Articles from "The reference handbook of the medical sciences" (William Wood & Co., New York, third edition, 1914) / by C. Judson Herrick.

  • Herrick, C. Judson (Charles Judson), 1868-1960
Date:
[1914?]
Catalogue details

Licence: In copyright

Credit: Articles from "The reference handbook of the medical sciences" (William Wood & Co., New York, third edition, 1914) / by C. Judson Herrick. Source: Wellcome Collection.

    6/44 (page 322)
    which he clearly drew between somatic and visceral systems of nerve components has been more and more evidently justified as time passed on. Fig. 1581 illus- trates the chief features of Gaskell’s analysis of the spinal nerves, each segmental nerve containing so- matic afferent and efferent fibers and also visceral affer- ent and efferent fibers, all of the afferent fibers and some of the efferent visceral fibers being ganglionated. The present conception of the composition of the spinal nerves is illustrated by Fig. 1582. _ Here it is seen that the typical spinal nerve contains two great groups of fibers, the somatic and the visceral, each with afferent and efferent subdivisions. The so- matic group is concerned with the adjustments of the body to the outside world (exteroceptive systems of Sherrington); the visceral group (interoceptive) is concerned with the internal mechanisms of circula- tion, digestion, etc., and its connections are made through the sympathetic nervous system (see further under Brain, Anatomy). The somatic efferent fibers are non-ganglionated and pass directly from their cell bodies in the ventral horn of the spinal cord through the ventral roots to end upon the fibers of skeletal muscles. The somatic afferent fibers are formed by the bifurcation of the single processes of the cells of the spinal ganglia which thus give rise to centrally and peripherally directed processes. The central processes form the larger part of the fibers of the dorsal spinal roots and termi- nate, after a longer or shorter course within the spinal cord, about cells of the dorsal horn or nuclei of the dorsal funiculi. The peripheral processes end in re- ceptive organs in or immediately under the skin (exteroceptors) or else in deep sense organs among the muscles, tendons, joints, etc. The latter are termed by Sherrington proprioceptors and serve for the in- ternal regulation of the somatic organs of response and not, like the interoceptors, for visceral reactions. The somatic motor and sensory roots of the cranial nerves are organized in essentially the same way as those of the spinal nerves just described. The visceral fibers of the spinal nerves are less numerous than the somatic fibers and they are chiefly efferent in function. These visceral efferent fibers arise from the intermediate zone of the spinal cord between the dorsal and ventral horns and espe- cially from an intermedio-lateral column of cells lying within and just external to the lateral horn (Bruce). They probably do not arise from the cells of Clarke’s column, as suggested by Gaskell. They do not reach their terminal organs (smooth muscles, glands, etc.) directly, but always end in some sympathetic ganglion, with whose cells they effect functional connection. The impulse is then carried on to the peripheral organ by axones of these sympathetic cells. The neurone of the first order, whose cell body lies within the central nervous system, is termed the pregang- lionic neurone; the neurone of the second order is the postganglionic neurone (Langley). The visceral sensory fibers of the spinal nerves are relatively very few in number. Some such fibers arise from cells of the spinal ganglia, whose periph- eral processes distribute through the sympathetic nervous system to mucous surfaces, etc., and whose central processes enter the spinal cord through the dorsal roots to terminate probably in the intermediate zone of gray matter at the base of the dorsal horn. It is probable that some cells of the sympathetic ganglia also send sensory processes centripetally through the rami communicantes to enter the spinal ganglia and thus after a synapse in the ganglion to discharge into the dorsal roots of the spinal nerves. But our knowledge of these fibers is stiff very incomplete. The cranial nerves exhibit a much more complex and diversified pattern than do the spinals._ The primary segmentation is obscure and there is stiff some difference of opinion as to the segmental rela- tions of the twelve pairs (of Sommering) as now com- monly enumerated. These twelve pairs are conven- ient anatomical units, but for physiological and clinical purposes a more useful unit is the functional system. Each system may be defined as the sum of all the nerve fibers in the body, which possess certain physiological and morphological characters in com- mon, so that they may react in a common mode. Morphologically, each system is defined by the ter- minal relations of its fibers—by the organs to which they are related peripherally and by the centers in which the fibers arise or terminate. The fibers of a single system may appear in a large number of nerves, repeated more or less uniformly in a metameric way (as in the cutaneous nerves of the spinal cord), or they may all be concentrated into a single nerve (as in the olfactory nerve). On the other hand, a single nerve may contain several components; i.e. its fibers may be- long to several of these systems. It becomes neces- sary, therefore, to analyze the root complex of each pair of cranial nerves into its components and to trace not only the central connections of these com- ponents within the brain, but also their peripheral courses as well. In other words, the description of any given ramus is not complete when we have given its point of origin from the nerve trunk or gang- lion, the details of its devious courses and the exact points where the several ramuli terminate. In ad- dition to this it is necessary to learn what functional systems are represented in the ramus and the precise central and peripheral relations of each system. The difficulty in determining this latter point is the chief obstacle in the way of researches on the nerve com- ponents; for, while the central connections of any nerve can be determined by the microscopical method and by various experimental procedures, the peripheral courses are usually studied by gross methods which reveal nothing of the precise relations of the several components and hence do not permit a knowledge of each system as a whole. To trace the components of the more complex cranial nerves continuously from their central nuclei of origin or termination by means of serial sections to their ultimate peripheral connections has never been attempted in the human body. In some lower vertebrates, however, this has been accomplished with a fair measure of success. The pioneer work in this field was done by Dr. Oliver S. Strong in 1895 on the frog tadpole, and this has been followed by many similar studies. _ All of the types thus far examined in this way exhibit certain broad general resemblances and permit the establish- ment of a schematic type of cranial nerve components which is presumably applicable to the vertebrates as a whole. From anatomical, physiological, and patho- logical data already in hand it is possible to compare the human nerves with this scheme and to infer the composition of most of the peripheral rami with more or less of accuracy. In the branchial region of the lowest fishes the gills are very numerous (as many as seventeen pairs in some cyclostomes) and each gill is supplied by a branchiomeric nerve. The segmentation of these gills does not coincide with that of the myomeres of the same region and it is a controverted point whether this diversity is primary or a secondary modifica- tion. But it is clear that in primitive vertebrates the entire branchial region contained greatly enlarged visceral structures, the gills, and also somewhat re- duced somatic structures, the myomeres, or body musculature, and the sensory organs of cutaneous and muscular sensibility. The somatic motor roots in the branchial region of fishes are usually enu- merated as spinal nerves. They all disappear in mammals, except one or more of the ventral roots which are represented in the hypoglossus nerve. In primitive vertebrates (cyclostomes) the typical nerve of this region contains sensory and motor somatic fibers arranged in series of dorsal and ventral roots as