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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF

The Pineal Organ (1940): 1 Introduction | 2 Historical Sketch | 3 Types of Vertebrate and Invertebrate Eyes | Eyes of Invertebrates: 4 Coelenterates | 5 Flat worms | 6 Round worms | 7 Rotifers | 8 Molluscoida | 9 Echinoderms | 10 Annulata | 11 Arthropods | 12 Molluscs | 13 Eyes of Types which are intermediate between Vertebrates and Invertebrates | 14 Hemichorda | 15 Urochorda | 16 Cephalochorda | The Pineal System of Vertebrates: 17 Cyclostomes | 18 Fishes | 19 Amphibians | 20 Reptiles | 21 Birds | 22 Mammals | 23 Geological Evidence of Median Eyes in Vertebrates and Invertebrates | 24 Relation of the Median to the Lateral Eyes | The Human Pineal Organ : 25 Development and Histogenesis | 26 Structure of the Adult Organ | 27 Position and Anatomical Relations of the Adult Pineal Organ | 28 Function of the Pineal Body | 29 Pathology of Pineal Tumours | 30 Symptomatology and Diagnosis of Pineal Tumours | 31 Treatment, including the Surgical Approach to the Pineal Organ, and its Removal: Operative Technique | 32 Clinical Cases | 33 General Conclusions | Glossary | Bibliography

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Chapter 17 The Pineal System of Vertebrates The Parietal Region

Before commencing the description of the pineal system of vertebrates, it will be necessary to define certain terms which are applied to structures included in the parietal region of the brain, namely, that part of the roof of the interbrain or thalamencephalon which lies between the paraphysis in front and the posterior commissure of the midbrain or mesencephalon behind. These structures are seen in their simplest form in the embryos of certain Teleostean fishes, e.g. Acanthias vulgaris (Fig. 130), or among reptiles, Lacerta muralis (Fig. 173, Chap. 20, p. 244).

Fig. 130. — Sagittal Sections of Acanthias Embryos, showing Structures in the Roof of the Diencephalon. (After C. S. Minot.)

A : 15 mm. embryo ; B : 28 mm. embryo ; C : 70 mm. embryo.

A 1 : Plane of section, Fig. 139, A.

B 1 : Plane of section, Fig. 139, B.

ds. : dorsal sac. hg. : habenular ganglion.

Ec. : Ectoderm. p. : paraphysal arch.

Ep. : Epiphysis. MB. : midbrain.

FB. : forebrain. pc. : posterior commissure.

he. : habenular commissure. p. int. : pars intercalaris.

V. : velum transversum.

From before backwards these parts include :

(1) The paraphysis.

(2) The velum transversum.

(3) The dorsal sac. Tela chorioidea superior.

(4) The habenular commissure (commissura superior).

(5) The pineal organs, including :

(A) The pineal eye or parietal organ.

(B) The " parapineal organ."

(C) The connections of these organs with the central nervous system.

(6) Pars intercalaris (Schaltstiick).

(7) The posterior commissure (commissura posterior).

When two pineal organs are present in or near the median plane, the term " Epiphysis I " has been used to denote the more anterior and " Epiphysis II " to denote the more posterior organ. When the term " epiphysis " is used alone it is generally applied to the primary embryonic diverticulum or to the pineal gland as distinct from the pineal eye or parietal organ. Thus the name " epiphysis " is usually applied to the deeply situated solid organ, e.g. that of the mammalian brain as contrasted with the superficial sensory vesicle or pineal eye of the lamprey or Sphenodon.

In addition to the above-mentioned structures, which are included in the parietal region of the brain, are accessory parts which comprise :

The fibrous coverings or capsule.

Pigment — (a) mesodermal ; (b) epidermal.

Pineal vessels and nerves.

The parietal cornea (Studnicka).

The parietal plug (Dendy).

The parietal spot (P. fleck).

The parietal scale (P. schuppe).

The parietal plate or pineal plate.

The parietal pit or impression.

The pineal- or parietal-foramen.

The meaning of most of these terms is self-evident. The parietal cornea is, however, defined by Studnicka as including all the translucent tissues lying over the parietal organ and between it and the superficial surface of the epidermis, whereas the name " parietal plug " was applied by Dendy to the conical mass of transparent tissue between the lens of the parietal organ and the deep surface of the corium or dermis in Geotria. The terms pineal plate or parietal plate are given to the bony plate which sometimes overlies the pineal organ in certain fishes and reptiles. The pineal plate may lie between the frontals, between the frontal and parietal bones, or between the parietal bones. It may or may not be perforated by a parietal foramen. It is perforated in the skull of Lacerta agilis ; not perforated in the arthrodiran fish Dinichthys intermedins found in the Upper Devonian of Ohio (Fig. 131) and in that remarkable extinct Elasmobranch fish Pleuracanthus (Fig. 141, B, p. 201). Examples of the latter have been found in the Carboniferous and Permian periods, and it would seem that a complete parietal foramen previously present in this plate had already disappeared at a date which preceded these two periods. The more primitive condition, namely, the presence of a parietal foramen, seen in the two examples cited, is thus exemplified in a living animal, the lizard ; whereas the closed foramen, indicating a complete loss of visual function and atrophy, is seen in the fossil fish. In the still more ancient fish Osteolepis, however, a well-marked pineal foramen is present in a plate formed by the fusion of the two frontals and lying between the orbits (Fig. 132).

