Book - The Pineal Organ (1940) 2
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The Pineal Organ
Chapter 2 Historical Sketch
The existence of the pineal organ was known by the ancient Greeks and Romans. Galen (a.d. 131-201) spoke of it as " scolecoid," or wormlike. His dissections were carried out on oxen, sheep, apes, and other animals, and he mentioned that it had been named by other writers the epiphysis. He also emphasized its intimate connection with the great vein which to this day is known all the world over by his name. Other classical writers who were impressed by its conical, pine-like form named it the conarium. Various ideas were held as to its function, among which was the notion that it acted as a valve or flood-gate, and regulated the quantity of spirit (? cerebro-spinal fluid) necessary for the psychological requirements of the individual. The theory that it functions as a gland seems to have originated with the Romans, who described it as the glandula pinealis. Little advance was made in the knowledge of the pineal organ in the centuries which followed, and in 1637 Rene Descartes taught that the human body was an earthly machine which was presided over by the " rational soul," which was situated in the pineal gland, " the little gland in the middle of the substance of the brain." This idea was ridiculed by Voltaire, who suggested a coachman sitting on his seat and holding the reins of the horses — which were supposed to be represented by the peduncles of the gland.
William Cowper (1666-1709), who considered the pineal to be a lymphatic gland, wrote " the glandula pinealis which we take to be a lymphatic gland, receiving lympha from the lymphe ducts which pass by way of the third ventricle of the brain to the infundibulum and glandula pituitaria." This idea of the lymphatic nature of the pineal body was shared by some others, among whom we may mention that pioneer histologist Jacob Henle (1809-85), who, being impressed by a general resemblance of its microscopic structure to that of a lymphatic gland, considered that it might also function as a lymphatic node. With modern microscopes and improvement in histological technique, however, the distinction between lymphatic tissue and the peculiar structure of the normal adult pineal organ has been rendered easy, and its developmental history coupled with a more exact knowledge of its comparative anatomy have shown that this conception of the nature and function of the organ is quite untenable.
The work on the pineal region in the latter part of the nineteenth century and the commencement of the present century was attended by a marked advance in our knowledge of the true nature of the pineal apparatus in all its aspects — geological, zoological, embryological, and phylogenetic. It is to this period that we owe the conception, which was generally held at that time, of the vestigial nature of the pineal organ. It was thought that the " parietal organ " or " pineal eye," which was found to be most highly developed in certain living reptiles, amphibia, and cyclostomes, was the vestige of an unpaired median eye or the persistent member of a pair of median eyes one of which was rudimentary or had completely disappeared.
Fig. 2. — Sagittal Sections through the Brain of Acanthias Embryos. (After Kupffer.)
A : 33 mm., showing the open neuropore. B : io mm., showing the cutaneous ectoderm still adherent to the neural ectoderm, at the site of the closed neuropore, which lies in front of the pineal diverticulum, pin.
ch. op. : optic chiasma. n.ch. : notochord.
ect. : cutaneous ectoderm. n. ect. : neural ectoderm.
inf. : infundibulum. np. : neuropore.
M. : midbrain. Rh. : rhombencephalon.
Before proceeding to the discussion of the more recent work on the pineal, we shall give a short account of some of the problems which arose during the period 1870-1915, and we shall mention in an approximately chronological order the names of a few of the principal authors, giving at the same time a brief note of the special contribution which each made tc the general knowledge of the subject.
Among the first of these was Leydig (1872), who discovered the parietal organ in the embryos of Lacerta agilis, L. muralis, and L. vivipara. He described its general position above the interbrain in the region of the third ventricle ; the external appearance of the pineal plate ; the disposition of the pigment ; and the microscopical structure of the vesicle ; but was doubtful of its nature and even contended that it was not a senseorgan. Goette (1875), described the epiphysis in Bombinator and stated that the site of its out-growth from the brain was identical with the position of the anterior neuropore, or the point where the medullary folds finally unite with each other in the formation of the neural tube. It was, however, shown by Mihalkovics that at the time of the first appearance of the evagination in birds (chick) (Fig. 200, Chap. 21), and in mammals (rabbit) (Fig. 208, Chap. 22), the medullary wall is separated by a considerable interval from the cutaneous ectoderm, and that the intervening space is filled in all amniote embryos by a layer of mesenchyme long before the pineal outgrowth takes place ; and it has also been shown by Kupffer in anamniota that the pineal evagination in Acanthias embryos lies behind the point of closure of the anterior neuropore and arises independently of it (Fig. 2).
Fig. 3. — Free-swimming Larva of Phallusia or Ascidia Mammillata, showing the Single Eye and Otocyst enclosed within the Cerebral Cavity (ventricle).
adh. pap. : adhesive papillae. med. : medullary tube.
al. c. : alimentary canal. n. ch. : notochord.
atr. : atrial opening. ot. cy. : statocyst.
oil. gr. : ciliated groove or funnel. sens. ves. : sensory vesicle.
end. : endostyle. stig. : earliest stigmata.
eye : right eye.
