Book - The Pineal Organ (1940) 20
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- 1 Chapter 20 Reptiles
- 1.1 Development of the Pineal System in Reptiles
- 1.2 The nerves and nerve Ganglia of the Pineal System of Reptilia
- 1.3 The Pineal Organ of Reptiles
- 1.4 Structure of the Pineal Organ of Reptiles
- 1.5 The Parietal Organ of Reptiles
- 1.6 Structure of the Parietal Organ of Sphenodon
- 1.7 A. The Retina
- 1.8 B. The Lens
- 1.9 C. The Vitreous Body
- 1.10 Nerve Supply of the Pineal System of Sphenodon
- 1.11 The Early Development of the Pineal Nerves in Sphenodon
- 1.12 The Connections of the Pineal Organs with the Central Nervous System
- 1.13 The Vascular Supply of the Pineal System of Sphenodon
- 1.14 The Parietal Foramen of Reptiles
- 1.15 Geckonidae
- 1.16 Agamidae
- 1.17 Draco volans
- 1.18 Jguanidae
- 1.19 Anolis
- 1.20 Plica umbra (Spencer)
- 1.21 Iguana tuber culata
- 1.22 I. The Parietal Eye
- 1.23 III. The Proximal Pineal
- 1.24 Lacertidte
- 1.25 Scincidae
- 1.26 Cyclodus gigas
- 1.27 Gongylus ocellatus
- 1.28 Rhipidoglossa
- 1.29 The Pineal System of Ophidia
- 1.30 The Pineal System of the Chelonia
Chapter 20 Reptiles
It is in the class Reptilia that the greatest insight into the nature of the pineal system has been obtained, and more especially with reference to the pineal eye of that archaic lizard, the Tuatara of New Zealand, or Sphenodon, which has sometimes been alluded to as a " living fossil."
Most valuable information has also been gained by the study of the parietal foramen in the various orders of fossil reptiles, which, like that in Pisces and Amphibia, not only indicates the antiquity and universality of distribution of the median eye in the widely separated orders of the class, but that, judging from the large size of the foramen in some of the more primitive examples as compared with their living representatives, the pineal eye itself is, both actually and relatively to the size of the animal, smaller in living species than it was in their remote ancestors.
As in fishes and amphibians, there is indubitable evidence of two organs being included in the pineal system of reptiles ; there is a difference of opinion, however, as to whether the two components are essentially separate and independent median structures or whether they are derived from a primarily bilateral pair of sense organs which are homologous with each other and have been secondarily shifted into the median plane.
There is in reptiles the same subdivision of the whole pineal organ into proximal part, stalk, and end vesicle, as in fishes and amphibians, and there are the same connections with the roof of the diencephalon. There is, however, much variability in the degree of differentiation of the pineal system in different orders of the class and in individual species.
Valuable information has also been gleaned from the study of accessory parietal organs in reptiles and of the degenerative changes which have been observed in some cases, e.g. the presence of pigment in the lens ; absence or inconspicuous appearance of the parietal spot ; and degenerative or pathological changes in the neuro-epithelial tissue.
In Crocodilia neither parietal organ nor epiphysis is present in the adult animal, although the habenular and posterior commissures are well-developed and distinct, being separated by a small part of the roof of the diencephalon ; the dorsal sac and paraphysis are also present — alligator (Sorensen, 1894) 5 Crocodilus madagascarensis and Caiman niger (Voeltzkow, 1903).
Development of the Pineal System in Reptiles
In addition to the pioneer work of Leydig, Strahl and Martin, Beraneck and others, the early stages of development of the pineal outgrowth have been studied by Nowikoff (19 10) in Lacerta muralis and L. vivipara, and by Dendy (1899-1911) in Sphenodon. According to Nowikoff, the first indication of the pineal outgrowth occurs as a thickening of the neural-epithelium forming the roof of the diencephalon. It is situated in the median plane and indicates the junction of the diencephalon with the midbrain. A transverse groove divides it into an anterior segment which will become the parietal eye, and a posterior segment (Fig. 172, A). At the 4-mm. stage (Fig. 172, B) the apex of the evagination is seen to be directed forward, and the groove now forms a constriction which completely surrounds the tubular evagination. In a 6-mm. example of L. muralis (Fig. 172, C) the anterior segment has become detached from the posterior or proximal part, and now forms a closed vesicle lying between the epidermis and the brain. Later in a 9-mm. embryo of L. vivipara (Fig. 172, D) the parietal vesicle is seen to be separated from the roof of the diencephalon by an ingrowth of mesoderm containing bloodvessels. Traversing the mesoderm is a bundle of nerve-fibres which connect the parietal eye with the right habenular ganglion. This lies in front of the posterior segment or pineal sac and it was considered by Graham Kerr (19 19) to have arisen by the development of nerve-fibres in the primary bridge which connected the wall of the vesicle with the roof of the brain, when these were in contact, as shown in Fig. 172, C. The connection of the nerve with the right habenular ganglion through the habenular commissure agrees with the description by Klinckowstroem (1894) of the connection of the " pineal " or parietal nerve in Iguana tuberculata, but differs from that in Sphenodon which was described by Dendy (1899), in which the connection is with the left habenular ganglion. A thickening of the superficial or distal wall of the parietal organ in Lacerta which was first noticeable in the 6-mm. stage becomes marked off later as the lens (Fig. 172, C), while the remaining portion of the vesicle becomes differentiated into the sensory or receptive part — the retina — the structure of which will be described later (p. 253). At the 25-mm. stage (Fig. 173) the parietal organ becomes further separated from the pineal sac by the interposition of the paraphysis and dorsal sac, around which the greatly elongated pineal nerve courses in its passage from the parietal organ to the habenular commissure.
Several views have been expressed in explanation of the appearances seen in the early stages of development of the pineal system ; briefly stated, these are :
- There is a single median evagination, the tip of which being cut off, forms the pineal eye, while the basal part retaining its connection with the brain, forms the epiphysis, a pineal stalk, sometimes represented by the pineal nerve, persisting between the two parts — Strahl (1884), Lacerta ; Spencer (1886), Anolis ; Moloch horridus.
- There are two separate and morphologically distinct evaginations, situated in the median plane one behind the other, of which the anterior gives rise to the parietal eye and the posterior to the epiphysis — Leydig (1891) ; Studnicka (1893).
- A single median evagination, which is subsequently divided into two : (a) an anterior vesicle, the parietal eye, having an independent nerve, the nervus parietalis, not connected to the epiphysis, but joining the habenular commissure ; (b) a posterior part, the epiphysis — Hoffmann, Lacerta agilis (1886) ; Strahl and Martin, Anguis, L. vivipara (1886).
- A single diverticulum, the distal part of which becomes divided by an incomplete transverse septum into an anterior and posterior part, in such a way that the lumina of the two end vesicles communicate with the lumen of a common stalk — Beraneck, Lacerta (1894).
- The parietal eye arises not as an independent structure but as an outgrowth from the anterior wall of the primary diverticulum — Klinckowstroem, Iguana (1894) ' McKay, Grammatophora muricata (1888) ; and Schauinsland, Sphenodon (1899).
- Two completely independent outgrowths soon unite with each other — Francotte, L. vivipara (1894).
- The rudiments of two pairs of serially arranged sense organs become fused in the median plane, the anterior giving rise to the parietal organ, the posterior to the epiphysis, and when present an accessory parietal or pineal organ.
- The parietal eye is phylogenetically bilateral. In ontogeny, indications of this condition are present in reptiles, e.g. Sphetiodon, and also in other classes of vertebrates, e.g. Geotria. Thus in Sphenodon the anterior diverticulum is at first situated to the left of the posterior part, and the nerve which arises from the retina of the parietal eye, at a later stage of development is connected with the left habenular ganglion — Dendy, Sphenodon, 1899 ; Gaskell, 1908 ; Patten, 1890 ; and others.
Fig. 172, A, B, C. — Sagittal Sections through the Pineal Organ of Embryos of Lacerta. (After Nowikoff.) A : Lacerta vivipara, 3 mm. B : Lacerta vivipara, 4 mm. C : Lacerta muralis, 6 mm. cutaneous ectoderm. pin. ect I. : lens. p.e. : pineal eye. p.s. ret. pineal nerve, pineal stalk, retina.