Fig. 131. — Head-shield of Dinichthys intermedius. (After A. S. Woodward.) A — Upper surface.

B — Intracranial surface, the latter showing a well-defined pineal impression, from the upper Devonian of Ohio, U.S.A.

p. imp. : pineal impression. p. pi. : pineal plate. orb. : orbit.

Definition of Terms applied to Structures in the Parietal Region of the Brain

1. Paraphysis (Fig. 130). — This is a sac-like evagination of the roof of the interbrain, which arises at the junction of the lamina supraneuroporica and the inwardly projecting transverse fold, the velum trans versum. The walls of the paraphysis which at the commencement of its development are simple are apt to become much folded by the ingrowth of vascular processes of the surrounding mesoderm, so that in the fully developed state the paraphysis has the appearance of a complicated glandular structure and on account of its resemblance to the choroid plexuses of the ventricles was termed by Sorensen the " plexus chorioideus superior." In some cases, e.g. in Sphenodon, it grows backwards over the dorsal sac between this and the pineal eye (Fig. 252, Chap. 24, p. 369). The paraphysis is frequently absent, e.g. in the adults of many mammals.

Fig. 132. — Dorsal View of Osteolepis macrolepidotus, one of the most Ancient Types of Fossil Fish found in the Upper Silurian and Devonian Strata, showing a Parietal Foramen in the Frontal Bone and between Large Orbital Cavities. (After E. S. Goodrich.)

e. : Jr.. io.c it. :

'• •• /. : m.c mx. n. : na. op. Pf.

ethmoid, included in rostral-shield, frontals fused and enclosing pineal foramen.

infraorbital canal, intertemporal, jugal. lacrimal.

main trunk-canal. maxillary, nostril.

• nasal included in rostral-shield, opercular, pineal foramen. p. op. : pre-opercular. p. op. c. : preopercular canal. pt.f. : post frontal. pt. o. : post-orbital, r. sh. : rostral-shield. so. : supra-orbital. 5. ob. c. : supraorbital canal. 5. op. : subopercular. sp. oc. : dermal supra-occipital. sq. : squamosal. st. : supra-temporal or pterotic. postorbital and supra temporal canal, tabular. c. : transverse occipital canal.

2. Velum Transversum. — A membranous fold which projects from the roof of the interbrain into the cavity of the ventricle. Its walls are usually smooth and simple, but it may be thrown into folds by the ingrowth of vessels, especially in its lower part. In some cases instead of forming a single transverse fold, it appears as two, paired folds, as in the Dipnoi, or is developed as an unpaired choroid plexus as in tailed amphibia.

3. The Dorsal Sac. — Fig. 252, Chap. 24, p. 369, and Fig. 130, p. 180, ds. This is an arched membranous fold which lies behind the transverse velum, the posterior layer of which forms the anterior boundary of the lower part of the dorsal sac. It projects towards the roof of the skull, between the pineal stalk and pineal nerve behind and the paraphysis in front. In some cases the dorsal sac grows backward on each side of the stalk of the epiphysis in the form of two blind pouches. The stalk in these cases may be enclosed in a tubular sheath of ependyma, or lie in a groove in the posterior wall of the sac. The wall of the sac is like that of the velum, in some cases smooth and simple, in others thrown into folds which may be vascular and form part of the tela chorioidea superior (Fig. 252).

4. Commissura Habenularis. — This, which is also named the superior commissure, appears later and is much smaller than the posterior commissure. It joins the right and left habenular ganglia. In some cases the habenular ganglia are fused in the median plane and the commissure is then enclosed in the centre of the mass, as in Petromyzon.

5. The Pineal Organs A . The parietal organ — (parietal eye ; pineal eye ; sensory-vesicle) (Fig. 133) : the principal or parietal sense-organ which is socketed in the roof of the skull may be derived from either the right (Cyclostomes) or left (Sphenodon) member of the primarily paired organ. In Cyclostomes it arises further back than the smaller vesicle which lies below it and to the left. The larger vesicle is therefore sometimes described as Epiphysis II whereas the term Epiphysis I is given to the small vesicle — " parapineal organ " of Studnicka — which springs from a point farther forward and is connected with the anterior part of the left habenular ganglion (see Fig. 134, p. 188, and Fig. 22, Chap. 3, p. 28). It must be borne in mind, however, that this attachment in Petromyzon is one which has been secondarily displaced forward along with the anterior part of the left habenular ganglion and that the primary outgrowth of the pineal diverticulum was immediately in front of the posterior commissure.

