past them, setting them in vibration, thus producing sound.
The nervous system consists first of all of the brain. This is composed of nervous matter and constitutes the mass contained in the skull. It is divided into three parts: the cerebrum, the cerebellum, and the medulla oblongata. These three parts seem to preside over different functions. Thus, the cerebrum is the seat of the faculties of the mind, while the cerebellum presides over the muscular actions. ”

“ When the surface of the human brain is inspected, it is seen to present a very unequal appearance, due to the fact that its substance is thrown into a large number of folds or convolutions (see Fig. 1) , as they are technically named by the anatomist. The brain, or cerebrum, is in reality a double organ, formed of two similar halves or hemispheres, which are separated by a deep central fissure, but which are also connected together below by a broad band of nervous matter known as the corpus callosum. ”

“ The cerebrum, or fore brain, is divided into two hemispheres, each of which is further divided into numerous lobes consisting of grey and white nervous substance and millions of nerve cells and fine blood vessels. The hemispheric ganglia are the instruments through which the intellectual powers manifest themselves. If these instruments have become imperfect in structure through retarded development by wrong environment and education, or have been damaged in any manner by severe shock, violence or disease, the manifestations of intelligence are affected in a corresponding degree. ”

“ Above, and on either side of the eyes, I saw the shadow of something white, soft, and spongy, in which I fancied I could detect a distinct likeness to a human brain, only on a large scale. There were the cerebral lobes, or largest part of the forebrain, enormously developed and overhanging the cerebellum, or great lobe of the hindbrain, and completely covering the lobes of the midbrain. On the cerebrum I even thought I could detect—for I have a smattering of anatomy—the usual convolutions, and the grooves dividing the cerebrum into two hemispheres. ”

“ Upon examining the contents of the cranium, the pia mater was considerably inflamed, and an extravasated blotch, about the size of a shilling, was seen upon that membrane, near the middle of the right lobe of the [Pg 49] cerebrum. There was no water between the membranes, nor in the ventricles, but a general determination of blood to the contents of the cranium. The medullary substance when cut into was full of bloody points. The consistence of the brain was natural. ”

“ An anatomist may tell you that when the human brain reaches its fifteen-power standard (its maximum growth, at the age of twenty) the cranium, or bone box that contains it, will not grow more. It delights me, lover of inequality that I am, to inform the anatomist that there is no standard at all for a few exceptional brains, that they go on increasing the number of the cells of the cerebrum until, at a great age, physical decay begins; and, in order to accommodate these new cells, the cranium itself expands and grows. ”

“ Intellect, Emotion, and Volition. Physiologists assign to the cerebrum its functions, and neurological, as well as phrenological writers, have located them as represented in Fig. 68. True, there is no structural division between the parts of the cerebrum to indicate this diversity of function, nor is there any perceptible limit between the sensory and motor filaments of the game nerve. As no one has any reason for denying that separate portions of the brain may manifest distinct functions of the mind, we shall assume it as a conceded proposition. ”

“ A part of the central nervous system, the spinal cord, runs longitudinally through the body on the dorsal side of the alimentary canal; the circulatory system is closed, the blood being always confined in the heart and in vessels called arteries, veins, and capillaries, and the blood is red in color owing to the presence of numerous red corpuscles or blood-cells. The nervous system is highly developed, with a large brain in all the typical forms, and with complex and usually highly efficient special sense-organs. ”

“ Alas for the dulness of human observers! Our naturalists and anatomists have said not one word of the most conspicuous fact that may be seen in the general survey of the animal kingdom.3
To return to the theory of cerebral development: The reader will understand by referring to the last chapter that the summit of the spinal system or great inferior ganglion of the brain, bearing the names of optic thalami and corpora striata, is the true beginning of the cerebrum, instead of the medulla oblongata, which does not contain the fibres of the cerebral organs. ”

“ By the ganglionic portion we now understand the nerve cells which lie in little groups outside of the medullary tube. These cells produce fibers, which grow in two directions—on the one side into the brain or spinal cord, on the other away from the brain and cord into other tissues and organs. ”

“ In the lower placental mammals, the cerebral hemispheres leave the proper upper and posterior face of the cerebellum completely visible, when the brain is viewed from above, but, in the higher forms, the hinder part of each hemisphere, separated only by the tentorium (p. 117) from the anterior face of the cerebellum, inclines backwards and downwards, and grows out, as the so-called "posterior lobe," so as at length to overlap and hide the cerebellum. ”

“ Sometimes, indeed, the two sets of filaments are combined into one nerve cord, but if traced to their origin these are found to arise from different brain centres. Thus it was clear that a hitherto unrecognized duality of function pertains to the entire extra-cranial nervous system. Any impulse sent from the periphery to the brain must be conveyed along a perfectly definite channel; the response from the brain, sent out to the peripheral muscles, must traverse an equally definite and altogether different course. ”

“ The fibres which emanate from them are very delicate, and, becoming mixed with others, do not admit of being further traced. Where the nervous tissue meets the enveloping epithelium it is connected with the latter from within, but differs widely from it; for the nerve-cells contain a longitudinally striated cylindrical or thread-like nucleus which carries on its peripheral end a delicate hair, while its central end is prolonged into a fine nerve-fibre. There are, besides these, two other kinds of cells which form a transition between the ganglion and the epithelium cells. ”

“ If the fluid be cerebro-spinal in nature, the natural inference is that the subarachnoid space is opened up to the exterior, either directly or indirectly along the course of a cranial nerve. A chemical analysis of the fluid will determine the nature thereof, provided that the fluid be collected EARLY.
Certain factors, however, must be taken into consideration which diminish the value of these chemical tests, for, though the fluid be cerebro-spinal in nature, the admixture of blood at once interferes with the delicacy of the test. ”