Fig. 172, D. — Sagittal Section through the Pineal Organ of an Embryo of Lacerta vivipara, 9 mm. (After Nowikoff.) The nerve-fibres were traced through the habenular commissure into the right habenular ganglion. According to Graham Kerr, they are formed at the time when the vesicle is in contact with the roof of the thalamencephalon and afterwards elongate as the vesicle becomes gradually separated from the brain (see Fig. 173).
/. : lens; pn. : parietal nerve; p.s. : pineal stalk and pineal sac; ret. : retina ; thai. : roof of thalamencephalon.
Fig. 173. — Longitudinal Vertical Section through Pineal Organ of Lacerta muralis (25 mm.) showing terminal vesicle. (nowikoff.)
d.s. : dorsal sac. p.e. : parietal eye.
h.c. : habenular or anterior commissure. p.n. : parietal nerve.
par. : paraphysis. p. st. : pineal stalk.
p.c. : posterior commissure. p.s. : pineal sac.
Fig. 174. — The End Vesicle and the Distal Part of the Stalk of the Pineal Organ,and a Part of the Parietal Eye of Pseudopus Pallasii. (After Studnicka.)
ace. : accessory pineal organ. pin. : end vesicle of pineal organ.
bl. : blood-vessel. r. sk. : roof of skull.
par. f. : parietal foramen. st. : stalk.
par. 0. : part of the parietal organ.
Dendy's view of the bilateral origin of the parietal sense organ, which was also held by Hill, Gaskell and others, is supported by the geological evidence of the existence in certain extinct fishes of two parietal impressions or foramina, placed side by side in the roof of the skull, as seen in Figs. 140, 229, 230 (Chap. 18, p. 200, and Chap. 23, pp. 329, 330), of Dipnorhynchus, Pholidosteus, Rhinosteus, and Titanichthys. It also receives strong support from the occasional development of an accessory parietal sense organ which lies close to or is in continuity with the tip of the pineal organ, as in Pseudopus palassii, described by Studnicka in 1905 (Figs. 174, 177), and Phrynosoma coronatum (Fig. 175) (after Ritter).
Fig. 175.— Sagittal Section through the Parietal Foramen of a Horned Lizard (Phrynosoma coronatum).
Lying in the foramen are a parietal eye and an accessory parietal organ. (After Ritter, from Die Parietal-organe Studnicka.)
P.E : parietal eye. A. P.O. : accessory parietal organ.
The nerves and nerve Ganglia of the Pineal System of Reptilia
A detailed study of the nerves of (1) the parietal sense organ and (2) the pineal organ, including the pineal sac or end vesicle with the stalk and the basal or proximal part, is instructive from several standpoints, more particularly (A) as an aid to the solution of the question as to its origin — namely, whether from two serially homologous parts placed one behind the other or from vestiges of primarily bilateral sense organs ; (B) ascertaining whether the pineal system shows signs, both phylogenetically and ontogenetically, of degeneration, or of the evolution of an endocrine organ.
An important point to bear in mind is that no nerve-fibres are found in connection with the rudiment of the parietal sense organ until after the vesicle has been completely separated from the basal part of the evagination, as indicated in the drawings of Strahl and Martin (1888), Anguis fragilis, Lacerta vivipara ; Beraneck (1892) in Anguis and Lacerta, and Nowikoff (1910), L. agilis, L. vivipara, and A. fragilis. The earlier observations made on the embryos of Lacerta and Anguis were later confirmed by Studnicka, and additional observations were made by Klinckostroem, Spencer, Dendy, and others on different species of reptiles, e.g. Iguana, Varanus, and Sphenodon. The general result of these researches may be briefly stated as follows. The parietal nerve originates from ganglion cells in the retina of the parietal sense organ ; it courses backwards over the dorsal sac and then turns downwards towards the brain in front of the epiphysis and between this and the posterior wall of the dorsal sac, and finally terminates in the roof of the diencephalon, where it has been traced into : the right habenular ganglion — Iguana tuberculata (Strahl and Martin) ; the left habenular ganglion — Sphenodon (Dendy) ; two parietal nerves, right and left, each of which ended in the habenular ganglion of the corresponding side — Iguana (Klinckostroem) ; a single nerve terminating in a nucleus in the roof of the diencephalon (formed according to Studnicka by the approximation of the right and left habenular ganglia) — Anguis fragilis . This was named by Beraneck le noyau parietal . Spencer described a case of longitudinal splitting of the parietal nerve in Varanus giganteus (Fig. 176), and he also described nerve-fibres passing in the remnants of the original stalk of the pineal eye to the epiphysis. Ritter in Phrynosoma coronatum describes the parietal nerve as extending between the posterior commissure and the parietal eye ; and cases have been described in which a nerve is absent. The failure to find a nerve in adult or later larval stages is explained by the degeneration of a nerve which, although developed as a transitory structure in embryonic life, has either disappeared altogether or is represented only by its fibrous sheath. The nerve-fibres when the whole nerve is at the height of its development are provided with medullary sheaths and there is an outer covering of neurilemma, the trunk being invested by a fibrous sheath which is continuous with the capsule of the parietal eye and with the pia mater.
The pineal nerve, 1 or tractus pinealis, or nerve of the epiphysis differs from the parietal nerve, or nerve of the parietal sense organ, in its position relative to the epiphysis and its termination ; arising from cells in the wall of the pineal sac or end vesicle, it passes towards the brain on the posterior aspect of the proximal part of the organ and ends in the posterior commissure (Figs. 166, 187, pp. 232, 264). The earlier description of nerve-fibres which were seen by Leydig in Platydactylus lying in the stalk of the epiphysis was considered by Melchers to be incorrect, the appearance of fibrillation being due to the persistence of connective tissue fibres in the degenerated stalk of the epiphysis. According to Klinckowstroem the tractus pinealis in an 18-day embryo of Iguana tuberculata sinks into the posterior wall of the epiphysis at the junction of its proximal two-thirds with the distal
1 The name " pineal nerve " is still frequently used to denote the nerve of the " parietal sense organ," or " pineal eye." The distal connection of the nerve with the parietal organ should, however, prevent confusion when the term " pineal " is used in this sense.
Fig. 176. — Sagittal Section through the Parietal Eye, and Parietal Foramen of Varanus giganteus, showing Deposit of Pigment in the Central Part of the Lens and a Division of the Parietal Nerve into Two Main Branches. (After Spencer.)
bl. v. : blood-vessel. par. 0. : parietal organ.
/. : lens. pig. : pigment.
n. par. : parietal nerve. r. sk. : roof of skull.
third, and from this point to its termination in the posterior commissure the tract lies free in the surrounding connective tissue. It is thus comparable with the pineal tract observed by Braem in Rana temporaria (Fig. 166, Chap. 19, p. 232). Whether there is also a connection with the habenular commissure or ganglia and the exact destination of the fibres which enter the posterior commissure are questions which seem to be unsettled. It seems probable, however, that degenerative changes similar to those which occur in the parietal nerve take place and that the tract disappears in late larval stages or during adult life.
The Pineal Organ of Reptiles
It is only in rare cases that the pineal organ of reptiles is seen to consist of all three parts — end vesicle, stalk, and expanded proximal or basal part, the latter of which represents the epiphysis or pineal gland of birds and mammals. The end vesicle is in most cases absent, but its existence in a rudimentary form and the presence in it of pigment in some species such as Pseudopus Palasii (Studnicka) (Figs. 174, 177) and the occasional occurrence of a fibrous connection between the tip of the pineal organ and the parietal eye such as occurs in the early developmental stages of certain Amphibia, e.g. Rana temporaria, Braem (Fig. 161, Chap. 19, p. 228), is of the greatest significance. It affords additional support to the hypothesis based on the ontogenetic development of the pineal organ and on geological evidence that the proximal part and stalk of the end vesicle represent the common stalk of a primarily bilateral sense
Fig. 177. — Sagittal Section through the Parietal Region showing a Large Parietal Foramen, containing a Parietal Organ, the End Vesicle of the Pineal Organ and Two Accessory Organs, from an Adult Example of Pseudopus Pallasii. (After Studnicka.) ac.', ac." : accessory organs. Hem. : hemisphere.
bl. : blood vessel.
c. hab. : habenular commissure.
d.s. : dorsal sac.
cp. : epiphysis.
