We have seen that a cavity is formed by the bones of the skull and continued through an opening in its base down through the centre of the vertebras to their extremity. It is to hold within its bony embrace that greyish white, soft and pulpy substance, called the cere-bro-spinal-axis. Of this, the part passing through the vertebrae, giving off branches to every part of the system, is generally known as the spinal cord, while the upper or enlarged portion, also giving off branches, is called the brain.
The brain, look at it closely, examine it minutely, bring to your aid, if you please, the powers of the microscope, and what do you see? A soft, pulpy, greyish white substance, and yet this is the seat of the deathless soul, here mind reigns and intellect forges those thoughts, which, like the thunderbolts of Jehovah, scatter ruin and death, rouse a nation to arms, shake a world, or infuse around an atmosphere of purity and love.
Here are conceived the glowing thoughts of the poet, the bright visions and gorgeous pictures of the artist, plans pure and holy, or debasing, polluting and devilish. This is the fountain of purity and greatness, and of filth, abomination and discord. And yet do we see any of these workings in that mass before us? Do we see how the thought is transmitted along those electric lines, the nerves, to the remotest extremity? Or is the fluid so subtile as to elude our gaze? We feel the blow which fells us to the earth, but can our eyes see that nervous stimuli which induces the action? Is it not natural to suppose, that when an agent so quick and subtile, so powerful in its influence on the system becomes diseased or deranged, it requires not the blow of the hammer to set it right, but rather a power quick and subtile, yet with sufficient strength to restore healthy action?
This cerebro-spinal-centre is enclosed in three membranes. The external one, forming the inside lining of the bone is called the dura mater. On the inner side of the dura mater is a thin serous membrane called the arachnoid. The serous fluid it secretes in a healthy state, seems to lubricate the brain; in a diseased state this fluid may increase in quantity and oppress that organ. The third membrane investing directly the brain, is highly vascular, composed of a net-work of innumerable vessels held together by cellular tissue. It is the nutrient membrane of the brain, and is called the pia mater.
Not only motion, but life itself depends on the healthy action of the nervous system. The cerebrospinal-cenntre in its bony cavity guides and controls every action by means of branches of nerves, which ramify throughout every part of the body. Of these branches the brain proper sends off twelve pairs. One pair ramifies upon the membrane lining the nasal passages, creating the delicate sense of smell, and are called the olfactory nerves. A second pair, penetrates the coats of the eye, and expands into the retina, forming a surface upon which every object is pictured, and creating the sense of sight. These are the optic nerves. The third, fourth and sixth pairs are distributed among the muscles of the eyes. The fifth pair has three important branches. One branch passes out from the skull at a notch distinctly felt about the middle of the eyebrow, and sends branches to the forehead, eyes and nose. The second branch supplies the teeth of the upper jaw and passing out through a notch in the malar (or cheek) bone, sends a branch to the eye and ramifies over the face. The third branch supplies the teeth of the lower jaw by sending a small branch to each tooth. It also sends branches to the muscles of the lower jaw, the ear and the tongue. It is generally in this nerve and its.branches that we have that most agonizing of all pain, prosopalgia or neuralgia of the face.
As we descend from the brain to the spinal cord, we find it giving off thirty-one pairs of nerves, each arising by two roots, one of which is called the motor, the other the sensitive root. The motor root arises from a narrow white band on anterior columns of the cord, while the sensitive start from the internal part of the cord.
Five pairs in the vicinity of the shoulder, unite, forming what is called the brachial plexus, and again separate in six nerves, which ramify on the muscles and skin of the upper extremities. For the sake of convenience, we may mention, that the first eight pairs of nerves, commencing at the top of the column, are called cervical, the next twelve dorsal, the next five lumbar and the last six sacral nerves. Besides the brachial plexus, above mentioned, there is also a lumbar and sacral plexus, formed in the same manner, supplying the lower extremities, the hips, the abdomen and the organs in the vicinity of the pelvis.