The parietal sense-organ in those animals in which it is best developed, such as in the Cyclostomes and certain lizards, consists of a hollow vesicle, the superficial wall of which is lens-like, while the proximal part is differentiated as a retina of the upright type, having receptive sensory cells, pigment cells, and ganglion cells, with nerve-fibres proceeding from these to form a parietal or pineal nerve which terminates directly in the habenular ganglion of the same side or in either the habenular or posterior commissure, or in both of these and so reaches the habenular ganglia and other nerve centres with which the commissures are connected. The nerve-fibres may pass by the stalk of the parietal organ as in Petromyzon, or they may course independently of the stalk, being formed secondarily as in Lacerta (Figs. 172, D, 173, Chap. 20, pp. 243, 244).

A Fig. 133. — Dorsal View of the Brain of Petromyzon marinus, showing the Right Pineal Organ and Large Right Habenular Ganglion. (After Ahlborn.) Cbl : cerebellum ; CN. I. : cranial nerve I. ; F. Rh. : fossa rhomboidalis ; M. Br. : midbrain ; JR. Pi. O. : right pineal organ.

The parietal sense-organ may be cut off and separated from the pedicle by the growth of the vault of the skull (Figs. 161, 162, Chap. 19, p. 228). In these cases it may persist throughout life as a functionless organ lying beneath the skin as in the frog, in which animal it remains as the " frontal organ " of Stieda, or it may fail to develop. In both cases the base of the stalk and other parts may persist as the pineal body (pineal organ ; pineal sac ; pineal gland ; conarium ; epiphysis), the detailed structure of which will be considered later.

B. The parapineal organ : this is the term which was applied by Studnicka to the anterior smaller end-vesicle Epiphysis I of Petromyzon in his description of the pineal system of Cyclostomes. In reptiles it is represented according to Studnicka by the anterior of the two organs, which in the Reptilia is the more fully developed organ and becomes the parietal eye. The posterior organ (pineal organ or Epiphysis) remains entirely within the cranial cavity, and though as in Sphenodon it may develop a terminal vesicle (pineal sac) (Fig. 252, Chap. 24, p. 369) it is cut off from all access of light and cannot function in any way as a visual organ. The same objection to the use of the terms " anterior organ " and " posterior organ " may be raised on developmental and morphological grounds as was made in discussing the terms Epiphysis I and Epiphysis II. The two organs in Sphenodon are developed from a single median diverticulum which afterwards is subdivided into two parts, an anterior vesicle which is separated off as a closed vesicle, the parietal organ or pineal eye and a posterior part which consists of the original stalk and an end-vesicle, the pineal sac. The stalk and pineal sac together form the pineal organ or epiphysis. On both developmental and morphological grounds Dendy regarded the anterior part of the diverticulum which becomes detached, to form the parietal sense-organ in Sphenodon as the representative of the primary left pineal eye, and the pineal sac as representing the primary right pineal eye and it would seem that the stalk which is common to both has arisen by approximation and fusion during phylogeny of the bases of a pair of bilaterally placed median eyes. If this interpretation is correct, the parapineal organ does not arise ontogenetically as a separate and distinct outgrowth from the roof of the interbrain, and does not spring from a separate cephalic metamere lying in front of the metamere which gives origin to the pineal organ. The separate anterior diverticulum which arises in front of the pineal diverticulum gives rise to the paraphysis and never develops a terminal sensory vesicle.

C. The connections of the parietal organ and the parapineal organ with the central nervous system will be described separately with reference to different types of animals.

6. The Intercalated Segment is a part of the roof plate which lies between the base of the pedicle of the pineal body and the posterior commissure. It is characterized by the absence in it of commissural nerve-fibres and it varies greatly in its extent in different types of animal.

7. The Posterior Commissure. This lies immediately above the anterior end of the aqueductus cerebri, and receives some fibres from the pineal organ, in addition to its nuclear connections, and the connections with the median longitudinal bundles. It varies greatly in its size and extent in different animals, Figs. 130 and 208, Chap. 22, p. 303 (rabbit embryo). In some it is spread out in a series of bundles, while in others it is compressed into a rounded cord. The fibres which it receives from the parietal organ in Petromyzon are said by Studnicka to divide in a T-shaped manner into branches which go to either side, but he was unable to trace their ultimate destination.

The Pineal Eyes of Cyclostome Fishes

The pineal eyes of this primitive class of vertebrates have been specially studied by Ahlborn, Wiedersheim, Studnicka, Dendy, Beard and Gaskell. The class comprises the marine and freshwater lampreys {Petromyzon) and the Australian form of lamprey (Geotria) ; also the slime-eels or hag fishes {Myxine and Bdellostoma), Bdellostoma being so named on account of its leech-like mouth. It is described as being blind, parasitic and degraded in type, and specimens of it have been taken from great depths of the sea. The lampreys, on the other hand, although the median eyes are probably little more than light-percipient organs, have welldeveloped lateral eyes in the adult animal and are active in their movements. They feed on the flesh of living fishes and are predatory rather than parasitic.

