Book - Contributions to Embryology Carnegie Institution No.33
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A Study Of The Superior Olive
By George B. Jenkins,
Professor of Anatomy in the State University of Iowa.
With two plates and one text-figure.
Some years ago, while working under the stimulating influence of Dr. Mall, I undertook at his suggestion the study of the morphology of the human inferior olive. During the progress of that work I determined to extend the investigation to include a detailed study of all the cell collections in the brain stem, both in man and in such of the lower animals as might be available. I now wish to present some of the results of this study as applying to the so-called superior olive, the literature upon which subject is so contradictory and confusing that one gets but a vague idea of this nuclear mass.
It seems probable, from a study of the earlier literature, that the term olive was applied to the inferior or bulbar olive because of the presence of the oval or olive-shaped prominence upon the exterior of the ventro-latcral surface of the medulla; and that later, when the underlying nucleus was discovered, it was quite naturally termed the olivary nucleus. The only apparent reason for calling the smaller pontine nucleus the superior olive was the assumption that it was related to and of similar cell content as the earlier-discovered inferior body. Neither of these conclusions has been borne out by my findings.
In order to arrive at a clearer comprehension of the superior olive, it was determined to use cat tissue for a preliminary study, since all investigators seem to agree that this animal has a large and beautifully developed nucleus. A number of other animals, including dogs, rabbits, field-mice, rats, and ground-squirrels, were used as checks, and all findings were compared with human tissue of varying ages. As a result of these stuches the more representative types were found to be the cat, dog, and man. The tissue figured herein consists of the brain-stem of an adult cat, serial No. Fel. 1, cut in transverse sections 20 microns thick and stained with hematoxylin and eosin; the brain of a dog-fetus, 115 mm., serial No. Can. 1, cut in transverse sections 20 microns thick and stained in Ehrlich's hematoxylin; the brain-stem of an adult dog, serial No. Can. 4, cut in transverse sections 20 microns thick and stained in hematoxylin and eosin; the brain-stem of a human fetus 168 mm. (cr), serial No. Hu. 28, cut in transverse sections 20 microns thick and stained with borax carmine and Lyons blue. All were reconstructed by the Born method, using 2-mm. wax plates. All of the sections of the human specimen were drawn at a magnification of 100 diameters; in the dog and cat specimens every alternate section was drawn at 50 diameters. The adult dog was not modeled, since the outlines of the nuclear masses in the two animals are identical in all essential particulars; and, owing to the comparative scarcity of associated fibers and the simplicity of the cell components, the younger animal offered fewer mechanical difficulties in the way of modeling. The adult was used as a control and for fiber and cell study. In each case one-half of the section was drawn and the median line was used as a straight edge to pile by, a wood form being used as a guide, as described in the study of the inferior olive. It was thus possible to i)ile by the external form of the pons and the ventricular floor, the perpendicular being the fixed plane.
The primary requisite, as shown above, was the selection of such ages and animal forms as would most accurately depict the typical external form of the nucleus. A second condition necessary was the selection of tissue of such a stage of development as would show the fully developed cells in a given animal; e. g., while the fetal dog presented a gross nuclear form and arrangement practically identical with that of the adult animal, the fiber arrangement and cell make-up were entirely different, the younger animal presenting the embryonic cell type, the adult showing the spindle cell which was found to be characteristic of this nucleus.
The superior olive is found in the lateral field of the pons, in the ventral portion of the roughly triangular interval between the nervus abducens and the emergent portion of the nervus facialis, and lies in an indentation in the dorsal surface of the corpus trapezoideum, ventro-mesial to the nucleus nervi facialis, and occupying the lower half of the vertical extent of the pons. The mass begins caudally at the lower hmit of the pons, the exact level varying somewhat in different subjects or even slightly on the two sides of the same subject, and extends cerebrally well up into the region of the nervus trigeminus, the upper limit likewise varying. It is placed definitely dorsal to the trapezium, the fibers of wliich curve ventrally around the nucleus, few, if any, passing dorsal to it. The nucleus nervi facialis, beginning caudally in the upper medulla, extends well up beyond the middle of the ohve and Ues almost in contact with its dorso-lateral surface. Where not in relation to the trapezium and the nucleus facialis, the superior olive is surrounded by the formatio reticularis of the pons. The nucleus nervi abducentis appears subjacent to the ventricular ependjTna, within the loop of the nervus facialis, and at the vertical mid-level of the olive.
Certain of these general figures vary considerably in the different animal forms, in keeping with the laws of development. We find, for example, in a comparative study of this area, abundant evidence that the degree of development of the pons is directly proportionate to that of the cerebellum; and further, that the relations of the various structural and contained elements vary in accordance with the degree of development of given parts. The human subject, with its well-developed cerebellum, jiresents large numbers of cortico-rhombic and cortico-spinal efferents (jiyramidal fibers), and especially large numbers of transverse pontine fibers, both superficial and deep to the cortical efferents; also proportionately large numbers of cells (the nuclei pontis) packed within the interspaces between these fibers. As a natural consequence, the superior olive, in this type, is much more deeply placed and farther from the ventral periphery of the pons; whereas in the lower animals, such as the dog and cat, iii which the cerel)ellum is not so well developed, we find small pyramidal bundles lying entirely superficial to much less numerous transverse pontine fibers, with comparatively few and scattered cells representing the nuclei pontis. Hence in these types the olive is much nearer the ventral surface of the pons. It was found to be more superficially placed in the cat than in the dog.
