Section I. OF THE NEVES.
The nerves are whitish cords made up of one or more threads composed of proper nervous matter enclosed within delicate processes or tubes of the membrane called neurilemma; each nerve is connected either directly or indirectly, by one extremity to the centre of the system and by the other to the skin, organs of senses, or muscles. In classifying the nerves anatomists have been influenced rather an attention to their numerical order, than to the essential properties by which they are distinguished.
Tltey have been very generally arranged since the time of Willis as follows:
I. Nerves of the Cerebrum.
- The olfactory nerves.
- The optic nerves.
- The common motor nerves of the eyes.
- The pathetic nerves of the eyes.
- The trigeminal nerves.
- The external motor nerves of the eyes.
- The seventh pair, consisting of the hard and soft portions.
- The vagum and its accessory nerves.
- The motor nerves of the tongue.
- The sub-occipital nerves.
II. Nerves of the spinal cord. III. Great sympathetic or intercostal nerves.
This arrangement is extremely defective, for it is not even correct according to the numerical principle on which it is founded. It was modified by Soemmering who admitted forty-three pairs of nerves; he increased the number of cerebral nerves to twelve pairs, by dividing the seventh pair of Willis into the seventh or facialis and the eighth or auditory nerves; the eighth pair was in a similar manner divided into the ninth or glossopharyngeal, the tenth or nervus trigeminus, and the eleventh or accessory; the ninth pair in the old enumeration became the twelfth, according to Soemmering. The sub-occipital was properly considered as the first of the spinal nerves, the number of which was thus made to thirty pairs.
The objections urged to both these classifications are of a more important nature than that which merely relates to numerical errors. The principles on which they are founded are altogether trivial and unnecessary, and they are still more objectionable in a physiological point of view, because they include under the same denominations nerves whose functions are totally dissimilar.
The division like that of Willis or Soemmering can only be serviceable in oral demonstration, or in those anatomical works that are intended to supply the face of it but in a treatise designed to illustrate the organization or the functions of the body, the arrangement of the various parts ought to be founded as strictly as it is possible with a reference to their structure and use.
The nerves of the cerebro-spinal system may be divided into those with double and those with single roots. The nerves of the first class are attached to the anterior and posterior columns of the spinal cord; they consist of thirty-one pairs, viz, the spinal nerves (with the exception of the accessory) and the trifacial nerves of the head. These nerves have been proved by Mr. Bell to be subservient to the production of sensation and of voluntary motion. They constitute in the aggregate what that distinguished physiologist calls 'the symmetrical intern of nerves.' The nerves with single roots are generally connected with one function only, viz., with sensation or motion: those of the former class are either sensory, the optic, and the auditory nerves; the motor nerves are the third, fourth, and sixth pairs belonging to the eye, and the sublingual or muscular nerves of the tongue.
There are certain other nerves which also have single roots, but whose offices have not been so satisfactorily determined; they consist of the hard portion of the seventh, the glossopharyngeal, the pneumo-gastric, and the hypoglossal accessory. These nerves, together with the fourth pair, the sublingual, the phrenic, and the posterior thoracic, are called by Mr. Bell the superadded or respiratory. The arguments and experiments by which this eminent physiologist supports his peculiar views are extremely ingenious, and at the time when they were published, they appeared to be perfectly conclusive. The subsequent researches of Mr. Mayo have thrown, however, considerable doubts on the correctness of some parts of this celebrated theory. Mr. Mayo concludes that the portion dura of the seventh, and that portion of the tenth pair which does not enter the Gasserian ganglion, are the voluntary nerves of those parts of the face which receive their sentient nerves from the ganglionic portion of the fifth. He also supposes that some of the branches of the glossopharyngeal, the pneumo-gastric, and the spinal accessory, are nerves both of sensation and motion.
As there is so much uncertainty concerning the nerves of respiration, it appears to be most correct to refer the cerebral and spinal nerves, according to their functions, to the three following classes: 1. Those of common sensation; 2. Of peculiar sensation; 3. Of voluntary motion.
The form of the nerves is in general cylindrical but some of them are flat, and others of a triangular shape.