The pineal system of the adult Petromyzon is situated close behind the single nasal orifice. It consists of two vesicles : one, the larger, called the parietal organ, is situated more superficially ; while the other, which is smaller and less differentiated, lies beneath the former (Fig. 22, Chap. 3, p. 28, and Fig. 134, p. 188). The latter was termed by Studnicka the parapineal organ. The larger vesicle is connected by a definite tract, the pineal nerve, with the right habenular ganglion (Fig. 14, Chap. 3, p. 19), and also sends fibres into the posterior commissure and right bundle of Meynert (Fig. 134). The smaller vesicle is connected by a few fibres with the left habenular ganglion, posterior commissure, and left bundle of Meynert. Owing to its anterior attachment to the roof of the thalamencephalon or interbrain, the smaller organ is sometimes termed Epiphysis I, while the larger organ, on account of its posterior attachment, is known as Epiphysis II. The right habenular ganglion, in correspondence with the greater size of the superficial vesicle as compared with the smaller deep organ, is proportionally larger than the left habenular ganglion, and the right bundle of Meynert is for the same reason larger than the left bundle (Dendy, A., 1907).

Owing to the connection of the larger vesicle, by means of its pineal nerve, with the large right habenular ganglion, and the similar connection of the smaller vesicle by nerve-fibres with the small left habenular ganglion, and also the position of the smaller organ slightly to the left of the larger, along with other evidence of the bilateral origin of the two organs, Dendy, Gaskell, and others regarded the two vesicles as right and left members of a primarily paired organ, rather than anteriorly and posteriorly arranged unpaired metameric organs arising in the median plane. Professor Dendy also assumed that the smaller vesicle having undergone greater regressive changes than the larger, has been displaced beneath the larger. In this position, where it lies under cover of the superficial vesicle, it is obvious that the smaller, less differentiated organ has become quite functionless as a visual organ and that the small size of the left habenular ganglion as compared with the right, is correlated with this displacement and loss of function. Lying directly over the larger vesicle, is a thick conical plug of translucent mesenchymatous tissue which is especially well developed in Geotria, the New Zealand lamprey (Fig. 134).

Fig. 134. — Sagittal Section through the Head of the New Zealand Lamprey (Geotria), showing the General Position and Connections of the Right and Left Pineal Eyes. (After Dendy.)

Aq. C. : aqueductus cerebri.

Cbl. : cerebellum.

Ch. PL : choroid plexus.

Cr. C. : cranial capsule.

Ep. : epidermis.

Hyp. : hypophysis.

Lam. Term. : lamina terminalis.

L. Hab. G. : left habenular ganglion.

L. Par. O. : left parietal organ (parapineal organ).

L. Pin. N. : left pineal nerve.

Muse. : muscle.

N. Ch. : notochord.

Opt. Ch. : optic chiasma.

Par. PL : parietal plug.

Pigm. C. : pigment cells.

Post Com. : posterior commissure.

R.F. : Reissner's fibre.

R. Hab. G. : right habenular ganglion.

R. Par. O. : right parietal organ.

IV. Ve. : fourth ventricle.

This fills a gap in the cartilaginous roof of the skull. Above is a layer of loose subcutaneous connective tissue and the epidermis. The hypodermal cells of the latter are peculiar in containing no pigment granules such as are present in this layer elsewhere, neither is there any pigment in the subepidermal tissue. All the tissues between the parietal organ and the superficial surface of the epidermis are translucent and constitute the " parietal cornea " as defined by Studnicka. The pineal organ can thus be seen through the cornea and appears as a central white spot (Fig. 48, Chap. 3), as described on p. 71.

Sections of the larger superficial vesicle (Fig. 45, Chap. 3) show that the " eye " is of the ocellar or upright type. It consists of a superficial or distal segment — the " lens " — termed by Studnicka the " pellucida " and a deep or proximal segment, the retina. These are continuous with each other at the circumference of the vesicle and they enclose between them a central cavity filled with a loose syncytial tissue which in the living animal is believed to enclose within its meshes a clear semi-fluid material. The whole tissue with the viscous fluid contained in its spaces is termed the vitreous. The cavity is in some specimens funnel-shaped (Fig. 45, Chap. 3, p. 69, and Fig. 14, Chap. 3, p. 19). The distal ends of the columnar cells of the retina converge towards the lumen of the " groove " or " tube " forming the narrow part or stalk of the funnel much in the same way as they do in the cylindrical upright eyes of certain arthropods, e.g. Acilius sulcatus, which in the large parietal eyes of young larvae, about 10-12 mm. long show a central cleft in the retina ; this, as pointed out by Gaskell, is strikingly similar to that in the parietal organ of Ammocoetes. The cleft in the Acilius larva, moreover, is directly continuous with the virtual cavity which occupies the central axis of the cylindrical " vitreous " segment of the eye of Acilius sulcatus (Grenacher). It must be borne in mind, however, that the bottom of the cleft in an Acilius or Dytiscus larva (Fig. 38, Chap. 3, p. 53) corresponds to the bottom of the optic pit, from which the eye is developed as a downgrowth of the surface epithelium into the sub-epidermal tissue ; whereas in Ammocoetes the blind end of the diverticulum from the roof of the interbrain is directed distally away from the stalk, and the cleft or atrium is in the stalk near the proximal end of the vesicle (Fig. 137, B). With reference to the supposed origin of the funnel-shaped prolongation of the cavity of the main vesicle into the distal end of the " optic stalk," it will be convenient to allude here to the position and relations of the atrium, which is a small accessory cavity in the wall of the main parietal organ. The atrium varies considerably in size and shape in different specimens ; it is usually situated behind and below the cavity of the main vesicle with which it communicates in the majority of cases, although it is in some completely cut off and appears as an independent closed vesicle lying between the proximal end of the funnel-shaped cavity and the exit of the pineal nerve. The wall of the atrium is composed of cells which are columnar in type. The free ends of these converge towards the central cavity, while their nuclei are peripheral (Fig. 45). Around these cells is an accumulation of irregularly disposed cells, the nuclei of which resemble those of the surrounding ganglion cells. The atrium therefore has been thought by some writers to represent a nerve ganglion situated in the wall of the vesicle and interposed between the sensory or visual cells and the tract of nerve-fibres which connect it with the habenular ganglion. Others, however, considering the similarity of the columnar cells lining its cavity to the columnar cells of the retina, regard the atrium as a part of the general cavity of the organ which has been either wholly or partially constricted off, and corresponds either to a secondary diverticulum from the main outgrowth or a part of the cavity of its stalk. According to Kuppfer (1894) tne primary outgrowth of the diverticulum from the roof of the interbrain is at first directed backward ; later an extension forward takes place, so that in a median sagittal section its cavity including that of the stalk appears T-shaped, and it seems possible that the posterior limb of the cross-bar of the T persists as the atrium whereas the distal part of the vertical bar remains as the stem of the funnel.