This area of the pons is very vascular, comparatively large vessels cutting into and between the portions of the nuclear complex. In the medulla the vagus and hypoglossal nerves embrace the inferior oUve, the latter nerve cutting the nucleus and partially separating the so-called accessory from the main nucleus. In the pons we find the same arrangement to some degree, with the abducent and facial-nerves embracing the superior oUve. In the cat and man, however, the abducens runs well medial to the superior olive; whereas in the dog it comes in contact with the medial margin of the nucleus, even cutting through it in one specimen examined (fig. 5). In man the emerging facial fibers run closer to the ohve than do the abducent.
Functionally, the superior ohve is to be classed with the cell masses which are developed in relation to the special sense organs. That this nucleus is a way-station in the auditory pathway is borne out by a study of its fiber relations, which will be discussed later on, and by pathological conditions produced experimentally. Baginsky claims that destruction of the cochlea in a new-born animal is followed by atrophy and disappearance of the superior ohve of the same side; and \'on Monakow found that sectioning of the lateral lemniscus in one of the lower animals (cat or dog) was followed by atrophy and disappearance of the dorsal portion of the superior ohve of the same side. He assumes from this that only a portion of the cells of this nucleus stands in relation to the lateral lemniscus. Other studies have estabUshed the fact that fibers, axons of the cells of the ventral cochlear nucleus, run transversely' across the pons, forming the corpus trapezoideum, and that a portion of them, at least, terminate among the cells in the superior ohvary nucleus. Flechsig suggested that the superior oHvary nucleus might be concerned with the innervation of the muscles of the ear, as it is considerably larger in animals with large, very movable ears. This contention is successfully disposed of by Spitzka, who states that he has found this nucleus highly developed in the cetaceans.
The superior ohvary nuclei in all of the animals studied present a sufficient degree of similarity in gross morphology, histologic constituents, and relations to enable us to establish a definite type of nuclear mass as a standard which can be used as a basis for further study and comparison. The conformity to type presented by this nucleus in each of the animals is as close as is to be found in comparative studies of any other structure common to all of the individuals in this group. While it is admitted that a certain amount of variation is to be found in the nuclei in the various forms of animal life, and even on the two sides of the same animal, these variations, in normal tissue, were found to be confined within fairly narrow hmits.
With these facts in view, therefore, we would expect to find, and do find, the inferior olive presenting one definite and constant type both in its gross and minute structure, while the superior ohve presents quite another type. While they may be said to possess some general features in common, just as is true of any two nuclear masses, they nevertheless differ so wddely in form, degree of complexity, foldmg, size, relations, cell and fiber-content, that they can be classed only as separate and distinct entities. In comparing the two in the different species an interesting feature develops. The simplest type of the superior olive was found in man, the most complex in the cat and rabbit. In the inferior olive these conditions are reversed, the most complex nucleus being found in man, the simpler types in the lower animals.
The pontine olive in most animals consists of two portions; a smaller, medially placed, bar-shaped mass, and a larger, laterally placed, S-shaped portion. The surfaces of these cell collections, seen en masse, are only slightly irregular, lacking entirely the crinkled, tortuous outhne which is so marked in the larger, bulbar olive. The nucleus is entirely surrounded by a very rich network of fine nerve fibers, which is in close relationship with the transverse fibers of the corpus trapezoideum, but there is no demonstrable intranuclear commissure between the olives of the two sides, such as is found so well developed in the medulla— the interolivary decussation. There seem to be no cross fibers aside from those to be identified with the corpus trapezoideum. The individual sections of both the medial and lateral portions of the superior olive are much broader and more robust than is the shell of the inferior nucleus, and show a much more densely packed noass of cells.
Kolliker, in his embryology, gives the most complete and accurate description of the superior olive extant. He states:
- " One can get a better understanding of the relations of this olive from the lower mammalia, where this organ is better developed than in man Whether this oli\-e appears simpler or more complex, it always possesses essentially the same structure in man and the lower animals, as Golgi's slides of young animals show."
With both of these statements my own findings are in entire accord, though Kolliker gives no results other than those obtained from a study of individual sections, and hence leaves much to be desired as to the gross morphology of this important nucleus.
A detailed study of the models of the upper olive in the three subjects chosen as the basis for this paper shows the general similarity of outline, as stated above; that of the cat being the most complex, the human the simplest, and the dog constituting an intermediate. The nucleus of the adult cat pnvsents a medially situated, bar-shaped mass which is separate and distinct from a laterally placed S-shaped mass. The latter is considerably less extensive vertically than is tlu' former, being overlapped from above by the outward-leaning medial mass. Both portions of the nucleus arc smooth in outline, rather comparable to the accessory mass of the inferior olive, and not at all resembling the wavy, crumpled outUne of the major nucleus.