MB. : midbrain.
par. 0. : parietal organ.
pin. : end vesicle of pineal organ.
st. : primary stalk of pineal organ.
organ, the end vesicles of which in some types are completely absent ; in others, one of the sense organs has become more highly evolved, and retaining its connection with the superficial structures becomes separated from the parent stem ; while the other, the growth of which early becomes arrested, either retains its original connection with the common stalk or, like the parietal sense organ, becomes separated off as an accessory pineal organ. Such an accessory organ may lie close to the parietal sense organ ; between this and the pineal organ (Fig. 177) ; or in continuity with the tip of the pineal organ. The frequent presence of pigment in the epithelium of the accessory organ and its location in the proximal segment of the accessory vesicle, or when continuity with the end vesicle of the pineal organ is preserved, its attachment to the distal extremity of this, form in our opinion, important evidence in favour of the accessory organ being the vestige of a light-percipient organ and of its homology with the parietal sense organ.
Structure of the Pineal Organ of Reptiles
The microscopic structure of the wall of the pineal sac of an adult Sphenodon is shown in Fig. 178, after Dendy. The appearances are very
Fig. 178. — Part of a Longitudinal Vertical Section through the Wall of the Pineal Sac of an Adult Male Sphenodon (after Dendy), showing details of Microscopic Structure, which closely resemble those of the Pineal Eye (Fig. 181).
b.m. : basement membrane. l.n.f.s. : layer of nerve-fibres of pineal sac.
B.V. : blood-vessel. m.l.i. : membrana limitans interna.
g.c. : ganglion cell. p.m. : pia mater.
haw. : hsematid. r.f. : radial fibres.
hue. : leucocyte. s.c. : sensory cells.
similar to those seen in the retina of the parietal organ, but there is in most specimens a lack of pigment in the inner epithelial layer and of large ganglionic nerve cells such as are present in the retina of the parietal sense organ. The pineal sac is invested by an outer fibrous sheath, continuous with the pia mater and containing blood-vessels. Beneath this is a basement membrane or membrana limitans externa, while internally next to the lumen is a less defined membrane, pierced by the inner ends of cells forming the internal epithelial layer. Between these two membranes is a supporting basis of ependymal glial tissue, the cellelements and fibres of which are arranged for the most part in a radial manner. In addition to the supporting glial element, nerve-fibres are present which form a definite layer running parallel to the surface and about midway between the two limiting membranes. Some of these fibres appear to be continuous with the tapering deep-ends of the sensory cells of the inner epithelial layer, and others with processes of cells, which resemble nerve-cells, belonging to the outer epithelial layer. In young specimens of Sphenodon the wall of the pineal sac is less differentiated and its structure resembles that of the stalk and proximal part. In all three parts of the organ, the wall consists of a single or double layer of ependymal cells, the oval nuclei of which are arranged radially to the axis of the tube, and there is a tendency where the tube is narrow, as in the stalk for the cells to form a single layer and become cubical in form, whereas in the expanded segments of the tube, two or more layers of nuclei are present and the cells become elongated. In adult specimens of some species of reptiles, e.g. Sphenodon (Spencer), Anguis (Ley dig), and Pseudopus (Studnicka) in which the proximal or " epiphyseal segment '* becomes expanded, the walls of this part become thickened and infolded ; the spaces between the folds being filled with vascular connective tissue, so that sections of it have a glandular appearance, which closely resembles that of the paraphysis and of the choroid plexuses. In older specimens there is a tendency towards degeneration in all segments of the pineal organ. This is also attended by folding of the walls and ingrowth of vessels. The degenerative processes are still more marked in the epiphysis of Ophidia and Chelonia, there being an early arrest of development and lack of differentiation which sometimes results in the disappearance of the cavity of the epiphysis or that of its constricted " neck " called by Studnicka the secondary stalk. In late stages the vessels may disappear and intracellular and intercellular spaces appear in a solid mass of degenerated glial tissue, as in Tropinodotus (Fig. 179) (Leydig).
There is the same difference of opinion with regard to the nature of the processes projecting through the internal limiting membrane of the distal part of the epiphysis that we mentioned in the case of teleostean fishes (Figs. 148, 154, Chap. 18, pp. 211, 219), namely, as to whether these represent secretory threads, cilia, or protoplasmic processes of the cells, similar to those present in the pineal eye of Geotria or Petromyzon.
A — The epiphysis of a young embryo of a water snake, Tropidonotus. The ependymal tissue is penetrated by ingrowing septa of connective tissue, containing capillary blood-vessels, and is subdivided into rounded lobules.
B — The epiphysis of an older specimen of Tropidonotus ; the organ now appears as a solid mass of tissue ; all traces of lobulation and plexus of capillary vessels having disappeared ; degeneration being indicated by the formation of spaces which are not lined by endothelium. (After Leydig.)
bl. v. : blood-vessel.
c. hab. : habenular commissure.
cap. : capillary.
caps', caps" : internal and external
roof of skull .
d.s. : dorsal sac.
sp'. : remnant of primary cavity Ep. : epiphysis. st. : secondary stalk.
They lack the regularity of shape and size which characterizes the smooth club-shaped ends of the sensory-cells of the pineal eye of Geotria (Fig. 45, Chap. 3, p. 69), but they give the impression of being degenerated processes of the epithelium rather than a secretion product or cilia.
The Parietal Organ of Reptiles
No parietal eye is developed in many of the lizards ; thus it is absent in Cyclodus gigas and in the tegu, a large lizard which frequents the forests of the Amazon ; there is also no parietal eye in certain geckos, e.g. Hemidactylus, Gehyra, Platydactylus ; nor in some Agamida:, namely Draco, Ceratophora, Lyriocephalus . A definite parietal eye is also absent in the snakes, tortoises, turtles, crocodiles, and alligators.
Fig. 180. — Reconstruction of the Structures on the Medial Aspect of the Diencephalic Region of the Brain of Testudo geometrica ( x 40) ;
SHOWING THE RELATIONS OF THE ANTERIOR EPIPHYSIS AND POSTERIOR EPIPHYSIS to the Dorsal Sac (Redrawn from G. W. H. Schepers.)
CD. : dorsal commissure. Ep. A.
C. Hb. : habenular commissure. Ep. P.
C. Op. : optic chiasma. H.C. :
C. Po. : posterior commissure. Hyp. :
C.V. : ventral commissure. N. Op.
E. Hyp. : hypothalamic eminence. S.D. 1 ,
E. Th. : thalamic eminence. S.EH.
anterior epiphysis. posterior epiphysis, cerebral hemisphere, hypophysis.
optic nerve. 2 , 3 : dorsal sac.
In some of the above-mentioned examples in which a parietal eye is stated to be absent, there is a subdivision of the primary pineal evagination into two parts, an upper and a lower. Thus in the tegu, there is a difference of opinion as to whether " the upper division of the embryonic evagination represents the rudiment of a parietal eye which at a later stage of development becomes incorporated with the end vesicle of the pineal outgrowth, and thus fails to form an independent parietal organ " (Klinckowstroem) ; or whether, as Studnicka believed to be more probable, the upper subdivision from the first represents the end vesicle of the pineal outgrowth, the parietal organ being absent.