The retina (Figs. 45 and 46, Chap. 3, pp. 69, 70) consists of (1) sensory cells ; (2) tall, columnar cells containing the so-called " white pigment " ; (3) ganglion-cells lying in a plexiform layer of nerve-fibres, with a few spindle-shaped neuroglial or connective tissue cells. There are also an internal and an external limiting membrane and a thin fibrous capsule continuous with the pia mater of the brain. The sensory or visual cells end distally in a flask-shaped or bulbar swelling covered over by a cap of clear finely granular material which frequently tapers at its extremity into a fine thread which is continuous with a syncytial fibre of the vitreous. The base of the knob or flask rests on the internal limiting membrane and is continuous through the membrane with the thin rod-like outer end of the body of the cell ; the oval nucleus is situated in a spindle-shaped swelling of the body near the base of the cell. Proximal to the nucleus, the body of the cell tapers into a fine nerve process which subdivides in a plexus of nerve-fibres containing the ganglion cells, the axons of which course in a tangential direction in the latter and join to form the pineal nerve.

The interpretation of specimens prepared by the silver impregnation methods is difficult. Cells which are heavily impregnated are found in the lens, and according to Studnicka's description of specimens prepared by Retzius many of the cells which are blackened in the retina are supporting ependymal cells which show typical sole-like thickenings, where their outer or proximal ends are attached to the external limiting membrane, whereas the inner or distal ends of the columnar retinal cells which contain the pigment remain unstained. On the other hand the nervefibre layer and the continuity of many of the nerve-fibres with the fibres of the pineal-nerve ; and also of the retinal cells of the parapineal organ and the connections of these with the anterior (left) habenular ganglion and the nerve-fibres of the pineal tract were well brought out by the impregnation process.

The columnar cells containing the " white pigment " (Fig. 45, B) are variously regarded as being : (1) ependymal cells, which function as supporting cells and are comparable to the " fibres of Miiller " in the retina of the lateral eyes (Studnicka) ; (2) " pigment " cells, which contain a crystalline deposit of calcium phosphate, guanin, and a small quantity only of melanin. The deposit of phosphate of lime was regarded by both Mayer and Gaskell as an indication of degeneration and comparable with the " brain sand " found in the pineal body of mammalia.

Fig. 135. — Diagram to show the Different Shapes of Head-shield, due to

the Forward Growth of the Somatic Musculature. (After Gaskell.)

A : Didymaspis ; B : Auchenaspis ; C : Cephalaspis ; D : Ammocoetes.

Ol. ap. : olfactory aperture ; Orb. : orbital cavity ; pJ. : parietal foramen.

The Parietal Foramen of Ammocoetes

The position of the narial and pineal openings in the muco-cartilage which forms the temporary head shield of an Ammocoete was studied by Gaskell, who compared the dorsal head-shield of the larval lamprey with the very similar head-shields of certain extinct fishes, belonging to the class Ostracodermata, and more particularly Didymaspis, Auchenaspis, and Cephalaspis (Fig. 135) ; in all of which the parietal foramen is placed behind the single nasal aperture and between the two orbital cavities for the lateral eyes. He also compared the microscopical structure of the head-shields of the Ostracodermata, as represented by Rohon, with that of the muco-cartilage of Ammoccetes, and found a marked similarity, more especially with regard to the disposition of the spaces relative to the matrix. A still further point of agreement which Gaskell emphasized is the relation of the somatic musculature to the head region. The somatic musculature seems to have advanced forward in successive stages of phylogeny and encroached upon the posterior border of the head-shield, causing a gradual absorption of this border on either side of the median hard plate which overlies and protects the brain, thus producing a notch on each side between the tongue-shaped plate which overlies and protects the brain and the lateral cornua. Gaskell further suggested that this plate on account of its position and general relations should receive the same name as the similar plate in the trilobites and be called the " glabella." Since Gaskell 's time the microscopical structure and general architecture of the head-shields of the Ostracodermata has been re-investigated, with the aid of modern methods of technique, by Stensio and Kiaer, and an account of their work will be found in the section on Geological Evidence of the Existence of Median Eyes in Vertebrates, Chapter 23.