The medial mass in individual sections begins about IG sections farther caudal-ward than does the lateral mass. It is rather club-shaped, curving somewhat like an italic letter /, its extremity pointing ventro-laterally and dorso-medially, the dorsally directed i)ole being slightly broader than the other. The highest level reached by this medial portion is 38 sections farther cerebralward than the highest point of the lateral mass. The medial bar trends gradually lateralward, farther fronri the median rai)he, and becomes thicker and more robust as we ascend the stem. The model of this medial portion is a long, band-like colunui occui)ying about the lower half of the ])()ns. It is somewhat irregular, presenting a wavy outline from ;ihove downward, with a decided lM(ei;il iiicliiint ion, so marked above that it completely overhangs the medial portion of the lateral mass. Figure 7, plate 2, shows the relative po.sitions of the nuclear complex in transverse section, and figures 2 and 3, plate 1, show the model of the entire nucleus, the two portions having been pinned together in the piling, in order that their proper relations might be retained. The model of the medial mass shows an antero-posterior concavity laterally directed, which corresponds to the rounded medial portion of the lateral mass, the two being separated by a narrow interval occupied by the fiber bundles, all of which appear to run parallel to the long diameter of the nuclear masses that bound them in transsection.
The laterally placed mass is more complex and of considerably greater bulk than is the medial portion. Beginning caudally as an irregular cell mass, this lateral portion soon assumes the typical S-sluiped double curve, the medial bar of the S pointing ventralward, the lateral bar dorsalward. Thus there are two hila, a medial one opening ventrally, a lateral one opening dorsally, the lateral limb of the S and its ventral coil being shorter and more robust than the medial one, the bar of which is slender and elongated. Its ventral tips in successive sections constitute the most irregular portion of the entire nuclear mass. At certain levels, corre- sponding to the more atypical portions to be found at the caudal pole of the mass, there are numerous small, irregularly shaped cell-masses, situated in the area between the ventral extremities of the medial bar and the medial Umb of the lateral mass, gi\'ing the impression that the'-e had at one time been a continuity of struc- ture which had been severed by the fiber-bundles so numerous in this area. The surface of the lateral mass is smooth and is more regular in outline than that of the medial bar. The thick lateral coil, which is ventrally directed, ends abruptly as a wide cell-mass at a point somewhat cephalad to the mid-point of the general nuclear mass; whereas the less robust medial coil, which is dorsally directed, extends farther cerebralward, becoming progressively smaller and ending in a blunt, rounded point almost in contact with the shelving lateral surface of the medial bar at the junction of its caudal three-fourths and cerebral one-fourth. This medial portion of the lateral mass is completely roofed in by the broad upper portion of the medial mass.
The superior oUvary nucleus in the dog presents the same general features as those found in the cat, though lacking in some degree the sj-mmetrical outhne which is so marked in the feline nucleus. The canine superior ohve also consists of two portions, a medial and a lateral portion, which are continuous for a short distance toward the caudal pole of the nucleus, where there is less regularity than is noted elsewhere in the nuclear mass. The medial mass begins caudally at a somewhat lower level than the lateral, usually one or two sections. Comma-shaped at first, it speedily becomes racquet-shaped, the handle of the racquet pointing lateralward ( fig. 8, plate 2), and the larger portion thinning out in the center until it consists of a central space surrounded by a narrow coil of cells. The lateral portion of the coil in turn thins out and disappears as the summit of the mass is approached; thus the coil becomes U-shaped in its cerebral one-fifth, the hilum of the U looking lateralward. The central cavity extends fully half-way down the cell-mass. The medial mass is perfectly smooth and regular in its upper one-fourth ; below tliis it presents the irregular, wavy outline noticed in the other nuclei. In its lower fourth it is markedly' irregular, and above the caudal pole its lateral extremity is contiiuious for a short distance with the ventral extremity of the medial limb of the S-shaped lateral mass, as shown in figure 2. The outline of the medial mass is distinctlj^ rounded excej^t laterally, where it presents in its lower three-fifths a decided antcro-posterior concavity to conform to the rounded medial portion of the lateral mass, as was noted in the cat. Wliile the upper portion of this medial mass extends farther lateralward, there is no tendency to overhang the lateral mass, from which it is separated by a narrow, fiber-filled interspace, except at the point where the two masses are continuous, as described above.
The lateral mass is bulky and S-shaped, and, wliile lacking the beautiful regularity of the cat specimen, presents the ventrally directed medial hmb and the dorsally directed lateral hmb of the S, and two hila — a medial one, ventrally directed, and a lateral one dorsally directed. The two portions of this lateral mass are coextensive in vertical measurement, the whole coming rather short of the cerebral fourth of the media' mass.
