Book - The Nervous System of Vertebrates (1907) 6
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Chapter VI. Somatic Afferent Division. General Cutaneous Subdivision
In the skin of all vertebrates certain fibers belonging to the dorsal nerves end by free branches between the cells of the epidermis (Fig. 48). These are the fibers of the sense of touch. In the trunk these fibers form the largest component of the dorsal spinal nerves and are distributed by way of both dorsal and ventral rami. The rami reach the skin in the myosepta and may be distributed forward or backward from the myoseptum in which they run, or both forward and backward, i.e. to the two adjacent segments. A diagram to show the central relations of the general cutaneous components is given in Fig. 49.
FIG. 48. General cutaneous endings in the cyclostome Lampetra Wilderi. B is taken from the border of a neuromast pit.
Passing forward into the head we find a region between the permanent first spinal nerve and the vagus in which general cutaneous nerves are apparently absent. In cyclostomes, however, the full number of cutaneous nerves are present, and the same is apparently true in the primitive selachian Heptanchus, as shown in Figure 2. In the embryo of other selachians there is one vestigeal ganglion. for each segment, which probably represents a cutaneous nerve. Even in mammalian embryos, as in the pig (Fig. 20) and in man (Fig. 32), rudimentary ganglia which may represent the cutaneous roots of this region accompany the spinal accessory roots. These remain in the adult as a few scattered clumps of ganglion cells among the vagus roots. The reason for the failure of these nerves to develop has been suggested in a previous chapter (p. 65). Since a certain number of somites in this region have disappeared and the trunk somites have shifted forward, it is probable that the cutaneous area has been lessened and one or more nerves have disappeared in most vertebrates. The arrangement of the cutaneous components in the cranial nerves varies in different vertebrates. In cyclostomes such fibers form large components of the X, IX and VII roots. In selachians, ganoids, bony fishes and amphibia general cutaneous components are found in the X and IX roots, but rarely (Kingsbury) in the VII root. In mammals and man a general cutaneous component in the X nerve has its cells of origin in the jugular ganglion and forms the ramus auricularis. A rudimentary root ganglion in the IX nerve probably represents this component (Figs. 20, 30, 31, 32). In all vertebrates the sensory trigeminus and the ophthalmicus profundus nerves are formed chiefly or exclusively of general cutaneous fibers. Finally, that small nerve which is found connected with the forebrain in many selachians, Amia and Protopterus, the nervus terminates, is probably general cutaneous in character.
FIG. 49. A diagrammatic representation of the general cutaneous components of a trunk segment.
The distribution of the general cutaneous components is relatively simple. The arrangement is shown in a generalized scheme in Figure 50. In cyclostomes (Fig. 51) the dorsal spinal nerves in the branchial region send dorsal rami to the skin of the back and their ventral rami join the epibranchial trunk. From this the cutaneous fibers are distributed by the posttrematic rami to the skin of the lateral and ventral surface of the gill region. The cutaneous component in the vagus root goes to the first posttrematic ramus of the vagus. In true fishes and all higher forms the cutaneous component of the X root is distributed by dorsal rami to the dorsal surface of the head. In fishes provided with an operculum general cutaneous fibers are not found in the branchial rami, but a ramus from X helps to supply the skin on the operculum. The cutaneous components in the branchial nerves of selachians have not been described. The cutaneous component in the IX nerve in cyclostomes goes by way of the ramus posttrematicus to the skin of the region of the first branchial arch. In other vertebrates only a dorsal ramus is present.
FIG. 50. A simple diagram of the general cutaneous components in the cranial and spinal nerves of a fish. The dotted lines (except R. opercularis) represent components present in cyclostomes but not yet found in other fishes.
The absence of a general cutaneous component from the VII nerve in most vertebrates requires to be explained. In forms provided with an operculum, the operculum is innervated by rami from the V and X nerves and rami from the V nerve supply the ventral surface in the gill region. In cyclostomes, where there is no operculum, a simpler and more primitive arrangement is found.