Quite recently an interesting paper by G. W. H. Schepers has been published in the J. of Anatomy (Vol. 72, 1938), in which he describes two epiphyseal vesicles in the brain of a South African land tortoise, Testudo geometrica. In this animal the epiphyses were situated in the usual position between the cerebral hemispheres and midbrain ; the smaller vesicle was antero-inferior in position, whereas the large vesicle was posterosuperior. They were enveloped in a large dorsal sac or parencephalon (Fig. 180), and were situated between the ependyma and pia mater. No stalk or nerve-fibres were found connecting the vesicles with the central nervous system. The larger, superficial vesicle lay in a depression in the roof of the dorsal sac, in close relation with the vessels of the choroid plexus ; while the smaller, anterior vesicle lay in relation with the anterior wall of the sac, below and between two anterior horns of the sac. The walls of both cysts, but more especially the anterior, showed signs of cellular degeneration. Schepers concludes that the vesicles are neural derivatives and that the larger structure is comparable with the pineal body ; the smaller with the " parapineal body."
Structure of the Parietal Organ of Sphenodon
A. The Retina
Beneath the thin inner fibrous capsule (Fig. 181), there is a well-defined external limiting membrane ; while next to the lumen is an internal limiting membrane pierced by the projecting inner ends of the sensory cells. Between the two limiting membranes is a supporting tissue formed by the elongated bodies of the ependymal cells, the outer ends of which are expanded and continuous with the external limiting membrane. The neurosensory elements consist of an inner layer of sensory cells, the cylindrical bodies of which extend from the internal basal membrane towards a middle layer of nerve-fibres. These course parallel to the surface and midway between the two limiting membranes ; as the cell bodies pass towards the middle stratum of nerve-fibres they taper into fine processes which appear to end in this layer. External to the nervefibre layer is found a stratum of nerve-cells, some of which are large and contain a clear vesicular nucleus with a well-defined nucleolus. These are the ganglion cells, processes of which join the nerve-fibre layer. Besides the nerve elements just described there are in the internal layer cells containing pigment granules, which are mostly arranged in a radial manner parallel to and between the sensory cells ; pigment granules are also found in and between the sensory cells and in the outer layer, where they are often deposited in large round cells called " pigment balls " ; small neuroglial cells with branched processes are also present. Moreover, a considerable amount of pigment is contained in the connective tissue around the parietal organ, namely, between the inner and outer layers of the fibrous capsule and in the loose areolar tissue beyond the capsule.
B. The Lens
The lens varies markedly in form both in different species and in different examples of the same species. It is usually biconvex, but one side may be more convex than the other or it may be approximately planoconvex. In the latter case the flatter surface may be directed superficially, or it may be directed towards the cavity of the parietal organ. Other lenses are irregular in form ; in some cases sections of the lens may show what appears to be a central notch, lying beneath a clump of rounded pigment cells as in Fig. 176, showing the parietal eye of a specimen of Varanus giganteus, described by Spencer. In a similar case in which pigment was present in the centre of the lens in a specimen of Anguis fragilis, described by Beraneck (Fig. 182), there was a peculiar seam on the superficial surface of the lens, which suggests that the lens was formed by the growth in contact with each other of the rudiments of two lenses. In some cases the lens may be flattened and very wide, more especially in those cases in which the whole parietal organ is wide and appears to be dorso-ventrally compressed. It is especially in this flattened, wide type of lens that central pigment cells have most frequently been observed. An exception to the above statement, however, occurs in an example of a very narrow, almost cylindrical parietal organ in Anolis, an American type of Iguana described by Spencer. In this specimen the vertical or axial diameter of the lens is little less than the transverse diameter, pigment cells were nevertheless present in the middle of the inner half of the lens. The growth of the whole organ appears to have been arrested at an early stage of development, and the retina to have been composed simply of elongated ependymal cells separated by wide intercellular spaces. It may be noted also that the parietal foramen in this specimen was especially narrow, being little wider than the parietal organ. With reference to these conditions it may be mentioned that in a nearly related type of Anolis Spencer found a dorso-ventrally flattened form of parietal eye, and that the occurrence of such opposite extremes in the shape of the parietal organ and its lens in the same genus merely exemplifies the tendency to variation which exists in degenerating organs, and does not contradict
Fig. 181. — Section of the Retina of the Pineal Eye of Sphenodon punctata.
br. a.p.a. : branch of anterior m.l.e. : membrana limitans externa,
pineal artery. m.l.i. : membrana limitans interna.
c.i. : capsula interna. n.r.f. : nucleus of radial fibre.
c.t.f. : connective tissue fibre. pig. : pigment.
g.c. : ganglion cell. pig. c. : pigment cell.
l.n.f.p.e. : layer of nerve-fibres p.s.c. : processes of sensory cells, of pineal eye.
Fig. 182. — Parietal Eye of a 76-MM. Embryo of Anguis fragilis. (After
Pigment is present in the central and deep part of the lens. The lens is very wide, and is continuous at its periphery with the retina. The nerve-fibre layer of the retina is interrupted below the central mass of pigment in the lens. Pigment is absent in the corium and epidermis of the tissues lying superficial to the eye.
the general statement with regard to the frequency of pigment cells in the centre of the wide type of lens.
Besides variations in the form of the lens in the parietal eye of reptiles, cases occur in which the lens is absent, its place being simply marked by a thin part of the wall of the vesicle which is destitute of pigment, and although not an homologous structure, it resembles the cornea of the lateral eyes ; a good example of absence of the lens was described by Spencer in a specimen of Moloch horridus. This was, however, a very exceptional case in which the parietal eye was almost completely filled with pigmented tissue, leaving only a small space in front, like the anterior chamber of a lateral eye. The parietal organ lay in the parietal foramen, and was connected by a solid stalk with the distal end of the epiphysis. The specimen appears to be a case of arrested development in which the parietal organ has failed to separate from the parent stem, and has failed to differentiate into lens and typical retina. If this interpretation is correct the end vesicle of the epiphysis has also failed to develop. It is possible, however, that the structure described by Spencer as the parietal organ, since it was directly connected by a stalk with the epiphysis, was itself the end vesicle of the pineal organ and it was the parietal organ which was lacking — a view which was first suggested by Studnicka.
The frequent appearance of pigment in the centre of the lens of the parietal organ, and the occasional presence in such cases of a notch on the under surface of the lens or of a seam on the superficial surface, may be of significance with reference to the theory of a dual origin of the parietal organ. A parallel condition is met with in cases of cyclopia, whether this anomaly occurs in single individuals or in double monsters, more particularly in certain cases of Cephalothoracopagus monosymmetros or Diprosopus triophthalmos. In these cases varying degrees of blending of the two eyes occur in the development of the cyclops eye, namely, between simple approximation of two eyes lying in a single orbital cavity and an apparently single eye in which a compound lens is found which is formed by the fusion of two halves. Thus, according to the classification of the various degrees in the development of cyclopia by Bock, with reference to the fusion of the paired lateral eyes, cases occur in which :
1. The two bulbs are united only by the sclera.
2. The optic nerves are approximated ; the common scleral tissue between the bulbs has become thinner ; the cornea, iris, lens, vitreous, and retina are double.
3. The cornea is single ; the iris, lens, vitreous, and retina double, the optic nerves separated merely by a narrow space filled with connective tissue.
4. One cornea ; two lenses united in the median plane of the compound eye ; sclera, chorioidea, retina, and optic nerve single.
5. An eye without visible doubling of any part may lack an optic nerve.
6. An apparently single eye, in which the lens is found to be compounded of two halves.
The possibility of the apparently single median or " third " eye of certain fishes and reptiles being compounded of elements of parts of two eyes growing in contact with each other is thus evident, and appearances such as those shown in Fig. 176, p. 247, and Fig. 182, p. 255, are readily explained. It is quite possible that the rounded pigment cells in the centre of the lens represent a vestige of interposed median parts of retinae which have become included between the two halves of the compound lens ; and the presence of a median notch on the under surface of the lens or a seam on the superficial surface affords additional evidence of the fusion, moreover, the deep brownish-black colour of the pigment and its deposit in large, rounded cells are typical of retinal pigment rather than a mere degeneration product ; although the element of degeneration is undoubtedly present, the localization of the pigment in the centre of the proximal half of the lens is strongly in favour of the cells in which it is deposited being retinal in origin. A reference to Fig. 175, p. 245, in which a " parietal organ " and an " accessory organ " are seen lying close together, in a sagittal section through the parietal foramen of Phrynosoma coronatum will show how if these two vesicles were fused, the opposed portions of the retinae between the two lenses would almost certainly be included in the centre of the compound lens. It may also be noted that the two vesicles are surrounded by a common fibrous capsule, and that pigment is less in the more degenerate accessory organ than in the more highly differentiated parietal organ.
