In the cerebro-spinal system there are three different kinds of reflex actions. (1.) Those of the spinal cord and medulla oblongata are performed without any consciousness or sensation on the part of the subject. (2.) The second class embraces those of the tuber annulare, where the perception gives rise to motion without the interference of the intellectual faculties. These are denominated purely instinctive reflex actions, and include all those operations of animals which seem to display intelligent forethought; thus, the beaver builds his habitation over the water, but not a single apartment is different from the beaver homestead of a thousand years ago; there is no improvement, no retrogression. Trains of thought have been termed a third class of reflex actions. It is evident that the power of reasoning is, in a degree, possessed by some of the lower-animals: for instance, a tribe of monkeys on a foraging expedition will station guards at different parts of the field, to warn the plunderers of the approach of danger. A cry from the sentinel, and general confusion is followed by retreat. Reason only attains its highest development in man, in whom it passes the bounds of ordinary existence, and, with the magic wand of love, reaches outward into the vast unknown, lifting him above corporeal being, into an atmosphere of spiritual and divine Truth.
The Cranial Nerves. From the brain, nerves are given off in pairs, which succeed one another from in front backwards to the number of twelve. The first pair, the olfactory nerves, are the nerves of the sense of smell. The second pair are the optic, or the nerves of the sense of sight. The third pair are called the motores oculi, the movers of the eye, from the fact that they are distributed to all the muscles of the eye with the exception of two. The fourth pair and the sixth pair each supply one of the muscles of the eye, on each side, the fourth extending to the superior oblique muscle, and the sixth to the external rectus muscle. The nerves of the fifth pair are very large; they are each composed of two bundles of filaments, one motor and the other sensory, and have, besides, an additional resemblance to a spinal nerve by having a ganglion on each of their sensory roots, and, from the fact that they have three chief divisions, are often called the trigeminal, or trifacial, nerves. They are nerves of special sense, of sensation, and of motion. They are the sensitive nerves which supply the cranium and face, the motor nerves of the muscles of mastication, the buccinator and the masseter, and their third branches, often called the gustatory, are distributed to the front portion of the tongue, and are two of the nerves of the special sense of taste. The seventh pair, called also the facial nerves, are the motor nerves of the muscles of the face, and are also distributed to a few other muscles; the eighth pair, termed the auditory nerves, are the nerves of the special sense of hearing. As the seventh and eighth pairs of nerves emerge from the cavity of the skull together, they are frequently classed by anatomists as one, divided into the facial, or portio dura, as it is sometimes called, and the auditory, or portio mollis. The ninth pair, called the glosso-pharyngeal, are mixed nerves, supplying motor filaments to the pharyngeal muscles and filaments of the special sense of taste to the back portion of the tongue. The tenth pair, called the pneumogastric, or par vagum, are very important nerves, and are distributed to the larynx, the lungs, the heart, the stomach, and the liver, as shown in Fig. 60. This pair and the next are the only cerebral nerves which are distributed to parts of the body distant from the head. The eleventh pair, also called spinal accessory, arise from the sides of the spinal marrow, between the anterior and posterior roots of the dorsal nerves, and run up to the medulla oblongata, and leave the cranium by the same aperture as the pneumogastric and glosso-pharyngeal nerves. They supply certain muscles of the neck, and are purely motor. As the glosso-pharyngeal, pneumogastric, and spinal accessory nerves leave the cranium together, they are by some anatomists counted as the eighth pair. The twelfth pair, known as the hypoglossal, are distributed to the tongue, and are the motor nerves of that organ.
THE GREAT SYMPATHETIC.
A double chain of nervous ganglia extends from the superior to the inferior parts of the body, at the sides and in front of the spinal column, and is termed, collectively, the system of the great sympathetic. These ganglia are intimately connected by nervous filaments, and communicate with the cerebro-spinal system by means of the motor and sensory filaments which penetrate the sympathetic. The nerves of this system are distributed to those organs over which conscious volition has no direct control.
Four of the sympathetic centers, situated in the front and lower portions of the head, are designated as the ophthalmic, spheno-palatine, submaxillary and otic ganglia. The first of these, as its name indicates, is distributed to the eye, penetrates the sclerotic membrane (the white, opaque portion of the eyeball, with its transparent covering), and influences the contraction and dilation of the iris. The second division is situated in the angle formed by the sphenoid and maxillary bone, or just below the ear. It sends motor and sensory filaments to the palate, and velum palati. Its filaments penetrate the carotid plexus, are joined by others from the motor roots of the facial nerve and the sensory fibres of the superior maxillary. The third division is located on the submaxillary gland. Its filaments are distributed to the sides of the tongue, the sublingual, and submaxillary glands. The otic ganglion is placed below the base of the skull, and also connects with the carotid plexus. Its filaments of distribution supply the internal muscles of the malleus, the largest bones of the tympanum, the membranous linings of the tympanum and the eustachian tube. Three ganglia, usually designated as the superior, middle, and inferior, connect with the cervical and spinal nerves. Their interlacing filaments are distributed to the muscular walls of the larynx, pharynx, trachea, and esophagus, and also penetrate the thyroid gland. The use of this gland is not accurately known. It is composed of a soft, brown tissue, and consists of lobules contained in lobes of larger size. It forms a spongy covering for the greater portion of the larynx, and the first section of the trachea. That it is an important organ, is evident from the fact that it receives four large arteries, and filaments from two pairs of nerves.
The sympathetic ganglia of the chest correspond in number with the terminations of the ribs, over which they are situated. Each ganglion receives two filaments from the intercostal nerve, situated above it, thus forming a double connection. The thoracic ganglia supply with motor fibres that portion of the aorta which is above the diaphragm, the esophagus, and the lungs.
In the abdomen the sympathetic centers are situated upon the coeliac artery, and are termed, collectively, the semilunar coeliac ganglion. Numerous inosculating branches radiate from this center and are called, from the method of their distribution, the solar plexus. From this, also, originate other plexi which are distributed to the stomach, liver, kidneys, intestines, spleen, pancreas, supra-renal glands, and to the organs of generation. Four other pairs of abdominal ganglia connected with, the lumbar branches are united by filaments to form the semilunar ganglion.