The Creation of God, Jacob Hartmann [e manga reader TXT] 📗

The spinal cord is a cylindriform column of nerve substance connected above with the brain, through the medium of the medulla oblongata, terminating below, about the lower border of the first lumbar vertebra, in a slender filament of gray or vesicular substance, the filum terminale, which lies in the midst of knots of many nerves forming the codæ equina. Through the center of the cord, running in a longitudinal direction, is a minute canal, which is continuous through the whole length of the cord, and opens above into the space at the back of the medulla oblongata and pons varolii, called the fourth ventricle; the aqueduct of silvius connects it with the third ventricle, lateral and fifth ventricles, near the base of the brain. The cerebro-spinal fluid circulates in the interior of these ventricles and spinal cord. What precise mechanical function it subserves is only surmised, not known.

The cerebro-spinal axis is protected by three membranes, named also meninges. They are: 1. An external fibrous membrane, named dura mater, which closely lines the interior of the skull, and forms a loose sheath in the spinal canal; 2. An internal areolo-vascular tunic, the pia mater, which accurately covers the brain and spinal cord; and, 3. An intermediate membrane, the arachnoid, which lies over the pia mater, the two being in some places in close connection, and in others separated by a considerable space.

The sympathetic nerves are distributed in general to all the internal viscera, and to the coats of the blood-vessels. Some organs, however, receive their nerves also from the cerebro-spinal system, as the lungs, the heart, and the upper and lower parts of the alimentary canal.

The great gangliated cords consist of two series, in each of which the ganglia are connected by intervening cords. These cords are placed symmetrically in front of the vertebral column and extend from the base of the skull to the coccyx.

With respect to the functions of the sympathetic nervous system, it may be stated generally that the sympathetic nerve fibers are simple conductors of impressions as those of the cerebro-spinal system are, and that the ganglionic centers have (each in its appropriate sphere) the like powers of conducting and of communicating impressions.

The general processes which the sympathetic appears to influence, are those of involuntary motion, secretion, and nutrition.

Nerve centers. This term is applied to all those parts of the nervous system which contain ganglion corpuscles, or vesicular nerve-substance—i.e., the brain, spinal cord, and the several ganglia which belong to the cerebro-spinal and the sympathetic system. Each of these nervous centers has a proper range of functions, the extent of which bears a direct proportion to the number of nerve fibers that connect it with the various organs of the body, and with other nervous centers; but they all have certain general properties and modes of action common to them as nervous centers. The brain does not issue any force, except when itself impressed by some force from within, or stimulated by an impression from without; neither do the other nerve centers without such previous impressions produce or issue motor impulses.

The more certain and general office of all the nervous centers is that of variously disposing and transferring the impressions that reach them through the several centripetal fibers. In nerve fibers impressions are conducted only in the simple isolated course of the fiber; in all the nervous centers an impression may not only be conducted, but also communicated; in the brain alone it may be perceived.

In all cases in which the mind either has cognizance of, or exercises influence on, the process carried on in any part supplied with the sympathetic nerve, there must be conduction of impressions through all the nervous centers between the brain and the part. But instead of, or as well as, being conducted, impressions made on nervous centers may be communicated from the fibers that brought them to others, and in this communication may be either transferred, diffused, or reflected. Along nerve fibers impressions or conditions of excitement are simply conducted; in nerve centers they may be made to deviate from their course, and may be variously diffused, reflected, or otherwise disposed of.

Function of nerves. The office of nerves as simple conveyors or conductors of nervous impressions is of a twofold kind: 1. They serve to convey to the nervous centers the impressions made upon the peripheral extremities or parts of their course; 2. They serve to transmit impressions from the brain and other nervous centers to the parts to which they are distributed. For this twofold office of the nerves two distinct sets of nerve fibers are provided, in both the cerebro-spinal and sympathetic systems. Those which convey impressions from the periphery to the center are classed together as centripetal or afferent nerves, or nerves of sensation—sensitive nerves. Those, on the other hand, which are employed to transmit central impulses to the periphery are classed as centrifugal or afferent nerves or motor nerves, conveying impulses to the voluntary and involuntary muscles, etc.

Nerves are constructed of minute fibers or tubules full of nervous matter, arranged in parallel or interlacing bundles, which bundles are connected by intervening connective tissue in which their principal blood-vessels ramify.

The size of nerve fibers varies, and the same fibers do not preserve the same diameter through their whole length, being largest in their course within their trunk and branches of nerves, in which the majority measure from 1⁄2000​ to 1⁄3000​ of an inch in diameter. As they approach the brain or spinal cord, and generally also in the tissue in which they are distributed, they gradually become smaller. In the gray or vesicular substance of the brain or spinal cord they generally do not measure more than from 1⁄10000​ to 1⁄14000​ of an inch.

The chemical composition of nervous matter. Like most of the other tissues of the body, the nervous substance contains a large proportion of water (from three-fourths to four-fifths of its weight). Of the residue which remains after the removal of this by evaporation or other means, the larger part consists of a phosphuretted fat, which may be obtained crystallized, and in this condition was termed protagon. The crystalline substance, however, is in reality a mixture of two other substances, lecithin and neurin. Cerebrin is also described as being frequently met with in conjunction with lecithin.