Structure of the Lens. — The lens of the parietal organ is formed of transparent elongated cells lying parallel to each other, and extending from its superficial to its deep surface. The cells are longer in the centre of the lens than at its periphery. Their form and general arrangement resembles that of the cells forming the posterior wall of the embryonic lenses of the lateral eyes ; there is, however, at no period of development any cavity or thin anterior lamina, as in the lateral eye, the lens of the parietal eye being solid from the first — it having been developed from a segment of the superficial wall of the vesicle, which was primarily directly continuous with the retinal segment, the two parts passing insensibly into each other, and a definite limiting membrane only being formed at a later stage.
The lenticular cells are cylindrical or prismatic in type, and show no special thickening in the situation of the nucleus. The nuclei are sometimes arranged in an approximately single plane, but in young embryonic specimens four or five nuclei may be present between the two surfaces, and they are irregularly disposed. There is a tendency towards degeneration seen in some of the older specimens, this being evidenced by the appearance of spaces between the cells and disappearance of the cell outlines. Leydig and others have described the presence of granules in the cells, producing an appearance of longitudinal striation. Leydig also described a radially striated layer on the surface of the lens in one specimen of Anguis fragilis, which somewhat resembled the layer of cuticular rods seen in some invertebrate eyes. The nature of these is obscure and the case seems to have been quite exceptional. On the other hand protoplasmic cilium-like processes on the inner surface of the lens, and projecting into the cavity of the vesicle are more common. In some cases these are continuous with syncytial strands of the vitreous tissue filling the cavity of the vesicle.
C. The Vitreous Body
In well-fixed preparations there is present in the cavity of the parietal organ a delicate syncytium which consists of an irregular network of fine protoplasmic strands showing a few oval nuclei imbedded in the thickened nodes of the reticulum. Peripherally it is continuous with the rodor club-shaped processes of the inner layer of sensory and pigmented cells of the retina and sometimes with similar tapering protoplasmic processes which project inwards from the inner surface of the lens. In one case a thickened band of protoplasm containing closely packed nuclei extended inwards from the retina with which it appeared to be continuous. It is probable that during life the meshes of the reticulum contain a clear albuminous fluid. In imperfectly preserved specimens this tissue may be condensed and form an almost uniform layer lining the inner surface of the retina, and appearing as a coagulum. In other cases it may be thrown down as a flocculent precipitate which occupies the whole cavity. The few cases in which a true syncytium has been demonstrated indicate that in the living animal there is normally a definite syncytium, enclosing spaces filled with clear fluid, and similar to that described in the median eyes of Petromyzon and Geotria and in the lateral eyes of vertebrates generally.
Nerve Supply of the Pineal System of Sphenodon
The nervous system of adult specimens of Sphenodon was minutely studied by Dendy in 191 1, who employed serial transverse and longitudinal sections stained with picro-indigo-carmine. He described (1) a left pineal nerve (n. parietalis) and (2) a right pineal nerve (tractus pinealis).
(1) The left pineal nerve commences in the nerve-fibre layer of the retina, passes from the proximal pole of the eye through the space which is enclosed by the outer fibrous capsule of the eye, being covered here by a sheath continuous with the inner fibrous capsule (Figs. 183 and 184).
Fig. 183. — Sagittal Section of the Pineal Eye of Sphenodon, showing the Pineal Nerve, Lens, Vitreous Body, Accessory Cavity, Capsula Interna, Capsula Externa, Parietal Plug, and Vessels including Branches of the Longitudinal Sinus. (After Dendy.)
A. Cav. : accessory cavity. /. ." lens.
accessory cavity. ce. : capsula externa. ci. : capsula interna. c. vit. : vitreous body.
n.p.e. : pineal nerve.
P.P. : pineal plug.
si. : longitudinal sinus.
It then passes into and through the dura mater, where it comes into relation with branches of the anterior pineal artery. Here it divides into two and then three branches and lies between two vessels formed by a loop of the sinus longitudinalis. It lies above the paraphysis. Hitherto the nerve has been approximately in the median plane just beneath the dense connective tissue of the cranial wall and in the dura mater. It now passes once more as a single strand to the left of the pineal artery, and, having left the dura mater, it reaches the anterior extremity of the pineal sac and passes beneath its ventral wall to the left of the median plane. The nerve accompanied by the artery is enveloped for some distance in folds of the pineal sac. Near the lower end of the sac its histological structure undergoes an abrupt change, the small oval nuclei
Fig. 184. — A, B, C. — Transverse Sections showing Some of the Relations of the Nerve of the Pineal Eye (N. Parietalis), of an Adult Sphenodon. The Sections Pass from before Backwards. (After Dendy, 191 i.)
A — Nerve enclosed in capsule of eye. B — Nerve divided into two strands. C — The subdivisions of the nerve have reunited to form a single nerve, which lies above the distal end of the paraphysis. D — Transverse section through the nerve of the pineal eye, showing its fibrous sheath, and nuclei of the non-medullated fibres. These disappear in the proximal part of its course.
a.p.a. : anterior pineal artery.
a.s. : saccular artery.
c.e. : external capsule.
c.f. : fibrous sheath.
d.m.e., and d.m.i. : external and internal layers of dura mater.
n.p.e. : nerve of pineal eye.
n.p.n. : nuclei of pineal nerve-fibres.
par. : paraphysis.
p.p. : parietal plug.
r .a.p.a. : branch of anterior pineal
artery. r.b.p.a : recurrent branch of anterior
pineal artery. r.p.a. : anastomosing branch of pineal
artery. si. : superior longitudinal sinus. so. c. : suboccipital cartilage.
which are present in the course of the nerve-fibres throughout the first part of its course cease and the single nerve breaks up into a number of separate non-nucleated fibres, which spread out in the narrow interval between the pineal sac and the posterior wall of the dorsal sac. Dendy was unable to follow individual fibres continuously to the habenular region,
Fig. 185. (After Dendy, 19 11.)
-Transverse section through the lower part of the tractus pinealis, and the habenular ganglia of an adult Sphenodon ; note the cavity in the pineal sac, with an ependymal groove in its floor. B — Transverse section through the same specimen as A at the junction of the tractus pinealis with the brain, showing the recessus pinealis, the subcommissural organ, and the fasciculus retroflexus of Meynert. C — Longitudinal vertical section through the pineal region of an adult Sphenodon, showing a bundle of nerve-fibres N.P.E. entering it from the posterior wall of the dorsal sac. D — Combined drawing from several sections of the same series as the preceding, showing the lower end of the tractus pinealis, the posterior, and the habenular commissures and the subcommissural organ.
cavity of pineal sac. fasciculus-retroflexus of Mey
B.V. : C.H. : C.P. : C.P.S.
nert. G.H.M., G.H.D., G.H.S., ganglion
habenulse medjus — dexter — sinistra?. N.P.E. : nerve of parietal eye. P.M. : pia mater.
but he found that numerous fibres entered the left habenular ganglion from the wall of the dorsal sac. Fibres from the same area were also traced to the median habenular ganglion and the right habenular ganglion, but the bulk of the fibres joined the left ganglion, and although some nonnucleated fibres from the nerve-fibre layer of the pineal sac joined the series of fibres coming from the left pineal nerve, Dendy considered that the greater number of fibres which entered the left habenular ganglion came from the left pineal nerve.
P.S. : pineal sac.
R.N. : cells of roof nuclei.
R.P. : recessus pinealis (" infra-pineal
recess ")• SCO. : subcommissural organ. T.P. : tractus pinealis. V.P.S. : vessel of pineal sac. K.3. : third ventricle. W.D.S. : wall of dorsal sac.
(2) The right pineal nerve (tractus pinealis) (Fig. 185). Dendy, using the term " tractus pinealis " in the wide sense in which this name was employed at the time, namely, as including the " entire band or cord of tissue which unites the lower extremity of the pineal sac to the roof of the brain," describes it as commencing in the nerve-fibre layer of the wall of the pineal sac, the greater number of its fibres leaving at its lower RE. C. P.S. Epd.