Although the nervous cords become successively smaller as they divide, yet the branches taken collectively are larger than the trunks from which they proceed; so that if the entire distribution of a nerve could be correctly represented, it would have the form of a cone, the point corresponding to the origin of the nerve and the base to its termination. We learn from this fact that the quantity of nervous matter increases as the nerve passes from the brain; this is an evident proof that the former is not a production of the latter, because in that case the reverse would be observed.
The disposition of the nerves presents three points for consideration: 1. Their origin; 2. Their course; 3. Their termination.
Definition of the Origin of a Nerve. We understand by the origin of a nerve the extremity which is attached to the encephalon, or spinal cord. Some anatomists consider the origin to be placed exactly at the point where the nerve completely leaves the external surface of the above organs; thus they say that the origin of the optic nerves is in the commissure of the tractus opticus, and not in any of those parts with which these tracts are connected behind. It is, however, necessary to trace the nervous roots beyond the mere surface, so that their real connections may be understood; and in order to accomplish this object the parts must be hardened by the continued action of alcohol. When a nerve is examined in this manner, its fibres may be followed as far as the grey substance of the medulla oblongata or spinalis. It is a very common opinion that the nerves arise from the fibrous substance and that they are, in fact, simply prolongations of it; but it is now ascertained that the nerves always communicate, more or less, with the grey matter. This connexion, which appears to have been noticed by Vicq-d'Azyr, has been particularly insisted upon by Gall, to whom we are indebted for a knowledge of its importance.
In order to render the description of the origin of the nerves intelligible, it is essential to offer a few observations on the structure of the spinal cord. The reader will recollect that in tracing the development of this part in the foetus, it was stated that it consisted, in the first instance, of two lateral columns which subsequently coalesced on the median line. In the adult there still remains an indication of the primitive condition, the nervous mass of the spine being composed of two similar and lateral halves which are separated to a certain depth by an abdominal and a dorsal furrow. These cords are united in the interior by some communicating nervous substance, which is named the commissure.
Each lateral column is marked by an anterior and posterior furrow, which receive the roots of the spinal nerves; there is also a third and shallow groove on either side between the posterior lateral and median furrows. In this manner it appears, as Mr. Bell some years since informed us, that each lateral half of the spinal mass is divided into three distinct longitudinal cords.
The spinal cord is enlarged opposite to the origin of each nerve, so that it exhibits a nodulated appearance.
Several writers contend, that all the nerves of the cerebro-spinal system may be traced to the spinal cord and the medulla oblongata; but from this statement the first, and perhaps the second pairs, ought to be excluded because they are certainly connected with the cerebrum. With respect to the other cranial nerves, it is well known that as high as the sixth pair or the motor extemi, they are immediately attached to the medulla oblongata, and the others may be shown to be connected either directly or indirectly with that important structure.
Gall considered the spinal cord to consist of a series of longitudinal cords which are united to each other by nerves over; in the manubrium, both nodules and to each other, they turn into a cord or sheath. This is the opinion of Gall, Sylluste Nerveux, p. 90. A similar opinion is supported by Mr. Mayo who compiles the white matter from which these spinal nerves proceed. In the manubrium, both nodules are interlaced together, they turn into a cord or sheath. This is the opinion of Gall, Sylluste Nerveux, p. 90.
Each nerve of the spinal cord has two roots, an anterior and a posterior, which being placed in the lateral furrows, are separated from each other by the dentinal canal ligament. The filaments which compose the anterior root are more irregular in their disposition than those of the posterior root; they are also smaller and more numerous. The posterior fibres are arranged in regular series, and pass out more abruptly from the spinal cord than the anterior fasciculi. There is a difference of opinion as to the respective volume of these two roots; it is stated by Gull that the posterior origin is the larger one but according to Desmonilios this only applies to the nerves of the brachial or axillary plexus; in the dorsal nerves, both roots are nearly equal while in those of the loins, the anterior root predominates in size over the posterior. The fibres of the two roots may be followed into the substance of the spinal cord when they are observed to be partly continuous with the fibrous matter of the corresponding column and partly with the terminal extremities of the grey matter. Each root performs the theca verticularia separately; a ganglion is then formed on the posterior fasciculus, and immediately afterwards the two roots unite as to form one nerve. The fact of the interior root being entirely unconnected with the ganglion was distinctly noticed by Monro.