In the adult dog, owing to the peculiar histologic structure of the superior olive, the relatively small number of cells, and the rich fibrillar network, the outlines of the nuclear mass are much less distinct, especially within the nucleus where the hilum is but faintly defined, making it difficult to distinguish the exact line of demarcation. The peripheral hmits are brought out more clearly by the encirchng coils of fibers, wliich everywhere separate the nucleus from the surrounding structures. The hila are very narrow, and the fibers witliin them are very delicate and not so numerous as in cat and human tissues, nor do they take the stain as kindly. The nuclear fold is coarser and more loosely woven, being much wider from hilum to periphery, and the loose meshwork contains a great deal of granular material.
Compared with the nucleus in the dog fetus, we find the general outUnes to be nearly identical. In the adult, however, with its greater area in cross-section with relatively little increase in the cell-content, there is a disproportionately greater increase in the richness of the fiber-network. This would lead to the assumption that the fiber element proper to a given nucleus is a large, if not the most important factor, in determining the form of the nuclear mass, which is somewhat more complex in the fully developed adult animal. The only modifj^ing element apijarent is the great number of relatively large sized blood-vessels which chaiuiel the nucleus in every direction.
The detailed study of the superior olive in man showed a much simpler jiicture than was found in either the cat or dog, as will be seen in figure's 2 and 3, plate 1. The nuclear mass as a whole is relatively smaller than in either of these animals. It differs from them also in shape and in the disposition of its component cell masses, the human nucleus consisting of three separate and distinct portions instead of two. The medial j^ortion is well developed and essentially like that in other animals. The lateral portions in the specimen figured herein consist of two cell- masses, one above the other, and separated by a small interval; the two, however, sustain the same general relations to the medial mass and to the surrounding parts as is characteristic of the more fully developed lateral masses. There was considerable variation in the size and form of the lateral portion of this nucleus in the human material examined, though in no case was it entirely absent. Other investigators appear to have had similar experiences. Kolliker, whose report is rather confusing in some respects, since it was based entirely upon a study of individual sections rather than upon reconstructions, states that in cross sections of this nucleus one can, as a rule, distinguish three portions more or le.ss distinctly: a larger, medial, more tape-hke portion, and two ventral, lateral, more cyhndrical formations; yet in man the resemblance to the folded membrane of the larger olive is entirely lacking. Weed, who includes a description of the superior olive in his study of the human brain-stem, says :
- "The nucleus olivaris superior begins just caudally to the middle of the nucleu-s nervi facialis and, sloping dorsally and slightly laterally, terminates cephalad in the region of the sensory enlargement of the ner\ais trigeminus The nucleus enlarges into a triangular nuclear mass, out of which three dorso-ventral cell colunms appear clearly defined. These are united at their ventral aspect and they spread out from this ventral point like spokes from a hub The mesial column arises from the ventral point of radiation in the caudal cell collection as a small continuous cell-mass. This .... enlarges into a thin sheet of cells which run cephalad This sheet of cells Ues in a general dorso-ventral plane, but its ventral border is placed more laterally from the mid-line than its dorsal margin. .... The lateral of the two cell-columns is really double throughout the middle portion of its extent, although it arises singly from the mesial surface of the dorsal union of the three primary radiate columns, .\i-ising from this union, the column extends as a triangular cell-colmnn, placed dorsally and somewhat mesially to the dorsal border of the mesial colunm At this point (the level of the superior pole of the seventh nucleus) the cell-column bends laterally and dorsally across the superior pole of the olive and then pursues a cephalo-lateral course to fuse quicklj' with its second portion."
This second portion he describes as arising caudal to the superior pole of the seventh nucleus and ascending, as an elongated oval with the long axis in the dorso-ventral plane, to join the first portion as stated.
Judging from these statements one must conclude that there is considerable variation in the different specimens examined. One point, however, seems clear; I. e., whatever the condition of the other nuclear parts, this medial portion is always present and well developed, and in comparison with the lateral mass presents a preponderance of the tj^pical spindle-cells. All this tends to confirm the assumption that this portion is the more essential part of the nuclear complex, at least for the proper performance of the functions common to all the types under consideration.
The medial mass is a large, band-Uke column of cells which begins caudally just above the ponto-bulbar junction and extends forward to the region of the nervus trigeminus, exceeding the vertical Umits of the lateral portion at both poles. It has a decidedly dorso-lateral incUnation from below up^-ard and is somewhat twisted upon itself in its long axis. Beginning below as a small oval mass, it speedily enlarges in both lateral and antero-posterior directions and, when fully developed, is semilunar in outUne on cross-section, being convex medially and concave laterally, one extremity of the demilune pointing ventro-laterally, the other dorso-laterally, the latter apiirnachiiip; the median line. The shifting in position of the successive sections, confirmed by careful measurements, involves changes in two directions — the rai^id, dorso-lateral inclination, which in ioto attains a considerable degree, the topmost section being at an angle of 45 degrees with the perpendicular of the most caudal section, and a twisting upon its caudal pole to such an extent that the extremities of the bar swing around a cjuarter of a circle, the long diameter of the individual section extcMiding almost transversely. The entire mass forms a robust cell-column of irregular, wavy outline in a cephalo-caudal direction, more marked upon the convex ventro-mesial surface. The dorso-lateral surface is concave, conforming to the adjacent medially directed surfaces of the lateral masses and overhanging the upper lateral mass to a certain extent above.