Fig. 186. — Semi-diagrammatic Representation of a Medial Longitudinal Section of an Embryo Sphenodon, showing a Constriction marking off the Anterior (left) Lobe, or Parietal Organ, from the Posterior (right) Lobe, or Pineal Sac The Anterior End of the Organ is Directed Forward and is to the left in the Figure. (After A. Dendy.)
C. : Constriction. L.N.F.R.: Layer of nerve-fibres in retina.
Epd. : Epidermis. P. Co. : Posterior commissure.
Le. : Lens. P.E. : Pineal eye.
L.N. F. P.S. : Layer of nerve-fibres in P.S. : Pineal sac. pineal sac. P. St. : Pineal stalk.
S.C. : Superior or habenular commissure.
pole and passing downwards in the stalk to the roof of the brain ; here, between the habenular commissure in front and the posterior commissure behind, some fibres pass to the right and others to the left of the infrapineal recess and, passing towards the posterior extremities of the habenular ganglia, some fibres appear to terminate in these ganglia, while others appear to end in the median habenular ganglion. Dendy also stated that some fibres passed backwards towards the posterior commissure and subcommissural organ and that possibly some fibres were continued directly into the bundles of Meynert.
The Early Development of the Pineal Nerves in Sphenodon
Dendy also studied the early stages of development in the parietal organ of Sphenodon. The earliest indication of the development of nerve-fibres was in embryos, of approximately Stage O when the visceral clefts have begun to close. 1 (Fig. 186).
1 Dendy, A., 1899. Q. J. Micr. Sc, 42, N.S.
In one case he was able to recognize nerve-fibres even before the two pineal vesicles had separated from one another. Both the habenular and the posterior commissures were recognizable. The nuclei in the ventral or future retinal wall of the pineal eye were just beginning to show an arrangement into principal layers separated by a nerve-fibre layer. These fibres were limited to the retinal part of the eye-vesicle, and he concluded that as, now, fibres were visible in the adjacent wall of the brain, the fibres appear first in the retina and from thence grow into the brain wall, as in the case of the ordinary paired eyes. As the pineal eye becomes separated from the roof of the brain by the development of intervening mesoderm, the left pineal nerve, which has now grown out from the retina, grows longer and longer, being attached at its ventral extremity to the brain roof. It is absolutely independent ot the pineal sac and its stalk.
In two series of transverse sections at a later stage of development, Dendy was able to trace the nerve continuously from the eye to the left side of the habenular commissure, and he also traced the nerve into the left habenular ganglion in a longitudinal series of sections, belonging to the same developmental stage. He never found it joining the brain roof in the middle or on the right side (see p. 261).
Development of the Right Pineal Nerve. — When the first nerve-fibres have become recognizable in the retinal wall of the pineal eye, a similar band of fibres is visible in the posterior wall of the pineal sac, close to where it joins the roof of the diencephalon in the region of the posterior commissure (Fig. 186). These fibres appear in the outermost zone of the wall, external to all the nuclei. From this point the nerve-fibres extend at a later stage of development upwards into the wall of the pineal sac and downwards into the posterior commissure. When later the nuclei in the wall of the pineal sac assume their characteristic arrangement in two principal layers, the nerve-fibres are found between the two layers of nuclei, as in the retina of the pineal eye. Also when the cavity of the proximal portion of the pineal sac becomes obliterated the nerve-fibres form the most important constituent of the stalk or " pineal tract." The nerve-fibres are associated here with the remains of epithelial cells, connective tissue, and blood-vessels, as in the adult.
Dendy believed that there was no doubt about the connection of the right pineal nerve with the posterior commissure, but that the right pineal organ (pineal sac) was also primarily associated with the right habenular ganglion ; and he considered also that the connection of the pineal sac with the posterior commissure was secondary and " possibly correlated with change of function which the pineal sac has evidently undergone." Several points of interest arise from this description.
First, the primary position of the nerve-fibres in the outermost zone of the wall of the pineal sac recalls the conditions found by Braem and Studnicka in Rana temporaria. In Rana the nerve-fibres commence deeply in the wall of the distal part of the pineal organ, but as they approach the stalk they converge and form a definite tract, which lies upon its dorsal aspect and finally leaves this to join the posterior commissure (Fig. 166, p. 232); while in Iguana tuberculata (Fig. 187), as shown by Klinckowstroem, the nerve quits the posterior wall of the pineal organ high up and runs an independent course through the connective tissue to the posterior commissure. Although a striking contrast is afforded between these cases and those in which the nerve-fibres pass to the posterior commissure within the stalk, it is probable that the difference in the route which is taken by the nerve-fibres in the two cases is not of morphological importance. Finally, it is probable that if Dendy had been in possession of the large amount of negative evidence with regard to the supposed function of the pineal gland which we possess at the present time, he would not have used the word " evident " with reference to the presumed change of function which at that time the pineal organ was thought to have undergone.
Fig. 187. A — The parietal region of a 26-mm. embryo of Anguis fragilis, showing the position of the parietal nerve in front of the epiphysis and passing towards the habenular commissure. Diagram constructed from three sections. (After Beraneck.)
B — The proximal part of the epiphysis from an 18-day embryo of Iguana tuberculata, showing the position of the pineal tract, which, emerging from the posterior wall of the epiphysis, passes to the posterior commissure. (After Klinckostroem .)
c. hab. : habenular commissure.
c.p. : posterior commissure.
d.s. : dorsal sac.
Ep. : epiphysis.
n. par. : nervus parietalis.
p. int. : pars intercalaris. par. : paraphysis. par. o. : parietal organ. tr. pi. : tractus pinealis. v. : velum
The Connections of the Pineal Organs with the Central Nervous System
The nerves of the pineal system in Sphenodon appear to end (Fig. 185) in the habenular and posterior commissures and in the habenular ganglia, and to be continued from the latter by means of the bundles of Meynert to the base of the brain (fasciculi retroflexi, v. tractus habenulo-peduncularis). Each of the fasciculi passes obliquely downwards and backwards in front of the recessus geniculi to the interpeduncular ganglion. The habenular and posterior commissures are recognized at a very early stage of development. The former, being much the smaller of the two, connects the right and left habenular ganglia and lies beneath the median habenular ganglion ; where it forms with the latter a projection into the third ventricle just above the pineal recess. The posterior commissure is closely connected from the first with the tractus pinealis, as well as with other structures outside the pineal complex. It grows rapidly in size and soon becomes folded transversely ; at the same time, the epithelium beneath it becomes converted into the subcommissural organ, which in its turn is connected with the anterior end of Reissner's fibre, that remarkable elastic filament which extends in Sphenodon, as in Geotria (Fig. 134, Chap. 17, p. 188), along the whole length of the central canal of the spinal cord.
The right and left habenular ganglia are situated above the posteromedian and dorsal part of the optic thalami and are prolonged upward on each side into the wall of the dorsal sac ; the prolongation is greater on the left than on the right side, the difference being associated with the termination on this side of the left pineal nerve (n. parietalis) in the apex of the habenular ganglion. Beyond this extension, however, there is no obvious difference in the size of the right and left habenular ganglia, as is the case in Geotria (p. 188), and there is no appreciable difference in the size of the bundles of Meynert as in Geotria. The lack of any marked difference in the size of a right and left habenular ganglia in Sphenodon, notwithstanding the absence of a right parietal nerve, is probably largely due to the presence of other connections of the habenular ganglia besides those with the pineal system, and the very small size of the pineal nerve.
The Vascular Supply of the Pineal System of Sphenodon
According to Dendy (Fig. 188), there is much individual variation among different specimens of Sphenodon, but the arteries concerned in the supply of the system appear to be always derived from the posterior
Fig. 188. — The Brain and Pineal System of Sphenodon punctatus. The right side of the skull has been removed, showing the brain and large subdural space. The latter is traversed by blood-vessels. The minute pineal eye (0-53 mm.) is seen in the parietal foramen, lying beneath the parietal plug, which is formed of translucent connective tissue. (After Dendy.)
