OF THE NERVOUS SYSTEM.
AFTER having described in the last chapter one of the After having described in the last chapter one of the appropriate powers of the animal system, contractility, and the organ by which it is exercised, we must proceed to the other exclusive property of animal life, sensibility. shall arrange our remarks on this subject into three sections ; in the first, we shall offer a brief description of the organ of sensibility, the nervous system, of its anato mical structure, and of its physical and chemical proper ties ; secondly, we shall inquire into its vital properties, and, particularly, we shall examine the nature of sensi bility ; in the third place, we shall treat of the use of the nervous system.
The ganglia are small masses of nervous matter, situat ed along the course of the larger nerves, frequently where two or three of them form ati angle, and especially in the different parts of the thorax and abdomen. They consist of a mixture of both cortical and cineritious matter, and are generally more plentifully supplied with blood-vessels than the nerves. With respect to their texture, we are informed that the filaments of the different nerves which compose the ganglia proceed without interruption, but that they arc all twisted together into an irregular bundle, and that filaments of different nerves are united together so as to compose one nerve as it proceeds from these parts ; the viscera of the thorax and the abdomen are principally supplied with nerves from the ganglia.
The organs of sense and the muscles of voluntary mo tion receive the greatest proportion of nervous matter, the viscera are supplied more sparingly, the glands still more so, while the membranous parts are nearly without nerves. For the most part, the organs of sense receive their nerves from the brain, and the muscles from the spinal marrow, while the viscera generally are supplied with nerves from the ganglia or the plexuses. These lat ter are connected with each other, and with the other parts of the body in a variety of ways, as if for the pur pose of insuring a communication between all the differ ent parts. If we conceive of the body as divided by a plane passing perpendicularly through its centre, the nerves of the two halves will be found almost exactly si milar to each other.
Anatomists have remarked upon the great quantity of blood that is sent to the brain ; although this organ is only about one-fortieth part of the weight of the whole body, yet it is estimated, that at least one-tenth of all the blood is immediately sent to it from the heart, and there are many curious contrivances in the structure of the brain, by which the force of the blood is diminished, or at least so modified as to prevent the danger of its pressing with too much violence upon any part of this organ, and thus injuring its texture.
The minute structure of the brain and nerves has been examined and described by different anatomists, but it must be confessed that considerable difficulty still attaches to the subject. When we inspect the brain shortly after death, we can readily distinguish in its medullary part a number of furrows with ridges between them, which run parallel to each other, and the same kind of fibrous or striated texture may be perceived in the nerves. It seems, however, that we are capable of detecting much more mi nute divisions in the nerves than in any part of the brain, those of the former, according to the microscopical obser vations of Fontana and Reil, being far too minute to be visible to the naked eye, whereas, it does not appear that we are able, by the aid of glasses, to separate the visible fibres of the brain into any smaller fibres.
For our knowledge of the chemical properties of me dullary matter, we are principally indebted to Fourcroy and Vauquelin. We learn from the experiments of the latter, that brain consists of a proper adipose substance, the peculiar animal principle called osmazome, a quantity of albumen, a minute portion of phosphorus, and some salts. The adipose matter is so connected or combined with the albumen, as to form a kind of saponaceous sub stance, so that when brain is agitated with water it forms an emulsion, which continues for a long time without be ing decomposed. The albumen is capable of undergoing an imperfect coagulation by heat, or by the addition of various re-agents, but it seems, independently of the pre sence of the other ingredients, to differ somewhat from the albumen of the blood. Brain contains about three fourths of its weight of water, and when this is removed by a careful evaporation, the residuum is converted into a half solid friable mass. The salts that are found in brain are chiefly the phosphates of lime, soda, and am monia; the muriates, which are in such large quantity in the blood, and many other animal fluids, appear not to be present in the brain, or at least to exist in much smaller quantity.