These specimens illustrate the production of a single eye or a single statocyst by the suppression (complete or incomplete) of one member of a pair of sensory organs. (From Korschelt and Heider, after Kowalewsky.)
Gotte's opinion with regard to the formation of the pineal diverticulum at the point where the roof of the brain remains latest attached to the external skin was also criticized by F. M. Balfour in 1885, who stated that he could find no indication in elasmobranchs of a process similar to that which was described by Gotte, and that his observations had not been confirmed for other vertebrates. Balfour also alludes to Gotte's comparison of the pineal gland or diverticulum to the " long-persisting pore which leads into the cavity of the brain in Amphioxus," and he comments : " We might also add that of the Ascidians " (Fig. 3).
Rabl-Ruckhard, in 1886, put forward the interesting suggestion that the function of the organ was to estimate the heat of the sun's rays, and that it was a thermal sense-organ rather than visual. Spencer, who in '87 experimented on the sensitivity of the parietal organ to light, stated that " In lizards, whose paired eyes are closed, no result is obtained by focusing a strong beam of light on to the modified eye scale, and thus on to the pineal eye ; in fact, strong light focused into one of the paired eyes merely causes the lid to be drawn down, without any further apparent result, whilst in the pineal eye there is no protecting lid, and no movement whatever takes place to remove the eye from the direction in which the light is coming." Nowikoff experimented similarly with electric lights and magnesium wire on lizards without producing movements. Francotte in 1887 experimented with Lacerta muralis and Anguis fragilis. He constructed a cage consisting of six boxes arranged round a central rectangular space. Each box communicated with the central space by a small opening. In one box he placed an electric lamp ; and in the central space a lizard was placed, with its lateral eyes covered by a red material. The top was then closed in by a covering lid. The experiment, which was repeated several times, showed that in eight cases out of ten, after a quarter of an hour the lizard was found in the lighted box. The same experiment was also tried with a " blind worm," Anguis fragilis, under similar conditions, and the animal was found in the lighted chamber three times out of ten.
Francotte considered that the experiment, without being absolutely conclusive, allowed one to think that in the lizard, at any rate, the unpaired eye is still capable of perceiving light. He thought, however, that the experiment hardly proved that the lizard was attracted by the light only,, but that it was also attracted by the warmth of the electric light. He accordingly modified the experiment by darkening one half of the terrarium and allowing a diffuse light to fall on the other half. After allowing a considerable period of time to elapse he found that the animals had shown no tendency to collect in the lighted half.
Experiments were also conducted with the object of ascertaining the influence of light in producing movement of pigment in the retina of the median eyes of both vertebrates and invertebrates. Thus Nowikoff in 1 910 investigated the action of light on the median eyes of Lacerta agilis and Anguis fragilis . He divided the experimental animals into two groups, one of which he kept for 2 to 3 hours in absolute darkness, while the other group was exposed for a similar period to full sunlight. In the subsequent histological examination of the retina of those animals which had been exposed to the light, he found the pigment granules tended to accumulate in the inner ends of the cells nearer the source of light and central cavity ; whereas in those animals which had been kept in the dark the pigment was found in the outer part of the retina, farther from the source of light. The subject of the presence of pigment in and around the pineal organ is of the very greatest interest with reference to (i) the origin of lightperceiving organs in general (Bernard) and (2) the occurrence of regressive changes and melanotic tumours in the human pineal organ. A more detailed description of the position of the pigment granules, their variations in type, and their significance will be given later in the appropriate places.
Another important period in the history of the pineal body is that in which its median situation between the two lateral eyes and the apparent similarity of its structure to that of the median eyes of invertebrates attracted the attention of some of the leading biologists and palaeontologists of the time (1880-1910), more especially with reference to the light that its study might shed upon the problems of the ancestry of the vertebrates and the connection of the pre-vertebrate stock with that of the invertebrates. Intimately bound up with this question was the controversy which arose as to whether the pineal body originated as a bilateral pair of light-percipient organs or whether as two separate median organs which arose serially one behind the other and belonged to two neural segments. Among those specially interested in these different problems we may mention : Baldwin Spencer, Lankester and Bourne, Kingsley, Beard, Gaskell, Patten, Dendy, Smith Woodward, Studnicka and Klinckowstrom. As in the case of pigment, the detailed consideration of these questions will be most appropriately dealt with in the section on the morphology of the pineal system. It may be mentioned here, however, that although much speculation, based on preconceived notions and insufficient evidence, occurred during this period, some of the most important observations were made on the structure and nature of the pineal system during this period and in some cases arose as a direct result of investigation which was stimulated by the controversy.