The Pineal Nerve, Chiasma and the Pineal or Habenular Tract

The nervus pinealis was first described by Ahlborn (1883) as the stalk of the epiphysis. A point of considerable developmental interest is the relation that the delicate pineal nerve of the adult Petromyzon has to the original hollow stalk of the pineal diverticulum in the embryo. In other words, does the pineal nerve of the mature lamprey correspond in its mode of development to the optic nerve of the lateral eyes of vertebrates ? Or is it developed independently of the stalk of the primary pineal outgrowth ? Also, is there anything in the pineal system of the lamprey comparable with the optic chiasma, optic tracts, and central connections of the lateral eyes of vertebrates ? These questions are intimately bound up with that of the development of the two pineal vesicles, the nerve-fibres arising from them and the central connections of these with the habenular ganglia and the anterior and posterior commissures. According to Kuppfer (1894) (Fig- I 38, A, p. 196) and Johnstone (Fig. 136), two separate pineal diverticula are developed ; a smaller anterior and a large posterior. Studnicka (1893) quite independently came to the same conclusion, but Kuppfer's figure shows the anterior diverticulum in front of the anterior commissure, a position which throws a grave doubt on the identity of the small recess which he depicts, with the future parapineal organ (Fig. 22, p. 28). On the other hand, Balfour (4-8 mm. larva of Petromyzon), Dohrn (Fig. 138, B, p. 196), and the well-known figures by Dohrn depicting the early developmental stages of the olfactory and pituitary sacs of Petromyzon, show only one pineal diverticulum. These authors and others describe only a single diverticulum which spreads forward, backward, and also laterally. In the later stages of development the stalk becomes elongated and its lumen disappears, the two terminal vesicles being carried forward away from the original site of origin of the diverticulum, namely, between the anterior and posterior commissures. On the left side a part of the left habenular ganglion is carried forward with the anterior or ventral terminal vesicle, and forms the " cushion " described by Ahlborn on which the vesicle rests (Fig. 22, Chap. 3, p. 28) ; the remaining part of the ganglion is left in its original position, the two parts of the ganglion being connected by a strand of nerve-fibres called the " pineal or habenular tract." The whole of the larger right habenular ganglion appears to remain in its original position. It is connected with the superficial vesicle or " parietal organ " by the " pineal nerve " and by some commissural or chiasma fibres with the posterior part of the left habenular ganglion ; these fibres constitute the habenular or anterior commissure. The right pineal nerve (Dendy, Studnicka) is connected with the posterior commissure and right bundle of Meynert as well as with the right habenular ganglion.

Fig. 136. — Schematic Diagram designed to show the Relations of the Two Epiphyses in Vertebrates. (After J. B. Johnstone.)

ca. : anterior commissure. np. 1 : nerve of anterior epiphysis.

cp. : posterior commissure. np. 2 : nerve of posterior epiphysis.

D.s. : dorsal sac. Par. : paraphysis.

Ep. 1 : anterior epiphysis. t. : tectum.

Ep.- : posterior epiphysis. vel. : velum.

The intermediate stages of development between the adult condition and the embryonic condition have, so far, not been completely filled in. Studnicka, however, showed that the pineal nerve of a 35-mm. Ammocoetes (Fig. 137, a) extended backward from the parietal organ to the posteriorcommissure, and that the nerve-fibres at this stage were enclosed throughout the whole length of the nerve in a nucleated sheath which was continuous anteriorly with the wall of the parietal organ . In transverse sections of ;he pineal nerve, obtained from an Ammocoetes of similar age, the proximal end of the nerve (Fig. 137, d) consisted almost exclusively of nerve-fibres, while at the distal end near the atrium (Fig. 137, b) the greater part of the section was occupied by epithelial cells ; the nervefibres here being confined to the lower and lateral parts in the periphery of the section. In the middle of the nerve (Fig. 137, c) the nerves were in the lower part of the section, the cells above ; the nerve-fibres were evenly distributed, not yet being collected together in cords. In the adult Petromyzon in the last part of the stalk, the nerve-fibres are arranged in definite layers ; moreover, outside the nerve-fibres in the fully developed pineal nerve, separate cells are still present which Studnicka regarded as being vestiges of the original tissue of the stalk. These cells are found either in the centre (Fig. 137, e) or at the periphery. The special piamatral sheath of the stalk which is continuous with that of the parietal organ, and covers a limiting neuroglial membrane is also considered by Studnicka as evidence of the pineal nerve being formed from and representing the stalk of the primary diverticulum. The pineal nerve of Petromyzon thus appears to differ from that of Lacerta (see p. 243), in which the nerve-fibres are developed separately and independently of the original stalk (Figs. 172, 173, Chap. 20). The explanation of this difference probably lies in the circumstance that while in Cyclostomes the stalk remains for a sufficiently long period to form a ready made pathway for the nerve-fibres to take, in their growth from the retina to the habenular ganglion ; in amphibia and in reptiles the parietal organ is cut off from the stalk before the growth of the nerve-fibres commences.

