AMERICAN

THE

NATURALIST.

VOL. XII. SEPTEMBER, 1878. No. 9.

THE SENSORY ORGANS-SUGGESTIONS WITH A VIEW TO GENERALIZATION.1

BY FRANCIS DERCUM, M.D., PH.D.

PERHAPS in the whole range of physiological and anatomical

studies, no subject is invested with a deeper interest than that of the sensory organs. It is by means of them that we first become aware of our own existence; it is by means of them, in other words, that consciousness is first awakened in us, and it is through them that we subsequently continue our acquaintance with the outer world. It would be of great importance, were it possible, to arrive at some general conclusion regarding their morphology-some general view regarding the essential elements or the essential conditions of their structure. Judging from our knowledge at present it may seem that an attempt in this direction would be fruitless. Notwithstanding there are several prime facts which do not seem to have attracted the general attention which their importance deserves, and these when placed in their proper relations may give a new character to the subject, and perhaps open new avenues for investigation.

It will be interesting, therefore, to hastily review what is known regarding the constant elements of the various sensory organs. Under this head, of course, it will be unnecessary to consider the more or less mechanical arrangements by which force is conducted, and we will adhere, therefore, to that part of the literature only which treats of the ultimate structures by which impressions appear to be received.

1 Read before the Alumni Society of the Auxiliary Department of the University of Pennsylvania, March 29, 1878.

It will be found most expedient to commence with the organ of hearing.

Here the percipient structure is either situated in, or constituted by, the epithelium of the maculæ acustica of the ampullæ and saccules, and by Corti's organ. Concerning this there can be no doubt, and were we to analyze the subject no farther, it would be sufficient for our purpose. The epithelium of the maculæ acustica, as we know, is made up of several kinds of cells. Prominent among these is one bearing a hair-like structure on its peripheral end, and possessed of central out-runners or processes. Now regarding the distribution of nerve-fibres to these acoustic spots we may hold the following views: either that the nerves end in plexuses and loops in or immediately beneath the epithelium, or that they are directly continuous with either the cylindrical, the stellate or the hair-bearing cells. Almost any one of these suppositions would suffice for the main points of the generalization I have in mind, but when we turn to the literature bearing immediately on this point, we learn that the constant or essential element of the maculæ acustica, and in fact of the organ of hearing, appears to be a cell bearing a central and a peripheral process. This cell is constant throughout vertebrates and, as far as known, throughout invertebrates. The entire literature makes it also more than probable that these cells are directly continuous with nerves. Thus, Max Schultze already in 1858 (Muller's Archiv), in speaking of the ultimate nerve termination in the ear of the petromyzon, ray, pike, etc., advanced strong grounds in support of this view. Deiters, in 1860 (M. A.), described the hair-cells of birds, and considered their connection with nerves probable, while in 1862 (Ibid.). he described the haircells of amphibians, and in the "lagena" believed them to be directly continuous with nerves. About this time, also, F. E. Schulze claimed to have seen the direct transition of nerve fibres into the auditory hairs of young sea gudgeons. In 1867, Hasse (Zeitsch. d. Wissensch. Zoologie, Bd. xvii) described the hair-cells of birds, and said that he clearly made out the entrance of the nerve-fibres into them. He said that he frequently traced the central processes of the cells for long distances in the course of the nerve-fibres, and vice versa, he traced the nerve-fibres in the direction of the central processes. Odenius, 1867 (A. f. M. A.), advocates for man the same thing. He says, that all comparative

researches point to that conclusion; that the fibres of the auditory nerve do not end in loops, that they do not become continuous with the blunt-ended cylindrical nor the stellate cells. The only remaining element is, therefore, the hair-cell, and this in every way supports the view of its nervous connection. It not only becomes stained with osmic acid (Waldeyer, Stricker's Hdb.), similar to nerve fibres, but its central process presents the same appearance as the latter. In 1868, Hasse (Z. f. W. Z., Bd. xviii) investigated the ear of the frog, and strongly expressed the same view as maintained by the previously mentioned authorities; in fact, in one instance he believes to have really seen the nerve terminating in the hair-cell (stäbchenzelle), but does not lay too much stress upon it. Waldeyer, 1868 (Stricker's Hdb.), claims the nervous continuity of the inner hair-cells of Corti's organ as a positive fact. He says, "the inner radial (nerve) fibres pass, as I have often been able to prove, directly through the granular layer and terminate in the pointed ends of the inner hair-cells." Gottstein, 1872 (A. f. M. A.), like Waldeyer, says, that the inner radial nerve-fibres enter the inner row of hair-cells, while the outer are supplied by fibres stretching directly across the tunnel formed by Corti's arch. Ladowsky, 1876 (A. f. M. A.), expresses emphatically the same conclusions maintained by Gottstein. He asserts notwithstanding the purely negative results of Nuel (A. f. M. A. viii), the nervous connection of the hair-cells strenuously as follows: "I assert, along with Boetcher, that the connection between nerves and cells can nowhere be demonstrated so clearly as in the cochlea . . Boetcher says, that he could, with some animals, as the hedgehog, find the connection of the nerves with the endcells in almost every section."