P.E. : pineal eye.
Pit. : pituitary gland.
. : dura mater interna.
P.P. : parietal plug.
P.S. : pineal sac.
SO. : supraoccipital bone.
O. : medulla oblongata.
SOC. : supraoccipital cartilage.
Hi., iv., v., ~\
vii., viii., >cranial nerves.
ix., x., J
cerebral artery and all the venous blood is returned via the sinus longitudinalis. The posterior cerebral artery divides on each side into an artery to the dorsal sac — saccular artery, and the superior cerebral artery. The superior cerebral artery gives off numerous branches to the hemisphere and the anterior choroidal artery. The saccular artery gives off, on either the left or the right side : (1) the anterior pineal artery, which goes to the pineal eye ; and (2) the posterior pineal artery which supplies the hinder part of the pineal sac. The anterior pineal artery in one case described by Dendy arose directly from the posterior cerebral artery of the left side. It was unpaired ; it passed upwards on the left side of the pineal sac, where it was hidden in folds of its walls and gave off branches to the sac ; it gave off a recurrent branch which passed to the right and finally accompanied by the left pineal nerve it entered the space between the inner and outer fibrous capsules of the pineal eye, where it broke up into capillary vessels which ramified in the connective tissue, but did not enter the eye itself. This fact is of considerable importance, as it is unlikely that an organ which has no demonstrable blood supply could be actively functional, and it indicates impending degeneration.
The Parietal Foramen of Reptiles
The parietal foramen is not present in all orders of the class Reptilia. When present it is usually situated in the anterior part of the interparietal suture. It is found in this position in Sphenodon (Fig. 189), which is the only living example of the Prosauria ; and it is present in the same situation in Palceohatteria and in Conodectes, one of the most primitive of the Permian reptiles. It is found also in the same site in the less primitive mammal-like reptiles, the Therocephalia and in the Plesiosauria.
Fig. 189. — Dorsal View of Skull of Sphenodon, showing the Parietal
cond. : condyle. p. mx. : premaxilla.
ex. p. : external pterygoid. pa : parietal.
jr. : frontal. pa. f. : parietal foramen.
jug. : jugular. pr.f. •' prefrontal.
mx. : maxilla. sq. : squamosal.
na. : nasal.
(From the Cambridge Natural History.)
In some extinct reptiles the foramen is farther forward, as in the Ichthyosauri, being situated at the junction of the two frontal bones with the two parietal bones (Fig. 190) ; or it may be far back near the exoccipitals as in Nothosaurus mirabilis.
In cases in which the lateral or supratemporal fossae are large, these may be separated by a median antero-posterior bony ridge. This is formed chiefly by the inner borders of the parietal bones, but the frontals may participate in its formation, as in Mesosuchus and Mixosaurus atavus (Fig. 191). In some types this ridge is reduced to a relatively slender sagittal crest, as in Trachyodon mirabilis and Iguanodon. In these cases the parietal bones become fused and the interparietal suture and the parietal foramen disappear. These animals were of enormous size ; they walked on their hind legs, using the tail also as a support for the body. Thus the fore-limbs were free and could be used as arms, as in the kangaroos and wallabies. Their temporal muscles were strongly developed, and encroaching on the sides of the skull they obtained an additional origin from the lateral aspects of the crest, which was formed between them, as is the case in the gorilla. A long median crest is also conspicuously developed in certain Ophidia (Python) and Chelonia (Trionyx gangeticus), in neither of which a parietal foramen is present. The formation of the crest will not only tend to reduce the width of the parietal canal, but it will increase the thickness of the skull in this situation and the distance of the cranial cavity from the superficial surface of the skull. Thus, although not the prime cause of the atrophy of the parietal organ and of its vascular and nerve-supply, the evolution of the crest when this is present would have probably acted as a contributory factor in the final obliteration of the parietal foramen and its contents.
Fig. 190. — Ichthyosaurus acutirostris, Owen. Upper Lias. Curcy, Calvados. A — Dorsal view of skull. B — Lateral view of skull.
na. : nasal bone. p. mx. : premaxilla.
nar. : external nares. scl. pi. : sclerotic plates.
orb. : orbit. st. f. : supratemporal fossa.
pa.f. : parietal foramen.
(After E. Deslongchamps, from Zittel.)
In some cases in which a " parietal eye " is absent, a parietal foramen is nevertheless present in the cartilaginous roof of the embryonic skull.
Fig. 191. — Dorsal View of Skull of Mixosaurus atavus, from the Trias, Germany, showing the Parietal Foramen, lying between the Temporal Fossae and at the Junction of the Parietal with the Frontal Bones. (After F. V. Heune.)
bo. : basi-occipital.
pf. : prefrontal.
fr : frontal
p. mx. : premaxilla.
/. : lacrimal.
po. : postorbital.
l.t.f. : lateral temporal fossa.
q. : quadrate.
mx. : maxilla.
qj. : quadratojugal.
na. : nasal.
so. : supraorbital.
orb. : orbital cavity.
sq. : squamosal.
pa. : parietal.
st. : supratemporal.
(Redrawn from E. S. Goodrich.)
Immediately beneath and partially occupying the foramen are the distal ends of the paraphysis, dorsal sac, and the end vesicle of the pineal organ, as shown in an example of a 33-mm. embryo of Platydactylus muralis, described and figured by Melchers (1899). The foramen is, however, usually absent, at any rate in the adult animal in those cases in which no parietal eye is developed, e.g. Gecko verus.
The size and shape of the parietal foramen differ considerably in different types. Apart from variations in the actual diameter of the foramen which accompany variations in the size of the animal, there is a wide difference in the proportional size of the foramen relative to the size of the skull. Thus in Procopholon (Fig. 171, A, Chap. 19, p. 237), in which the skull is only 5 cm. in length, the diameter of the circular parietal foramen is 3-3 mm. ; whereas in Ichthyosaurus, which has a skull 50 cm. long, the parietal foramen is 16 mm. long and 8 mm. wide. The foramen is thus actually much larger in Ichthyosaurus than in Procopholon, although it is relatively smaller. In both animals the orbits are very large relative to the size of the head, but they are exceptionally large in Procopholon. The shape of the foramen also varies, the superficial opening usually being either oval or circular in outline. In Mixosaurus the anterior margin of the opening is slightly concave forward, but it is not definitely heart-shaped. In section the parietal canal may be tubular with straight sides, or the edges may be bevelled, the central part being narrow (Lacerta agilis). In other cases it is funnel-shaped, the narrow end of the funnel being either superficial, as in Pseudopus Pallasii (Fig. 177, p. 248), or the smaller opening may be below and the larger above, as in Varanus giganteus (Fig. 176, p. 247). The opening is usually closed superficially by a fibrous membrane continuous at the margin of the foramen with the periosteum and the membrane lining the canal. The membrane is destitute of pigment, as are the structures which lie superficial to it ; moreover, the superficial part of the parietal eye is firmly adherent to the membrane, so that if traction is employed in removal of the organ from the foramen the eye is liable to be damaged. Exceptionally, the lower aperture may be closed by membrane continuous with the dura mater, so that the parietal organ is, strictly speaking, outside the cranial cavity, as in certain amphibia (Fig. 161, Chap. 19, p. 228). This condition is found in chameleons. The foramen in the adult animal is in bone ; in the embryo it is in cartilage, as in the Selachia. Its contents vary ; as a rule the parietal organ is situated superficially in contact with the external closing membrane and its diameter is about one-third the diameter of the foramen. It is surrounded by a loose, fibrous connective tissue, often containing pigment and sometimes fat. In Sphenodon there is a special inner and outer fibrous capsule containing blood-vessels and traversed by the parietal nerve. In addition to these structures, the parietal recess may contain a part of the end vesicle of the pineal organ and one or more accessory organs (Fig. 177). In some instances it has been noted that in the embryo the parietal organ does not lie in the foramen, but below this, and it is only at a later period that it is found actually in the foramen, e.g. Lacerta vivipara (Owsjannikow, 1888), and Iguana (Klinckostroem, 1894). In one instance in Sphenodon (Spencer) the central axis of the parietal organ was directed obliquely forward, so that rays of light entering through the foramen could only fall directly on one part of the retina. This condition is, however, exceptional and does not appear to have been described in other adult specimens. Dendy's figures show a less pronounced degree of obliquity in two embryos of Sphenodon, but it is probable that this condition would have been rectified by the time the animal had reached maturity.