From what has been already said it will be perceived, that the nerves of the spine as well as the brain are of two kinds, the motor, or nerves of motion, and the sensitive, or nerves of sensation. The nerves of motion communicate principally with the muscular tissues and the various organs of the body, while those of sensation ramify more particularly on the skin. Hence the pain in cutting through the skin is much more severe than in the tissue beneath. If the nerves of motion are injured so that they cannot act on either side, there is an entire paralysis of that side or organ, notwithstanding the sensation may be as acute as ever. So also when the nerves of sensation are paralized, notwithstanding there may be no diminution of motion in the part, with which they communicate, yet all sense of feeling is entirely gone. Sever the optic nerve, which is a nerve of sensation, and sight would be entirely destroyed, notwithstanding the motion of the eye would remain uninjured. Yet if the nerve of motion were severed, the sight would remain uninjured, but the eye becomes fixed and motionless; and so with the other nerves of the head and spine. Hence we are often unable to move some particular part of the body in which feeling may be acute, and on the contrary, sensation may be absent and motion perfect.
Besides the nerves already enumerated, there is still another, which performs a most important part in the economy of life, viz.: The sympathetic nerve. It consists of a chain of ganglia, or knots, extending the whole length of the spinal column on each side. It communicates by its branches, not only with the spinal but cranial nerves, and all the internal organs of the head and trunk. Every part of the body is more or less under its influence, as filaments from it, accompany all the blood-vessels throughout their course. Thus a sympathetic chain is kept up throughout the body both in health and disease.
Digestion. A proper understanding of the organs connected with digestion is of course important, as their derangement is the fruitful source of a large share of our physical sufferings.
Situated around the mouth, are six salivary glands, three on each side of the jaw, the parotid, submaxillary, and sublingual. Their secretion called saliva, is discharged through small ducts into the mouth. Back of the cavity of the mouth, and connecting with it, is a large passage connecting above with the passages of the nose, and below with the larynx, by means of which air is passed to the lungs, and the oesophagus, through which food passes into the stomach, called the pharynx. The (oesophagus is a muscular tube extending behind the larynx, heart and lungs, through the diaphragm into the stomach. The diaphragm is a muscular curtain, separating entirely the organs of the chest from those of the abdomen. It rises and falls with every respiration. The stomach is located on the left side of the abdomen, just below the diaphragm. it has two openings the upper or cardiac orifice connecting with the oesophagus, and admitting the food, the lower or pyloric orifice, connected with the intestines, through which the food passes after undergoing the action of the stomach.
The intestines are divided into large and small. The small intestine, which is about twenty-five feet in length, is divided into the duodenum, jejunum, and illium. The large intestine, about five feet in length, is divided into the coecum, colon and rectum. The duodenum commences at the pyloric orifice of the stomach, and is about twelve fingers in length. Into it open the ducts from the liver and pancreas. The jejunum is a continuation of the duodenum, as is the illeum of the jejunum. These forming the small intestines, pass in convolution from one side to the other, until the illeum terminates in the colon by a valvular opening near the right hipbone. The coecum is a blind pouch at the commencement of the large intestines. From this point the large intestine or colon ascends on the right side, crosses the abdomen just below the liver and stomach, descends on the left side to the hip-bone, where curving on itself it passes downwards from the rectum and terminates in the anus.
The peritoneum is a serous membrane adhering to the inner surface of the abdominal cavity, and is reflected over, invests, and supports the viscera of the cavity.
After having completely invested an organ it passes double to the walls of the abdomen, to be there expanded. These duplicatures confine the organs in their place and support them. That which supports the intestines is called the mesentery, and a very large one, hanging loose before them, keeping them warm, is called the oementum.
The lacteals are small vessels commencing on the mucous membrane of the small intestine, in the upper portion of which they are the most numerous, passing between the membranes of the mesentery to several successive ranges of glands, diminishing in number and increasing in size at each successive range until they open into the enlarged portion of the thoracic duct.
This duct passes through the diaphragm to the lower part of the neck, where it opens into a vein which passes directly to the heart. The food then, after it has been digested by the stomach, passes into the duodenum, and there mingling with the secretions of the liver and pancreas, is taken up in the form of chyle, as it passes along the intestines, by the innumerable mouths of these little absorbents, the lacteals, by them conveyed to the thoracic duct, and through it passed into the circulation, and thus conveyed to every part of the system.
The liver is a large gland, weighing three or four pounds, situated on the right side below the diaphragm. On the under surface is the gall-bladder, which acts as . a reservoir for the bile.
The 'pancreas is a long gland situated behind the stomach transversely across the posterior part of the abdomen. From it as well as from the liver a duct passes into the duodenum.
The spleen is a small body situated in the left side. Its use is unknown.