Fig. 137. A — a. : Longitudinal section of the pineal nerve of a 35-mm. Ammoccetes ; b, c, d : Transverse sections through different parts of the pineal nerve from a similar specimen ; b : near the atrium ; c : about the middle of the nerve ; d : through the proximal part ; e : transverse section through the pineal nerve of an older Ammoccetes. (After Studnicka.) Sagittal section through the parietal region of the brain of an older Ammoccetes. The atrium of the parietal organ is exceptionally large, and is situated at the same level as the lumen of the parietal organ. atr. : atrium. po. : parietal organ.

cp. : commissura posterior. pp. : paraphysis.

c.p.a. : commissura pallii anterior, pp. o. : parapineal organ.

B

ch. hob. : chiasma habenularis.

M. Br. : midbrain.

N. pin : nervus pinealis.

o. r. hab. : right habenular ganglion. tr. hab. : tractus habenularis.

On the left side the statement that true nerve-fibres pass from the left terminal vesicle to the anterior part of the left habenular ganglion is said by Studnicka not to have been confirmed, but whether the fibres joining the two parts are true nerve-fibres or not, the connection between the vesicle and the ganglion obviously represents the stalk of the left pineal organ, in which, if the vesicle should retain any sensibility to light, the nerve-fibres would be developed.

With regard to the term " pineal tract " which is rightly applied to the strand of nerve-fibres connecting the two parts of the left habenular ganglion (Fig. 137, B, and Fig. 22, Chap. 3) ; if the ganglion cells of the right parietal organ of Petromyzon are regarded in the same light as those in the retina of a lateral vertebrate eye, namely as retinal, then the right pineal nerve of the lamprey, like the " optic nerve " of the human eye, is strictly speaking a " nerve tract," similar to inter-ganglionic nervetracts in the central nervous system generally, and the distinction implied between the terms " pineal nerve " and " pineal tract " is misleading. From the morphological standpoint the ganglion-cells around the atrium and elsewhere in the retina of the right parietal organ of Petromyzon probably correspond to those of the anterior part of the left habenular ganglion and the pineal nerve of the right pineal organ represents and corresponds to the pineal tract of the left organ. On the right side the anterior ganglion cells appear to have been incorporated in the retina ; on the left side they have remained as a separate ganglion outside the vesicle, but connected with it by a wide stalk of neuroglial tissue, which is surrounded by a constricting fold of pia mater.

The Development of the Parietal Organ of Petromyzon

According to Studnicka's description of the development of the two parietal organs, the " pineal organ " is the first to develop (Fig. 138). It appears at the posterior part of the roof of the interbrain in the form of a small, caudally directed diverticulum, the wall of which is formed by a single layer of cells. This soon becomes bent forward, and after having considerably increased in size two parts may be distinguished, a proximal hollow stalk immediately in front of the posterior commissure, which has meanwhile been developed, and a dorsoventrally flattened vesicular endpart which lies over the stalk, namely the " terminal vesicle " (Fig. 139). The terminal vesicle at this stage of development lies directly over the ganglia habenulae, which are already present, and it raises the skin over it so as to form a slight prominence. The whole organ at this period is, relatively to the brain, exceptionally large ; its transverse diameter being little less than that of the interbrain. The terminal vesicle which at first had a large lumen becomes transformed into a " loaf-like " structure. Its upper wall remains thin and single layered, whilst its lower wall becomes thickened, its superficial surface being arched upwards so that the floor and roof of the vesicle are in close contact with each other, and in transverse section the lumen appears as a crescentic cleft, convex superficially (Fig. 139). After a time, however, the lumen again enlarges and the two walls of the vesicle become separated again. Still later (Fig. 137, B), the lumen of the stalk disappears, except below where a small part persists as the recessus pinealis, which lies in front of the posterior commissure, and above in the situation where the stalk becomes continuous with the terminal vesicle. From the lower wall or retina of the terminal vesicle nervefibres grow into the stalk, the cells of the latter being pushed aside or penetrated in the same way as in the development of optic nerves of the paired [lateral] eyes, and it is in this way that the nervus pinealis is developed. At the upper end of the stalk, there is formed at a later stage a fairly large hollow cavity, the " atrium," the walls of which " remind one " of the structure of the retina, and as a rule the atrium opens into the cavity of the end vesicle near the centre of the retina, where there is a funnel-shaped recess. Subsequently the parietal organ is shifted forward from its point of origin in the region of the ganglia habenulae to a position, in the fully developed brain, in the region of the hemispheres.