The lateral mass is much less developed in man and consists of two portions, one just above the other. The lower, slightly larger portion, begins 19 sections above the caudal limit of the medial bar and extends somewhat above the mid- point of this bar, where it ends in a pointed extremity. The second mass begins 5 sections cephalad to the former and ends in a blunt pole 21 sections below the cerebral Umit of the medial bar. These lateral portions are simply irregular cell- masses presenting no hila and no definite modeling comparable to the beautiful S-shaped structure found in other animals. Both masses are channeled by blood- vessels, and are situated very close to the medial bar, almost in contact with its dorsally directed margin; whereas the point of continuity in the dog and that of closest proximity in the cat are with the ventral margins of the two portions.
The superior oUvary nucleus presents an enormous number of nerve-cells which have been variously described by different authors, the majority of whom, however, are perhaps agreed that the cells in the superior and inferior oUves are alike. This is well demonstrated in Barker's summary, which is to the effect that Held, Kolliker, and Cajal describe their results in a general way as follows: The cell bodies in the superior olivary nucleus markedly resemble in tj'pe those found in the inferior olive and the dentate nucleus; they possess numerous, much branched dendrites which are turned toward the interior of the nucleus, the axons in the main Ijeing directed toward the periphery of the nucleus. Still others describe the.se cells as being spindle-sha]ied or club-shaped. One reason for this diversity of opinion, perhai)s, is the fact that })rior to comi)lete develojjment and differentia- tion the great majority of nerve cells resemble each other rather closely, since all are of the embryonic type; and in studying a given cell-mass in an embryo, even one comparatively well advanced in other respects, one is liable to be led into error as to the definite mor])hology of a given tyj^e of cell unless one studies the same cell area through several successive stages of develo]nnent. It is even more satisfactory to. study the fully differentiated adult stage as well. Amuch morecomjjletepictureof a given cell group can be obtained by comparative study in a number of animals.
In a comparative study of the inferior olive (unpublished) it was found that there was a striking uniformity in .shajie, size, and structure of the cell peculiar to this nucleus. There is also, of course, a similarity in the gross morphological features of the nuclear masses as well, but the conformity to type of the gros.s outline is neither so striking nor so significant as is the likeness of the cells charac- teristic of this nucleus. These features were noted throughout the series of animals used in that study. The same morphologic similarity obtains in the case of the superior olivary nucleus, both for the mass of the nucleus and more particularly for its constituent cells. It will be found that when complete adult differentiation has been attained these two groups of cells— those of the inferior and those of the superior olive — will each have developed into a type of cell pecuhar to and charac- teristic of the given collection or nuclear mass within which it is found, each type being sufficiently definite as to morphology to enable the investigator to positively determine its proper repository without difficulty, as shown in figure 1, a camera- lucida tracing of the various cells under discussion. (Compare these with the microphotographs of the different sections, figs. 4, 5, and 6, plate 1).
Text-Figure 1.-Outline tracings of the cells in the two olives: cells .'V-E inclusive are from the human inferior olive; cells F-I are typical cells from the superior olive of an adult dog; cells J-M are from the medial ma,ss of the superior olive of an adult cat; oeUs N-P are from the lateral mass of the same; cells Q-U are typical cells from the superior olive of a human fetas. The outlines were made with camera lucida, A-P being made with a 4 mm. objective and No. X ocular, Q-U being made with a 1/12 objective and No. X ocular.
The cell type which is characteristic of the superior olive is more or less definitely sjiindle-shaped, a great number being true spindles with a narrow, long- drawn-out cell body, the nucleus being compressed laterally (ovoid) to conform to the shape of the cell body, and the processes coming off from the two extremities of the spindle. Another type of cell, especially abundant in the ohve of the adult cat, presents a large cell body with a large vesicular nucleus, the cell body tapering sharply at its two poles. Unless studied carefully through a series of sections, these cells may be thought to be club-shaped. Indeed, Kolliker classes them as such. A careful serial study, however, shows them to present two sharp poles with a rather fat intervening body, a modified spindle. A third type of cell found in but not limited to the superior olivary nucleus in all animals is a small, rounded granular cell, scattered irregularly through the nuclear area and the surrounding parts. The spindle-cells are closely packed, side by side (like small fish in a tin), their extremities pointing toward the perijjhery of the nuclear fold, the long axis of the cell being placed at right angles to the long axis of the nuclear coil. This condition is especially apparent and regular in the medial mass of the nucleus, as this portion corresponds more nearly to a straight line than does the S-shaped lateral mass, though in the latter the spindles extend crosswise of the fold, thus conforming to type.