There is a large cutaneous component in the VII nerve which is distributed by way of the posttrematic ramus to the skin of the hyoid segment, just as in the more caudal branchial segments. The skin of the corresponding dorsal area, however, is innervated by rami from the V nerve. From this it would appear that the hyoid segment had originally its own cutaneous innervation and that when the operculum was formed the component in VII disappeared and the V nerve supplied the parts of the hyoid segment remaining exposed.
FIG. 51. A reconstruction of the cranial nerves of a cyclostome fish, Petromyzon dorsatus, to show the arrangement and distribution of the several systems of nerve components. Sym., sympathetic trunk; 2 g. c., second gill cleft.
FIG. 52. The principal sensory collaterals in the spinal cord of the new-born rat. From Cajal (Textura, etc.). A, visceral sensory collaterals to the intermediate nucleus; B, collaterals to the somatic motor column; C, collaterals to the somatic sensory column, dorsal horn; b, branches of somatic motor reflex collaterals'ramifying in the intermediate nucleus (visceral motor); c, deep collaterals to the substance of Rolando.
The trigeminus gives small branches to the dorsal surface of the head behind the eye, and in fishes it gives a larger ramusophthalmicus superfacialis trigemini to the skin above the eye. It then forms the rami maxillaris and mandibularis which supply the skin of the upper and lower jaws and the lining of the stomodaeum.
The ophthalmicus profundus nerve supplies the area in front of the eye to the tip of the snout. In tailed amphibians the maxillary nerve is greatly reduced and the profundus takes on the innervation of the territory of the maxillaris.
The Central Apparatus For Cutaneous Impulses
This consists of the dorsal horn of the gray matter in the spinal cord and secondary tracts and centers connected with it, and of corresponding structures in the brain. The cutaneous fibers have their ganglion cells in the spinal ganglia. The central processes of these ganglion cells enter the dorsal part of the cord and there bifurcate in T- or Y-form. The two branches run one cephalad and one caudad, forming the dorsal tracts of the cord. In man it is known that the cephalic branch is the longer and this seems to be the case in lower vertebrates as well. In man and mammals the cephalic branches of fibers in the more caudal roots are pushed toward the median plane by the incoming fibers of the successive roots farther forward, so that the long cephalic branches come to form a definite bundle (the mesial funiculus) mesial to the fibers of the more cephalic roots (the lateral funiculus). Each branch of a cutaneous fiber as it runs forward and backward in the cord gives off fine collateral branches. These are distributed to the same and to the opposite side, to the ventral horn, and to other parts of the cord as shown in Figs. 41, 52. The collaterals going to the ventral horn serve for the simplest reflexes. The other collaterals spread the impulses widely through the adjacent regions of the cord and so make possible more ample responses. The ascending and descending branches end finally in the dorsal horn. While the collaterals make connections which serve for relatively simple and direct reflexes, the connections by way of the dorsal horn serve for complex reactions and for sensation.
The structure of the dorsal horn and its secondary connections are best understood in the region at the junction of the spinal cord and brain where the dorsal tracts of the cord have their ending and where a part of the cutaneous fibers of the head also end. In this region the dorsal horn is considerably enlarged in all vertebrates and in man it forms the two large nuclei of the dorsal funiculi. In lower vertebrates there is a siagle large nucleus of the dorsal funiculus. This nucleus consists of large and small nerve cells imbedded in a very rich and intricate interlacing of their own dendritic branches and of the end-branches of cutaneous nerve fibers. The small cells have relatively short and simple dendrites and send their neurites among the cells of the dorsal horn itself either at the same level or farther forward or backward. Such cells are seen in Figs. 53, 56. The function of these cells is to spread the incoming impulses more completely to all parts of the nucleus. A part of the neurites of these cells pass across the median plane dorsal to the canal, forming a part of the dorsal commissure of the cord, and end in the dorsal horn of the opposite side (Fig. 53).