Among the observations brought out in this way we may specially mention the discovery and accurate description of the nerves of the pineal eye by Nowikoff, Beraneck, Dendy, Klinckowstrom, and others, more especially in cyclostomes, fishes, amphibians, and reptiles. Moreover, it will be appropriate to draw attention here to the significance of the connection of the right and left pineal nerves, with the corresponding right and left habenular ganglia (Dendy), and also of the relation of the pineal nerves with the habenular and posterior commissures (Studnicka). We may further emphasize the significance of the existence of a nervus pinealis during one phase of development and the disappearance of this nerve at a later stage as evidence of a regressive character being manifested in the ontogenetic history of the organ.
In recent years the interest shown in the pineal body has been revived, more especially in connection with its supposed endocrine function. This has led to a more exact study of its histological structure and experimental work on animals along the lines of extirpation, feeding with the whole gland or desiccated preparations of the gland, injection of extracts, and grafts. Details of this work will be discussed later, it being only necessary to mention here that although much has been learned by the use of special neurological methods of histological technique and from the clinical observations of pathological cases occurring in the human subject, the results recorded by various authors of the experimental work are often conflicting, and the clinical syndrome described by Pellizzi, namely macrogenitosomia precox, does not always accompany destructive lesions of the pineal body, and the symptoms have sometimes been present but on post-mortem examination no abnormality of the pineal has been discovered. On the other hand, the careful observation and record of the special pressure-symptoms which are produced by enlargement of the pineal organ, accompanied by X-ray examination, has been of great value in the diagnosis of tumours originating in or near the gland.
Significance of the Pineal Body, considered from the Standpoint of Comparative Anatomy
Towards the end of the nineteenth century the opinion expressed by Dendy, Gaskell, Patten, and others, that the pineal eye of vertebrates was primarily bilateral in origin, led not only to further work on the pineal eye of vertebrates but also to a careful comparative study of the position and microscopical structure of the median (paired and unpaired) eyes of invertebrates ; and it was considered by Gaskell that the pineal system formed one of the most important clues to the origin of the vertebrates. These, he believed, originated from a pre-vertebrate ancestor which had affinities with one of the higher invertebrate phyla, and more particularly the Arthropoda. The similarity in general form of certain fossil fishes belonging to the class Ostracodermi — e.g. Cephalaspis and Pteraspis — to the living representatives of the Xiphosura, namely the king crabs (Limulus polyphemus), and also to some small living crustaceans, e.g. Apus cancriformis and Lepidurus, seemed to indicate that the whole vertebrate stock had arisen from a remote fish-like ancestor which was related to the arthropods and more particularly to the Xiphosura, Trilobites, and the gigantic " sea scorpions " Eurypterus and Pterygotus. Recent embryological and palacontological work has, as we shall show later, done much to confirm the view that the pineal organ was primarily a bilateral structure, but it is now believed that the vertebrate stock branched off from the invertebrate ancestors at a very remote period, before the special characters which are typical of the higher classes of invertebrates and vertebrates had been established. Many of the points of resemblance which have been observed between the higher types of invertebrates and vertebrates may be explained by assuming that certain fundamental characters, such as a general bilateral symmetry involving the nervous system and senseorgans, and certain common characters in the structure of the genitourinary system, have been retained in both. While the differences between these classes may be accounted for on the assumption that a gradual differentiation has arisen in the course of time in adaptation to varying needs and possibilities of development, and that these changes, taking place along divergent lines, have eventually led to the formation of more fully evolved organs, which while retaining certain common characters inherited from the ancestral stock, yet differ in important respects, such as having eyes with an upright or an inverted type of retina. Some idea of the antiquity of the vertebrate kingdom and, by inference, the still greater antiquity of the common ancestor of the higher types of invertebrates and vertebrates may be gained by a consideration of the evidence which is afforded by the existence of the pineal organ in some of the most ancient types of fossil fish, e.g. Cephalaspis, Pteraspis, and other species. In these the small shallow pit which is situated between the orbits and believed to mark the position of the pineal organ is on the inner aspect of the cephalic shield. The existence of this depression on the internal aspect of the vault of the skull indicates that even in these archaic types of fish the pineal organ, although retaining its connection with the skull, must have already been withdrawn from its primary superficial position beneath the skin into the cavity of the skull, or that in the course of ontogeny the " parietal organ " or " pineal eye " had been severed from the stalk of the pineal outgrowth and that the distal end of the latter had caused the impression on the inner aspect of the vault of the skull, as is seen in many living types of fishes, Amphibia, and reptiles. In either case it is evident that the organ in these extinct fishes must have already reached a retrogressive or vestigial stage in its evolutionary history. Since the " parietal organ " had already been cut off from its connection with the brain as well as from the source of light, it is obvious that at this very remote period in the history of the vertebrates it must have ceased to function, at any rate as a light-perceiving organ.
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Cite this page: Hill, M.A. (2020, October 21) Embryology Book - The Pineal Organ (1940) 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_2
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