Some of the cells present irregularities in outhne, though all are sufficiently close to type to justify one in classifying them as belonging to the same general C(>11 family wliich is characteristic of this nuclear mass. The different animals exhibit some minor variations. In the adult dog, for example, in proportion to the fiber network, the cells are less numerous than in other animals, and thus the difference between the spindles and the tapering oval cells is more clearly defined. The sjiindles are quite deeply stained, while the tapering, oval cells are very pale and rather indefinite. In the cat the staining reactions are similar, though the ovoid cells are much more in evidence and take the stain better, and the total cell-content is vastly' greater than in the dog.
In human tissue, aside from a few granular cells, only true spindle-cells are found. These present the characteristic shape, position, and relations as those described for other animals. The cells are deeply staining, very numerous, and closely packed together. The medial bar, which is the more constant and essential portion of the nuclear complex, presents these cell conditions to the best advantage. These facts, coupled with the finding of great numbers of true spindles to the practical exclusion of other cell types, strengthen the assumption that the spindle- cell is the one pecuUar to and characteristic of this nucleus. The lateral masses show the same general cell characteristics, though they are not as regular in the medial mass. Wliile the nucleus is very richly supplied with blood-vessels in all the animal forms, the human nucleus is especially vascular.
A careful study was made of the cells in the areas adjacent to the superior oUve in order to definitely deUmit the nuclear mass under consideration. There are several groups of these cells to be considered, and for purposes of description these may be cla.ssed as follows: (1) The cells of the nucleus faciaUs, which, though situ- ated in the formatio reticularis, for obvious reasons require specific consideration ; (2) the cells in the formatio reticularis; (3) the cells in the corpus trajiezoideum.
(1) The nucleus facialis is a large nuclear mass located in close ])r()ximity to the dorso-lateral surface of the superior olive, but the cells making up this nucleus are as characteristic of their kind as are those of the superior ohve. These two types of cells differ so widely that confusion is hardly probable, although Weed ff)und it difficult to differentiate the lower pole of the ohve from the facial nucleus in the tissue which he studied. A study of the fib(>r relations of thes(> cells is al.«o an aid in chstinguishing them.
(2) The formatio reticularis presents a great number of cells which are, for the most part, scattered irregularlj- throughout the brain-stem. At times a few of these cells will be found grovijied together, though at no place constituting definite nuclear masses save such as have long since been identified and named. While in every section examined some of these scattered cells were found in close relation to the superior olive, in no case is it especially difficult to exclude them from the make-up of the nucleus proper, since morphologically they differ widely from those proper to the olive.
(3) There are a great number of cells in the area of transverse fibers ventral to the superior olive. The difference in the disposition of the cells comprising the nucleus pontis found in the lower animals as compared with man, makes their study rather more difficult, and it becomes quite a problem (in the human as well as in the animal forms) to separate the scattered cells into two classes, one to be designated as the nucleus pontis and a second as constituting certain smaller nuclei which have been described as being closely associated with the superior olive. A number of these nuclear masses, such as the nucleus preolivaris, the nucleus semi- lunaris, and the nucleus corpus trapezoideum, will be discussed in a subsequent paper. In this connection we find, in the lower animals especially, that the great majority of these cells are grouped within a rather limited area adjacent to the ventral surface of the superior olive, although e^'idently not forming a part of this nuclear complex, since these cells have, for the most part, rather large, irregularly rounded, darkly staining cell-bodies with deeply sttiining nuclei. In this they differ widely from the typical olivary cell.
There is a quite noticeable cell group to be found at some levels, in cat tissue especially, immediately ventro-mesial to the medial mass of the olive ; and in one area, extending over several sections, there are numerous irregular clumps of the.se cells between the ventrally directed medial limb of the S and the ventral pole of the medial mass, surrounded by streams of fibers. Thus, in a sense, they seem to piece out a connection between the two portions of the major nuclear complex. These groups, and others less numerous, found along the ventral surface of the olive, may be genetically related to the oUvary cells, though there is but the faintest morphological resemblance. There are also many fine collaterals passing between the trapezial fibers and the olivary cells proper, which stream around and among these cells and possibly establish relations with them.
Since, as has been noted above, the medial nuclear mass is well represented in every animal and, conforming more closely to type, is the more constant portion of the nucleus; and as the spindle cell has been seen to be the type of cell characteristic of the superior olive, we find, as would be expected, additional support for the idea of the conformity of structure to function in the fact that the best-defined and the greatest number of spindles are found in the medial nuclear mass, which may therefore be assumed to be the most essential part of this nuclear complex.
The Related Fiber Element
There are great numbers of nerve cells in the superior oUve. Associated wath these is a correspondingly rich network of both afferent and efferent nerve fibers. These, as Cajal observed, are so numerous and so exceedingly fine and the network so intricate as to rciuk'r tlicir study a very difficult task. Mucli work has been done in an effort to determine the fiber relations of this portion of the brain-stem, and certain facts have been fairly definitely established; among others, the manner in which the fiber element of the superior oUve is related to the corpus trapezoideum, the lemniscus lateralis, and the tractus olivo-nudeus abducentis, and throufih these indirectly to the various parts with which these tracts are connected. \\'liile it is impo.s.sible at this time to give a complete study of these fiber elem(>nts and their relations to other parts of the central nervous system, a few of the nujre perti- nent facts may be recorded.