FIG. 53. Some cells in the dorsal horn of the chick embryo of five days. From Cajal (Textura, etc.). A, dorsal root; B, transverse cell of the substance of Rolando; C, cell of the substance of Rolando; D, cell of the posterior horn whose neurite goes to the dorsal commissure; E, interstitial cell whose neurite goes to the dorsal commissure; F, dorsal commissure.
FIG. 54. Transverse section of the substance of Rolando in the cervical cord of the new-born cat. From Cajal (Textura, etc.). A, cells of the vertex of the dorsal horn; D, C, 'cells of the substance of Rolando; E, deep collaterals; F, endings of same; a, neurites.
The large cells have larger dendrites with long branches which spread through the dorsal horn and the dorsal tracts (Figs. 54) 55> 57)- The neurites go ventrally beyond the dorsal horn, either in the gray or the white matter; and, crossing the median line ventral to the central canal, form what appears in transverse section as a loop. They are therefore spoken of as arcuate fibers. The greater number of them run deeply imbedded in the wall of the cord (or brain) and are known as internal arcuate fibers to distinguish them from the remainder which have a part of their course on the surface and are known as external arcuate fibers. The internal arcuate fibers after reaching the opposite side of the cord or brain bend forward and help to form the ventro-lateral fiber tracts. Continuing forward these fibers collect into a bundle which is more definitely limited in higher vertebrates than in fishes. In man this bundle is known as the ascending lemniscus. In fishes the fibers go to the nuclei in the tectum mesencephali and form what is known as the tractus bulbo-tectalis. Its relations in mammals are more complex and will be treated later (Chap. XVI).
FIG. 55. Cells in the dorsal horn of the cord of a chick embryo of nineteen days incubation. From Cajal (Textura, etc.). A, large marginal cell; B, giant cell of the center of the horn; C, cell of the interstitial nucleus; a, b, c, neurites.
In man the cutaneous fibers in the cranial nerves (trigeminus and vagus) form a longitudinal tract in the medulla oblongata which corresponds to the dorsal tracts of the spinal cord and continues caudally to the level of the first cervical nerve, the tractus spinalis trigemini. A small part of the fibers of the trigeminus, however, run directly to the cerebellum. The tractus spinalis trigemini is accompanied by an end-nucleus which is directly continuous with the gelatinous substance of the dorsal horn of the cord. This substantia gelatinosa is made up of small and large cells. The neurites of the small cells are either short, ending in the immediate vicinity, or are directed forward or backward within the substantia gelatinosa itself. The neurites of the large cells go as internal arcuate fibers to the opposite ascending lemniscus or to a part of the lemniscus system on the same side. The structure of the substantia gelatinosa of the rabbit is shown in Figure 56. The primitive condition of the general cutaneous centers as seen in lower vertebrates differs considerably from this. In fishes the trigeminus and ophthalmicus profundus fibers may or may not bifurcate on entering the medulla oblongata. When they bifurcate one branch goes to the cerebellum and the other either into the tractus spinalis trigemini or into the deeper part of the tuberculum acusticum. When they do not bifurcate the fibers may take any one of these courses. Further, in selachians, ganoids and amphibians a bundle of very coarse fibers enters the brain with the sensory trigeminal root and ends in the tectum mesencephali. This may be called the -mesencephalic root of the trigeminus and corresponds in position to the bundle which in man is usually described as a motor root. Thus in lower vertebrates the cutaneous fibers are widely distributed through all that most dorsal column of gray matter which connects the dorsal horns of the cord with the cerebellum and the dorsal part of the mesencephalon. Indeed, in cyclostomes there is no marked distinction in structure between the different parts of this column.
Fig. 56. Transverse section through the spinal V tract and the substance of Rolando of a new-born rabbit. From Cajal (Beitrage u. s. w.). A, ventral part of the root; a, interstitial cell; c, border cell; d, cell-islands of the substantia gelatinosa; e, small cells of the islands; /, stellate cells not arranged in islands; g, cells between islands; h, a cell whose neurite seems to go into the white substance or toward the root.
FIG. 57. A transverse section of the spinal cord of a cyclostome fish, Lampetra Wilder i to show the cells of the dorsal horn (d. h.}.