Fig. 138. A and B : Median sagittal sections of the parietal region of the brain, showing early

stages in the development of the pineal organs of Petromyzon. (A after

Kuppfer ; B after Dohrn.) C — Transverse section of the rudiments of the pineal organs of a 37-day-old

embryo of Salmo fontinalis. (After Hill.)

ch. : habenular commissure.

cp. : posterior commissure.

/. po. : left pineal organ (anterior).

/5. ; lamina supraneuroporica.

MB. : roof of midbrain.

po. : pineal organ.

pp. : parapineal organ (Kuppfer and Studnicka).

r. po. : right pineal organ (posterior).

st. : common stalk of right and left pineal diverticula.

Fig. 139. — Transverse Section through the Pineal Organ of a 6-mm. Embryo of Petromyzon planeri. (After Studnicka.) Epd. : epidermis. po. : pineal organ. hg. : habenular ganglion. st. : stalk.

Studnicka's description of the development of the parapineal organ is not so explicit and it is probable that the recess which is shown in Kuppfer's diagram in front of the anterior commissure does not give origin to the parapineal organ, but that this arises as a subdivision of the primary pineal diverticulum.

From the foregoing description it will be evident that the pineal system of the Cyclostomes may be considered as a vestigial apparatus, which consists of an imperfectly developed pair of median eyes of the upright type, of which one, the left, is more degenerate than the other, and has been displaced beneath the other less degenerate right organ. Since the latter is placed immediately beneath a transparent " cornea " and has a retina provided with sensory cells, the inner ends of which are connected with the dendrites of ganglion cells, and by nerve-fibres arising from these with the habenular ganglia and posterior commissure ; the right organ may be regarded as capable of receiving impressions of light and transmitting these to the central nervous system. But owing to the imperfect development of the dioptric system no distinct image of an object on the retina is possible. It may be conjectured, however, that in the ancestors of the Cyclostomes both eyes were equally developed, were structurally more highly differentiated and reached to the surface of the body. The absence of a cuticular or epidermal lens in the living representatives of the class may be accounted for by the withdrawal of the eyes from the surface. This supposition being granted, the structure of the pineal eye does not differ very greatly or essentially from the upright median eyes of certain invertebrates.

The development of the lateral eyes of Petromyzon is similar to that of vertebrates in general ; an optic cup being formed by inversion of the distal part of the primary optic vesicle to form the sensitive part of the retina, while the proximal segment gives rise to the outer or ensheathing layer of the cup and becomes pigmented. The lens also is derived from the cutaneous ectoderm in the usual manner, a hollow vesicle becoming separated from the superficial epithelium by an ingrowth of mesoderm (Fig. 252, Chap. 24, p. 369).

Variations of the Pineal System in the Different Orders of the Class Cyclostomata

It is unnecessary to give a detailed description of the variations which occur among the different species of Petromyzon. It is of interest, however, to note that in Mordacia mordax, the pineal organ of which was described by Spencer in 1890, he found no trace of a parapineal organ. Also in the hag-fishes, belonging to the Order Myxinoidei, although several authors have described a single vestigial diverticulum without any thickening of the lens, or pigment in the retina, there seems some doubt as to whether these outgrowths were true pineal diverticula, and according to Kuppfer, who carefully examined a series of specimens in 1900, the parietal region behind the velum is quite smooth and he found no trace of an epiphysis. Both Myxine and Bdellostoma are said to be blind, to a large extent parasitic and of degraded type. Like the lampreys, they feed on the flesh of fishes, but instead of clinging on to the surface of the body, they actually bore their way into the flesh.

The description of the pineal system of the extinct Ostracoderms, which have been shown by Stensio and Kiaer to be very closely related to the living representatives of the Cyclostomata, will be referred to in Chapter 23, on the Geological Evidence of Median Eyes in Vertebrates and Invertebrates.

The Pineal Organ (1940): 1 Introduction | 2 Historical Sketch | 3 Types of Vertebrate and Invertebrate Eyes | Eyes of Invertebrates: 4 Coelenterates | 5 Flat worms | 6 Round worms | 7 Rotifers | 8 Molluscoida | 9 Echinoderms | 10 Annulata | 11 Arthropods | 12 Molluscs | 13 Eyes of Types which are intermediate between Vertebrates and Invertebrates | 14 Hemichorda | 15 Urochorda | 16 Cephalochorda | The Pineal System of Vertebrates: 17 Cyclostomes | 18 Fishes | 19 Amphibians | 20 Reptiles | 21 Birds | 22 Mammals | 23 Geological Evidence of Median Eyes in Vertebrates and Invertebrates | 24 Relation of the Median to the Lateral Eyes | The Human Pineal Organ : 25 Development and Histogenesis | 26 Structure of the Adult Organ | 27 Position and Anatomical Relations of the Adult Pineal Organ | 28 Function of the Pineal Body | 29 Pathology of Pineal Tumours | 30 Symptomatology and Diagnosis of Pineal Tumours | 31 Treatment, including the Surgical Approach to the Pineal Organ, and its Removal: Operative Technique | 32 Clinical Cases | 33 General Conclusions | Glossary | Bibliography

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