It has been supposed by some anatomists that there is a hypoplasion between the roots of the nerves; in section it shows, however, that this intercrossing of the fibres does not exist and also that there is not any communication across the median line, between the nerves of the two sides of the body. We must except from this statement the optic nerves which according to the best authorities partially decussate in their commissure. The pathetic ought also to be distinguished because they are frequently united at their origin by transverse bands and likewise the auditory, which are occasionally connected by some white strie in the floor of the fourth ventricle.
The nerves take their course in a direction which corresponds, more or less, with the long axis of the body; but near their origin they pass differently; thus the spinal nerves run outwards nearly at a right angle with the trunk. The nerves which are called the respiratory are more irregular in their disposition and as a consequence of their direction, they often cross the preceding nerves so as to produce a very complex appearance in the face, throat, and neck. The nervous trunks, as they pass towards their destination, furnish branches which commonly separate at an acute angle; they also form numerous communications with nerves of the same and of different classes.
The mode in which the nerves terminate is not known, with the exception of the optic and the auditory nerves, each of which expands into a delicate and transparent pulp that becomes opaque after death. When the nervous fibres are traced towards the different organs it is observed that the neurilemma either becomes extremely thin or is altogether lost so that there is little else than the nervous matter, which being very soft cannot be followed to its ultimate termination. It is impossible to ascertain the nature of the connexion that exists between the nerves and the parts they supply; all that can be seen by a careful inspection is that the fibres just before they appear to end are generally conjoined and rather fattened; they then suddenly escape observation while their size is still considerable, being about twelve times larger than the muscular filaments.
<Callout type="important" title="Key Classification">The division of nerves into cerebral and spinal systems helps in understanding their functions.</Callout>
<Callout type="tip" title="Nerve Origin">Tracing the origin of a nerve beyond its surface attachment is crucial for understanding its full function.</Callout>
<Callout type="risk" title="Respiratory Nerves Uncertainty">The uncertainty surrounding respiratory nerves highlights the need for caution in applying historical medical knowledge to modern practices.</Callout>
Key Takeaways
- The nervous system is divided into cerebral and spinal systems based on their functions.
- Each nerve has two roots, an anterior and a posterior, which are crucial for its function.
- Tracing the origin of nerves beyond the surface attachment provides important insights.
Practical Tips
- Understand that the nervous system can be divided into cerebral and spinal systems to better grasp their functions.
- Always trace the full path of a nerve from its origin to termination, as this will provide critical information for medical applications.
- Be cautious when applying historical techniques in modern survival medicine due to potential uncertainties.
Warnings & Risks
- The uncertainty surrounding respiratory nerves indicates that some knowledge may not be fully understood or accurately applied today.
- Historical anatomical classifications can be misleading and should be cross-referenced with current medical literature.
- Always verify the accuracy of historical techniques before applying them in a survival context.
Modern Application
While the classification and structure of nerves remain relevant, modern medicine has advanced our understanding significantly. Techniques for tracing nerve origins and functions are now more precise, but the foundational knowledge from this chapter still provides valuable insights into the nervous system's organization and function.
Frequently Asked Questions
Q: What is the significance of dividing nerves into cerebral and spinal systems?
Dividing nerves into cerebral and spinal systems helps in understanding their specific functions. Cerebral nerves are associated with the brain, while spinal nerves connect to the spinal cord, each serving distinct roles in sensation, movement, and other bodily processes.
Q: How can one trace the origin of a nerve accurately?
To trace the origin of a nerve accurately, anatomists recommend examining beyond the surface attachment. This involves hardening the tissue with alcohol to follow the fibers deeper into the brain or spinal cord, providing insights into their full function.
Q: What are the risks associated with applying historical knowledge about respiratory nerves?
Historical knowledge about respiratory nerves may be uncertain and could lead to misapplication. It's important to cross-reference this information with current medical research to avoid potential hazards in survival scenarios.