FIG. 58. A, transverse section of the tuberculum acusticum of Lampetra; B, a sagittal section of the cerebellum of the same animal, ac., acusticum.
The structure and secondary connections of these centers in cyclostomes may now be described as showing the most primitive known condition of the general cutaneous centers in the brain of vertebrates. The nucleus funiculi, although simple, has essentially the structure above described. The tuberculum acusticum differs from it in being somewhat more voluminous and in possessing a larger number of cells. The neurites of the small cells form a thin layer of fine fibers covering the outer surface of the acusticum and cerebellum. This represents the molecular layer and the cell layer corresponds to the granular layer, which are such conspicuous features of the cerebellum of higher forms. The large cells are larger and more conspicuous and are arranged with more regularity, their bodies being near the fourth ventricle and their dendrites spreading in the fiber layer. Then neurites of these cells go as internal arcuate fibers to join those from the nucleus funiculi and run forward. The large cells of the acusticum and of the cerebellum are shown in Fig. 58. The cerebellum is very small and consists of a continuation of the acusticum forward, upward and mesially so that the two halves of the cerebellum meet over the fourth ventricle. Here there occurs a commissure formed by the neurites of the small cells. By means of these fibers the cutaneous impulses are carried from the centers of one side to those of the other. In histology also the cerebellum differs little from the acusticum. Its large cells are not specialized as are the Purkinje cells in the cerebellum of higher forms, although by comparing them with the Purkinje cells in other fishes one can see that they are the forerunners of such cells. The neurites of the large cells go as internal arcuate fibers to the opposite side of the brain, but whether they join those from the nucleus funiculi and acusticum is uncertain. The latter fibers pass forward and upward to enter the roof of the mesencephalon, forming the tractus bulbo-tectalis.
The roof of the mesencephalon is thus a secondary center for general cutaneous impulses. Its structure and the differentiation of centers in it will be described in the chapters on the visual apparatus and on the higher correlation centers (Chaps. VIII and XVI). Here it is necessary only to state the course of tracts arising in the tectum mesencephali which are of importance for the cutaneous apparatus. The first is a tract of fibers which arise from the cells of the tectum and run in a direction opposite to, but parallel with that of the tractus bulbo-tectalis and have their endings in the medulla oblongata or spinal cord. The tract is called the tractus tecto-bulbaris et spinalis. It descends from the tectum over the outer surface of the mesencephalon and bends backward along the ventro -lateral surface of the myelencephalon.
FIG. 59. A diagram representing the centers and fiber tracts related to the general cutaneous components in fishes.
Here the fibers give collaterals inward and themselves turn inward to come into relation directly or indirectly with the cells which give origin to the motor nerves. A part of the tract, when it descends over the lateral face of the mesencephalon, instead of going backward along the same side of the brain crosses to the opposite side through the ventral wall of the mesencephalon, helping to form the large ventral commissure of this region of the brain. The fibers then join the tract of the opposite side and continue with it to similar endings. The tractus tecto-bulbaris et spinalis thus consists of a crossed portion and of an uncrossed or direct portion. Another tract from the tectum descends over the side of the mesencephalon and bends forward to end in the inferior lobe, the tractus tecto-lobaris. By means of tracts which run from the inferior lobes to the forebrain and to the hindbrain and spinal cord, probably wider connections are set up and more complex correlations provided for. The several central tracts and nuclei belonging to the cutaneous apparatus in fishes are shown in the accompanying diagram (Fig. 59). The outline of a selachian brain is drawn schematically and the various tracts are shown as seen from the left side and as if projected upon the median plane. The cutaneous components in the roots of the V, VII, IX, and X nerves are shown, but the ganglia of those nerves are omitted. In Fig. 60, the cutaneous fibers and the ascending lemniscus in man are shown.
FIG. 60. A diagram representing the general cutaneous centers and fiber tracts in the human brain.
