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SPECIAL PHYSIOLOGY OF NERVES.
The thirty-one pairs of nerves which leave the vertebral canal by the openings between the vertebræ are called spinal nerves, in contradistinction to the cranial nerves, which pass through the base of the skull. They are attached to the spinal marrow by two bands, the anterior and posterior roots, which unite together in the intervertebral canal to form the trunk of the nerve. Just before the junction of the two roots the posterior one is enlarged by a ganglionic swelling.
The spinal nerves are all “ mixed nerves,” that is to say, they contain both efferent and afferent fibres ; but these two sets of fibres are separate in the roots of each nerve, the posterior root containing only afferent, and the anterior only efferent fibres. The spinal nerves are thus joined to the spinal marrow by two nervous cords, each one of which is functionally distinct. About seventy years ago Charles Bell discovered that the anterior roots were motor, and the posterior sensory channels. Hence, the anterior are commonly spoken of as the motor roots, and the posterior as the sensory roots of the spinal nerves. The experiments to show this difference are simple, but require delicate manipulation.
If the anterior roots of the nerves supplying the hind leg of a recently-killed frog be divided, the muscles of the limb are cut off from the centres in the spinal cord, and the leg hangs limply, and does not move if pinched when the frog is suspended ; whereas the limb on the sound side, upon which the anterior roots are intact, will move energetically when the motionless one is irritated. If the distal ends of the divided anterior roots be stimulated, the muscles of the paralyzed limb are thrown into action; but stimulation of the proximal end gives no result. the two webs of this frog be compared, the blood vessels running across the transparent part of the web on the injured side will be found to be fuller than those in the web of the other limb, but if the distal ends of the motor roots be stimulated, the dilated blood vessels return to their normal calibre. By these experiments we are shown that efferent fibres carrying impulses to the muscular walls of the vessels are contained in the anterior roots of the spinal nerve, together with fibres to the skeletal muscles.
Posterior Roots.—The fact that when the leg on the side where the anterior roots have been severed is stimulated the animal moves the other, is sufficient to show that the sensory connections between its surface and the cord are not destroyed by cutting those anterior roots; and we may conclude-taking the other facts just mentioned into account—that the afferent fibres are situated in the posterior roots.
We can confirm this result by cutting the posterior roots on one side of a recently-killed frog, and repeating the stimulation of the feet.
Pinching the limb whose posterior roots are cut, gives rise to no response, because the impulses cannot reach the spinal cord; but stimulation of the sound foot causes obvious movements of both legs. This shows that the section of the posterior roots of one limb cuts off the afferent (sensory) communication on the side operated on, but that the efferent (motor) impulses can pass freely to the muscles, even when the posterior roots are divided, for the limb moves on pinching the other foot. If the proximal ends of the cut posterior roots be stimulated, motions are produced showing that the centres in the spinal cord are influenced by the afferent impulses carried by those posterior roots. distal ends of the cut roots be stimulated no movement results.
Recurrent Fibres.-It has been sometimes found that stimulation of the anterior roots seemed to cause pain, as shown by the motion of other parts besides those to which this root was distributed ; and it was believed that some sensory fibres must run in the anterior roots. But it has been found that if the corresponding posterior roots be cut these signs of pain when the anterior roots are stimulated are not shown. From this it has been concluded that the apparent sensory channels of the motor roots
are nothing more than some sensory fibres which pass from the nerve trunk a little way up the motor root, and then turn back and descend again to the junction of the roots, whence they pass along the posterior root to the cord. These fibres are named the “recurrent sensory fibres," and the recurrent sensibility of the anterior roots is not regarded as any serious departure from Bell's law.
The course of the secretory, etc., nerves probably follows that of the motor channels at their exit from the cord. Their peripheral distribution, and that of the vasomotor nerves, are
Two cells from the former
seen under a high power, Section through spinal ganglion of a cat, showing ganglion cells showing the fine protointerspersed between the fibres. (Low power.)
plasm here and there retracted fromthe cell wall.
intimately connected with the sympathetic system, and will be considered further on.