The fibers that arc distinguishable in relation to the superior oUve may be roughly grouped into three classes: (1) Those that can be traced from the ohve to the nucleus of the abducent nerve, the olivary peduncle (shown in figs. 7 and 10, plate 2). These fibers are cjuite fine and are grouped in small bundles which can be traced vertically from the dorsal surfaces of both medial and lateral masses of the superior olive, from a point near its middle portion to the nucleus nervi abdu- centis. The bundles run parallel to the emergent portion of the nervus facialis. The oUvary peduncle forms a connecting link between the auditorj^ pathw^ay and the nerve supply of the ocular muscles. Santee believes that a part of the fibers from the olivary peduncle go to the nucleus abducentis, and part go by way of the medial longitudinal bundle to the trochlear and oculo-motor nuclei, thus correlat- ing all the nerve nuclei supplying the ocular group of muscles with the auditory pathway. This may serve to explain some of the results of Ferrier's experiments upon monkeys. This observer claims that the animals will, when anesthetized, turn the eye in the direction from which a sound is perceived, a similar movement being observed when the cortical center for hearing is stimulated. This action will necessarily call into pla}^ the lateral rectus muscle through stimulation of the nervus abducentis.
(2) This group would include all those fibers which extend between the ven- trally situated corpus trapezoideum and the ventral surfaces of both parts of the nuclear mass of the superior olive. These vary in length; some can be traced for only a short distance, others extend well out toward the periphery of the section. All are better develoi)ed in adult tissue. Some of the fibers arc terminal, but the major part is undoubtedly collateral. In the trai)ezium they can be seen to branch, T-shaped, one collateral passing dorsalward toward the olive. The olive rests in a bay in the dorsal surface of the corpus trapezoideum, the fibers of which curve around ventral to the nucleus. The strands of trapezial fibers are observed to occup3' a much greater antero-posterior extent in the interval between the olives of the two sides (fig. 5), but curve sharjily forward at the medial margin of the nucleus to decussate in the median line with those from the opposite side, none of them apparently entering the nuclear mass. Indeed, one commonly finds the collaterals, not the terminal fibers, entering the nucleus, though the fibers of these strands appear to give off no collaterals in this region. Kolliker claimed to have foimd, in a study of frontal sections, fibers coming from the diri-ction of (he mtnlian raphe to end about the cells in the dorsal part of the oli\e. Barker suggests that the olive also receives fibers from the forniatio reticularis. It is entirely probable that both collaterals and terminals pass in both directions between the superior oUve, a way-station in a specific tract, and that the cells in the formatio reticu- laris constitute a part of the general association system.
(3) This group includes the fine fibers surrounding the periphery of the nucleus in well-defined bundles and filling the interval between the medial and lateral masses, and also the hila of the lateral mass. IVIost of these fibers are i)robably axons of the cells making up this nucleus. While fibers are seen to turn into the substance of the cell-mass at all parts and levels, no bundles can be observed to cut into and through the substance of the nuc'ear fold, which is so noticeable in the fiber arrangement of the inferior olive; though in the cat the medial mass appears at some points to have been separated from the lateral mass ventrally, and a still greater tendency to group into bundles has been noted in the fibers of human tissue.
Cajal describes the axons arising from the cells in the superior olive; as passing in three different directions: (1) The majority of them, after giving off collaterals in the nucleus itself, pass to the dorsal surface of the nucleus, where either by bending or bifurcating they turn to run vertically in a longitudinal bundle con- tinuous with the lateral lemniscus of the same side. (2) A certain number of the axons, much curved inside of the nucleus, leave the latter at its lateral border to enter the trapezoid body, where they can be followed nearly as far as the ventral cochlear nucleus. (Held describes these as actually terminating inside of this nucleus.) (3) Other axons, arising from the cells of the superior olive, pass out at the medial side of the nucleus, entering the plexus of the preolivary nucleus to mingle there with the trapezial fibers. Held adds to these the group of axons which make up the olivary peduncle described above.
It is to be noted that the number of these fibers is greatly augmented in the upper part of the nucleus. In the cat specimen, especially, great, dense whorls of fibers are seen around and above the superior pole of the olive, where they form the beginning of the lemniscus lateralis. In none of the tissues studied could any connection be found between the cell-mass of the superior olive and the nucleus of the lateral lemniscus, though such a condition is claimed by some investigators. Bruce asserts (p. 48) that these two nuclei are continuous, and Cajal states that the lower nucleus of the lateral lemniscus is anatomically continuous with the superior ohve. Nevertheless, it is to be sharply separated from the latter, for its constituent cells are very different in shape and the axons are entirely different in distribution.