In higher vertebrates the acusticum and the cerebellum become more and more higher specialized in their histological structure and secondary connections. Since this specialization is largely due to the fact that these are the centers for the special cutaneous (acustico-lateral) system of nerves, the special structure and the course of differentiation will be described in the next chapter. At the same time that the specialization is going on the general cutaneous fibers for the most part cease to enter the acusticum and cerebellum and come to be concentrated in the spinal V tract, so that in higher vertebrates by far the greatest part of the cutaneous fibers end in the nuclei of the spinal V tract and of the dorsal funiculi. While this concentration of the primary tracts and nuclei is going on, a change takes place in the secondary connections as well. The secondary tract from the spinal cord and the nucleus funiculi, the lemniscus, now ends only in part in the roof of the mesencephalon. A larger part of its fibers end in nuclei situated in the lateral walls of the diencephalon. (See Chapter XVI.)
We should expect the general cutaneous system of nerves to be the most primitive and widespread part of the nervous system, unless the general visceral nerves be excepted. And so it is, but there is reason to think that even in the lowest vertebrates the system has undergone considerable modification. A pair of cutaneous nerves and corresponding centers is to be expected in each segment, but in cyclostomes the first five segments of the brain are without cutaneous roots and the corresponding area of the skin is supplied by nerves whose roots enter the brain in the sixth neuromere. The fact that one of these, the ophthalmicus profundus, arises in the embryo from the fifth neuromere is taken as evidence that it once held that position in the adult. Even so, the first four segments of the head are without cutaneous nerves of their own and are supplied by cutaneous rami coming from more caudal segments. In selachians, however, the N. terminalis connected with the first neuromere is probably the cutaneous nerve of that segment. Reasons will be given in a later chapter (Chapter VIII) for thinking that the cutaneous nerves of the second, third and fourth segments have been modified into visual organs, while the ophthalmicus profundus has come to supply the corresponding area of skin. In higher forms the differentiation of the gills and the development of an operculum have led to changes in the general cutaneous components, such that the VII and in some cases the IX nerve are without cutaneous fibers. Then in the occipito-spinal region a variable number of cutaneous nerves have been gathered into the single vagus root and others have disappeared on account of the shifting of the mesodermal organs and shortening of the cutaneous area. Within the brain correlated changes have taken place. A general cutaneous center probably persists in the forebrain of selachians but little is known of its structure or relations. In other vertebrates the most cephalic cutaneous center is the tectum mesencephali. The ophthalmicus profundus nerve once arose from this segment of the brain and the trigeminus from the cerebellar segment. Both of these regions still receive general cutaneous fibers, at least in lower vertebrates, but both these and the tuberculum acusticum have lost most of the fibers of this component which once entered them.
All these facts may be expressed or implied in a word by saying that there has been a process of concentration of the tactile apparatus of the head toward the caudal part of the cranial region. This has been due in part to the usurpation of the cephalic part of the brain by highly specialized somatic sensory organs, the eyes and acustico-lateral system, and in part to some undefined advantage that is probably gained by the concentration of a system of nerves and centers instead of their being equally distributed segmentally. Such is the general cutaneous system; the most primitive and the least specialized system of nerves and centers, yet progressively more and more modified in its arrangement, chiefly through the influence of more highly specialized organs.
Demonstration of Laboratory Work
- Review dissections of the dorsal spinal nerves and the trigeminus.
- Trace the position and relations of the dorsal tracts, acusticum, and cerebellum in the brain of a large fish, bullfrog and a mammal, by dissection.
- Trace the spinal V tract in Delafield haematoxylin or Weigert sections of a fish, frog or mammalian brain. Note its relations with the dorsal tracts of the cord, and the relations of the substantia gelatinosa of medulla oblongata and cord.
- In Weigert sections of the brain of a selachian or frog, look for the sensory fibers of the trigeminus running to the tectum mesencephali and to the cerebellum.
- Study the nerve elements in the dorsal horn, acusticum, substantia gelatinosa and cerebellum in Golgi sections of the fish brain.
- Trace the ascending lemniscus in fish, frog or mammal in transverse or sagittal sections by the method of Weigert.
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