Of the function of the ganglia on the posterior roots of the spinal nerves but little is positively known. There is no evidence of their being centres of reflex action, nor can they be shown to possess any marked automatic activity. From the fact that when a mixed nerve is divided the end cut off from the ganglion degenerates after a few days, these ganglia are supposed to preside over the nutrition of the tissue of the nerve itself. And if the roots be cut, that part of the posterior root attached to
the cord degenerates, while the piece connected with the ganglion is well nourished. This is not the case if the anterior root be divided, but, on the contrary, that portion next the cord is well nourished, while that connected with the posterior root is degenerated.
It would thus appear that the trophic function of the ganglia is restricted to the sensory nerves, while the nutrition of the motor nerves is provided for by nervous centres situated higher up.
THE CRANIAL NERVES. The nerves which pass out through the foramina in the base of the skull must be considered separately, as the function of each of them shows some peculiarity. Some are exclusively nerves of special sense, some are simple, being purely motor in function, while others are exceedingly complex, containing many kinds of fibres. They may be taken in the order of their functional relationships, motor and mixed. Those which relate to the special senses will be considered in future chapters.
III, THE MOTOR OCULI NERVE. The nerves of the third pair are efferent, being the chief motor nerves of the eyes. They arise from the gray matter on the floor and roof of the aqueduct of Sylvius, pass out of the brain substance near the pons from between the fibres of the peduncle, and run between the posterior cerebral and superior cerebellar arteries. They pass into the orbits in two branches, and are distributed to the following orbital muscles : (1) elevator of the eyelid, (2) the superior, (3) inferior, and (4) internal recti, and (5) the inferior oblique. They also contain fibres which carry efferent impulses to (1) the circular muscle of the iris, and to (2) the ciliary muscle. The latter branches reach the eye by a short twig from the inferior oblique branch, which goes to the ciliary ganglion, and thence enters the ciliary nerves.
The action of the orbital muscles is, in the main, under the control of the will, though they afford good examples of peculiar coördination and involuntary association of movements. The reflex contraction of the pupil by the action of the circular muscle (sphincter pupillæ) is a bilateral act, the afferent impulse of which originates in the retina, passes along the optic nerves, and is transmitted, from the corpora quadrigemina, to both the third nerves. The central extremities of the third nerves must have an intimate connection with each other and with the optic nerves, for the diminution in size of both pupils follows accurately the increase in intensity of the light to which even one of the retinæ is exposed. In retinal blindness and after section of the optic nerve the pupil becomes dilated from loss of the retinal excitation. The action of the ciliary muscle may be said to be voluntary, since we can voluntarily focus our eyes for near or far objects. Contraction of the sphincter pupillæ and of the internal rectus is associated with the contraction of the ciliary muscle in accommodation.
Section of the third nerve within the cranium gives rise to the following group of phenomena : (1) Drooping of the upper lid (Ptosis). (2) Fixedness of the eye in the outer angle (Luscitas). (3) Dilatation and immobility of pupil (Mydriasis). (4) Inability to focus the eye for short distances.
IV, THE TROCHLEAR NERVE. This thin nervous filament arises under the Sylvian aqueduct, and passes into the superior oblique muscle, to which it carries voluntary impulses, which are involuntarily associated with those of the other muscles moving the eyeball. Paralysis of this muscle causes no very obvious impairment in the motions of the eyeball when the head is held straight, but it is accompanied by double vision, so there must be some displacement of the eyeball. When the head is turned on one side the eye follows the position of the head instead of being held in its primary position. In paralysis of this nerve a double image is seen only when looking downward, and the image on the affected side is oblique and below that seen by the sound eye.
VI, THE ABDUCTOR NERVE OF THE EYE. This arises in the floor of the fourth ventricle, and appears just below the pons Varolii. It is the motor nerve of the