The cell and fiber complex in the adult dog presents some features which are altogether different from those observed in the other forms studied, there being a disproportionately greater amount of fibrillar element as compared vnih the other animals, the fine intranuclear fibrillar network making up the bulk of the nuclear mass, the cells being widely scattered and enmeshed in the plexus. The extra- nuclear fibers surrounding the nuclear mass are less abundant than in the cat. The individual section of the superior olive of the dog is much coarser, presenting a greater transverse measurement and a less sharply defined hilum. The adult cat is the best subject for fiber study. Quite bulky strands of fine fibers can be seen flowing around tlie ])erii)hory of tlie nucleus and in the intorval botwoon the medial and lateral masses, filling in the hila and forming dense whorls around the superior pole, making altogether a much richer network than is found in any other form.
In man there is a well-develoi)cd fiber comple.x, both intranuclear and extra- nuclear; the latter, in particular, shows a tendency to group into bundles. These can be seen at places cutting the substance of the nuclear mass in the same manner, though in much less degree, than is commonly found in the inferior olive.
Baoinsky, B., 1890. IVbcr den Urspning und den con- tralcn Vorlauf der Ner\'us acu-sticus des Kanin- ohens und dor Katze. Arch. f. path. Anat., vol. 119, p. 81-92.
B.\RKER, L. F., 1899. The nervous system and its con- stituent neurones.
Brcce, a., 1892. Illustrations of the nerve tracts in the mid- and hind-brain, and the cranial nerves arising therefrom.
C.UAL, Ramon y, 1896. Beitrag zum Studium der Medulla oblongata, etc.
Ferrier, D., 1890. Croonian lectures on cerebral localization. Delivered before the Royal College of Physicians, London; vol. 6.
Fleichsig, p., 1886. Zur Lclire vom contralen Verlauf der Sinne.sncrven. Neurol. Ceiitralbl., Bd. 5, p. 97-100.
— -, 1890. Weitere Mittheilungen ueber die Bezio-
hungen des unterpn Vierhiigels zum Horner ven. Neurol. Centralb., Bd. 9, p. 98-100.
v. KoLLTKER, A., 1896. Handbuch der Gewebelehre.
v. MoNAKOw, C, 1890. Strise aousticac und untere Schlcife. Arch. f. Psychiat., Bd. 22, p. 1-26.
Santee, H. E., 1915. Anatomy of the brain and spinal cord.
Weed, L. H., 1914. A reconstruction of the nuclear masses in the lower portion of the human brain-stem. Carnegie Inst. Wa.sh., Pub. No. 191.
Description of Plates
Fig. 2. — Photograph of the ventral aspect of three models of the superior olive: A, adult cat; B, dog fetus; C, human fetus. A and B are oriented to conform to their natural position in the stem. C has been placed to show the relations of its different parts to each other; the picture accordingly shows the ventro-lateral view of the vertically placed nucleus, whereas in its proper position it should lean sharply to the right with some twisting upon its long axis. In A and B the S-shipcd lateral mass can be clearly seen, separate from the medial bar in A (cat nucleus) but continuous with it in the lower part for a short distance ventrally in B.
Fig. 3. — Dorsal view of the same models. In A the interval between the two masses is well shown. In B the marked irregularity of the lateral bar of the S ind the absence of continuity of the two masses are noted. In C the three component portions are clearly shown, though the interval between the medial and lateral masses is slight.
Fig. 4. — High power microphotograph of the superior olive in an adult cat, showing the characteristic spindle-cells. All of the sections in figures 4-6 were cut 20 microns and are too thick to give sharp pictures.
Fig. 5. — Spindle-cell in the adult dog.
Fig. 6. — High-power photograph of the human superior olive in embrj'o No. 28, in which the closely packed arrange- ment and large number of spindle-cells are evident. In all of these the same character of cell (the typical spindle) will be found.
Fig. 7. — Lower power microphotograph of a section of the pons of an adult cat. Here the superior olive can be seen to consist of a bar-shaped medial portion and an S-shaped lateral por(ion, together forming a eompact m.i-ss lying on the dorsal surface of the traiiezoid body between the emergent fibers of the adbucent and facial nerves.
Fig. 8. — Microphotograph of the superior ohvary region in a 115 mm. dog fetus. The olive consists of a racquet- shaped medial portion and an S-shaped lateral portion. The cluster of large cells dorsal to the latter is the nucleus of origin of the facial nerve.
Fig. 9. — Microphotograph of a Yamagiwa preparation of the brain-stem of an adult dog, showing the C-shaped medial portion of the superior olive with the emergent fibers of the abducent nerve passing through it. The lateral portion is O-s^li'TC'l.
Fig. 10. — Pholograph of a Pal-Weigcrt preparation in the region of the superior olive in a new-born infant, showing the fiber elements. To the right of the section is the thick strand of fibers of the facial nerve, and just medial to this is it« nucleus of origin with fibers streaming upward toward the genu. Between the facial and abducent nerves is the superior olive, lying medial and ventral to the nucleus of the facial. The fine fibers streaming dorsally from it constitute the ohvary peduncle.
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