The Organ of Smell.-This, when analyzed, appears likewise to consist of a cell with a central and a peripheral process. The outline of the literature is as follows:

In 1857, Ecker described the epithelium of the olfactory region (Z. f. W. Z. Bd. viii), but did not claim any nervous connection with cells. Later investigators, however, J. L. Clark and Max Schulze, strongly advocated the view of the direct nervous. continuity of the so-called "olfactory" cells. Clark, 1862 (Z. f. W. Z. Bd. xi), traced the nerves into a fine sub-epithelial plexus, and thence believed them to become continuous with the central processes of these cells. Max Schultze, 1863, (Abh'dl'g. d.

Naturges. zü Halle, vii), described the olfactory cells of fishes, amphibians, birds and mammals, and strongly argued for their direct continuity with nerves. His theory rests on the complete chemical and morphological analogy between the central ends of the olfactory cells and the nerve fibrilla. Prof. Babuchin, 1868 (Stricker's Histology), says that he possesses a chloride of gold preparation from a tortoise, in which can be observed the immediate passage of the nerve fibrilla into the epithelial layer, "where they can be followed into the nuclei of the olfactory cells." He says, “this might raise M. Schultze's hypothesis to an actual fact if we possessed in the chloride of gold a substance which stained only nervous elements, and if this re-agent were not so very uncertain in its action." Von Brunn, 1875 (A. f. M. A. xii), describes, in addition to the epithelial and olfactory cells described by Clark and Schultze, a homogeneous membrane covering the olfactory region of mammals, and which is pierced by the peripheral processes of the olfactory cells. This arrangement would seem to exclude the epithelial cells from immediate contact with any substance inhaled into the nostril, and confine such contact entirely to the peripheral ends of the olfactory cells. This would go very far to establish the fact that the olfactory cell is the essential or percipient structure of the organ of smell. At any rate, it is a cell peculiar to the olfactory region, is constant throughout vertebrates, and like the corresponding structure in the organ of hearing, the weight of evidence is in favor of its direct nervous continuity.

The Organ of Taste.-Here we meet with a peculiar difficulty at the outset, and one to which the consideration of other organs is not subject. It is at first a matter of doubt as to what structure really constitutes the organ of taste. From the literature of the subject, however, I think it may be fairly attributed to the so-called taste-buds, as is almost unanimously done by the investigators of these organs. The taste-buds are distributed most thickly where the sense of taste is most acute, and less thickly on parts of the tongue where the sense of taste, per se, is less but where the general sensibility is greatest, as the tip and anterior part of the dorsum. What gives the most ground for doubt regarding their gustatory function, is that Verson has found them on the lower surface of the epiglottis and Krause on the dorsum of the same. The latter, however, does not consider this

an objection to their being concerned in the sense of taste, while C. Davis (1877 A. f. M. A.), who describes these organs as existing in the upper parts of the larynx, says that whatever we consider as their function we must regard them as terminal organs of the glossopharyngeal nerve. He says that Vingtschau and Hönigschmied have given experimental evidence that the buds at the dorsum of the tongue are terminal organs of the glossopharyngeal, though he neither says in what the experimental evidence consisted, nor gives any references where it might be found. Leaving you to form your own views, I will briefly pass over the pertinent literature of these organs. They have been found in fishes, amphibians and mammals. Waller appears to have been the first to investigate the epithelium of the fungiform papillæ of the frog, while Leydig first described the "taste-disks of fishes, and was disposed to consider them as tactile organs (1851, Z. f. W. Z. Bd iii, also 1857, Lehrb. d. Hist.). Billroth, 1858 (M. A.), and Hoyer, 1859 (M. A.), both described the peculiar epithelium of the taste-papillæ of the frog, and while the former thought a connection between it and nerve-fibres probable, the latter described the nerves as terminating bluntly beneath it. Axel Key, however, 1861 (M. A.), described the same structure and pictured the nerve-fibres as directly entering certain cells, which are designated by the term "taste-cells." His results were attacked by Hartman, 1863 (M. A.), who, although he could assign no definite termination to the nerves himself, supposed them to end in plexuses beneath the cells. Beale, however, 1865 (Phil. Transactions), strongly supported Axel Key in the essential points. He showed that Hartman had destroyed the finer structures by his method of examination. Speaking of the nervous connection of these cells, he says: "In many specimens I have seen, and most distinctly the delicate network of fibres (in the body of the papilla) continuous with the fine nerve-fibres in the summit of the papilla, and I have demonstrated the continuity of these fine fibres with the matter of which the outer part of these peculiar cells consists. . . Upon the whole, I

am justified in the inference that there is a structural continuity between the matter which intervenes between the masses of germinal matter at the summit of the papilla and the nerve-fibres in its axis, and I consider that an impression produced upon the surface of these peculiar cells may be conducted by continuity of