More important from the standpoint of interference with function is the occurrence of pigment in the substance of the lens — Anolis (Spencer), Anguis fragilis (Beraneck), Varanus (Spencer) ; and the encroachment from the surrounding tissue of black pigment, which may grow inward over the area occupied by the subjacent lens, as in a case described by Leydig of Lacerta muralis (1891) ; also, irregularity in form and structural degenerative changes, previously alluded to as occurring in the lens of a specimen of an adult Iguana described by Klinckowstroem (1894). Calcareous concretions have also been found in the parietal foramen, which, although in relation with the end vesicle of the pineal organ rather than the parietal organ, indicate localized degenerative processes occurring within the area of the pineal system. These concretions have been noted by Leydig (1891), who described four rounded, calcareous nodules lying over the tip of the epiphysis, and by Studnicka, in Varanus nebulosus, who found similar structures in the neighbourhood of the pineal bud. In this case there was one large and several small calcareous balls, the latter being arranged in a semicircle around the pineal outgrowth. These conditions indicate not only interference with function and degenerative changes in the living animals in which they have been observed, but loss of function of the parietal eyes and accompanying degeneration of structure, which must have commenced in quite remote ancestors of the living generations of reptiles. The parietal cornea, corneal scale, and parietal spot of reptiles have already been alluded to in the general description of these structures.
Notes on the Pineal System of Special Examples of Living Saurian Reptiles
Further reference to the significance of the parietal foramen of extinct Saurian reptiles will be made in the chapter on the morphology of the skull in so far as it is concerned in the geological evidence of the antiquity of the median and lateral eyes of vertebrates. The following notes will deal chiefly with the pineal system of embryonic and mature examples of different types of lizard.
In the geckos no parietal organ is developed and the pineal organ consists of a long, tubular outgrowth springing from the usual site between the habenular and posterior commissures and passing upwards to the roof of the skull behind a well-developed dorsal sac and paraphysis. In a 31-mm. embryo of Gehyra oceanica, described by Stemmler, the base of the outgrowth is constricted so as to form a secondary stalk in which the lumen has become obliterated and the distal end is slightly expanded into an end vesicle, the tip of which is prolonged forward as a beak-like process ; this is connected to the dura mater of the skull roof by a fibrous cord. The lumen of the end vesicle is smooth, there being no folds or lobulation. The conditions are, therefore, very similar to those which are found in the tubular type of pineal system present in the Selachia. In older specimens, e.g. Hemidactylus verruculatus, brown pigment has been observed in the epithelium of the distal end of the epiphysis (Ley dig). The same author described in Platydactylus muralis fine striated strands in the stalk of the epiphysis near the posterior commissure, which he was inclined to believe were nerve-fibres passing from the posterior commissure into the epiphysis. The existence of a true pineal nerve in geckos, however, appears to be very doubtful, and in certain specimens, e.g. Hemidactylus mabouia, it has been definitely stated that it could not be demonstrated.
The epiphysis in older embryonic specimens may show a certain degree of differentiation with thickening of its wall ; in late stages of development and in mature animals kinking and irregularity of the wall has been present, accompanied by penetration of vessels. The epiphysis has, moreover, been found separated from the roof of the brain, presumably by constriction of its stalk, attended by loss of its lumen and final rupture, as in the case published by Melchers in 1899.
The changes which have been observed in the late stages of development of the epiphysis of geckos include : elongation of the cells ; disappearance of cell contours ; vacuolation of cellular bands ; penetration of bloodvessels ; deposit of pigment in and around the capsule and the separation of the whole organ from its connection with the roof of the brain. Taken together, and more particularly the complete severance of the epiphysis from the central nervous system, these changes indicate that the epiphysis in the mature animal shows indubitable signs of retrogression.
This family includes lizards of widely varying types. One of the most remarkable is the little flying dragon Draco volans, in which the hinder ribs are expanded and covered by a thin membrane ; this forms a kind of parachute or gliding apparatus by which the animal is enabled to pass by long leaps from tree to tree. There is also the frilled lizard, Chlamydosaurus, which has a whig-like expansion growing out around the head, neck, and shoulders. It is found in sandy deserts, has a long tail, and walks with long strides on its hind legs. The family includes the horned-lizard, Ceratophorus aspera ; Calotes versicolor, which, like the chameleon, is capable of changing its colour ; the thorny-tailed lizard, Uromastix ; and the Australian lizard, Moloch horridus, which is covered all over with large conical spines.
In this family a parietal foramen, parietal scale, and parietal spot are described as being present in most of the species that have been examined, though these differ in their degree of development. Thus, in Ceratophorus the foramen is only indicated by vessels traversing the bone, while in Calotes ophiomachus all three are specially well developed.
In Agamidae a parietal organ and end vesicle are not as a rule both present in the same individual ; hence it is often difficult to determine whether the organ found lying in the parietal foramen is the parietal sense organ or the end vesicle of the pineal organ, more especially as the " eye " is in most cases not separated from the stalk of the epiphysis ; moreover, in some cases there is no distinct differentiation into lens and retina, and no pigment. A differentiated eye is, however, definitely present in Calotes, Agama, Phrynocephalus, and Grammatophora. Moreover, in an adult specimen of the latter described by McKay (1888) the parietal eye was separated from the epiphysis, as in Lacerta vivipara or Sphenodon. It showed a biconvex lens and a well-differentiated retina, in which McKay distinguished rod-like cells, round cells, a molecular layer, a layer of spindle-formed elements, and special triangular cells (? multipolar nerve cells) ; pigment was deposited in horizontal layers in the rod-like cells, and a corneal scale was present. Speaking generally, in those cases in which a well-developed parietal foramen, parietal scale, and parietal spot are present, the parietal eye is well developed. In cases such as Ceratophora, in which the existence of a parietal foramen is only indicated, it is doubtful whether the imperfectly developed organ beneath it really represents a parietal sense organ or is a modified end vesicle, the parietal organ being absent.
Spencer described a parietal organ having the form of an ovoid vesicle, the longest diameter of which lay in the median plane. There was no differentiation into lens and retina. Pigment was absent in the vesicle itself, but was present in the connective tissue membranes behind the vesicle. It lay in a parietal foramen, the position of which was in most 18 cases indicated by a modified scale and a transparent parietal spot. According to Studnicka's observations on one specimen of Draco, there is no parietal eye, but the pineal organ ends in a dorso-ventrally compressed end vesicle which lay beneath a parietal pit closed superficially by a lamella of cartilage. The end vesicle was not pigmented.
In a specimen described by Spencer (1886) there was a pentagonal corneal scale (Fig. 192, B) with the broad end anterior ; a little behind the centre of this was the parietal spot which was slightly convex superficially
Fig. 192. — The Parietal or Corneal Scale and its Surroundings in ThreeForms of Reptiles
A : Varanus giganteus (after Spencer) ; B : Calotes versicolor, a tree lizard so named on account of its power or changing its colour (after Spencer) ; C : Lacerta muralis, var. cozrulea (after Leydig). In the centre of the parietal plate is seen the parietal eye spot ; through this is seen in Calotes, in which this part is definitely transparent, the parietal eye. In Lacerta muralis pigment granules encroach on this area.
and bounded by a circular rim. The parietal eye was a dorso-ventrally flattened vesicle having its longest axis in the median plane. Some of the cells of the lens contained pigment at their inner or central ends. The parietal organ was separated from the epiphysis. The latter consisted of two parts, a vertical basal segment and a horizontal terminal part which was directed forward, its tip ending at the posterior border of the parietal foramen near the parietal eye.
Agama hispida. Spencer (1886).
The parietal eye lay in a parietal foramen, beneath a pigmentless parietal cornea and modified scale, through which could be seen a remarkable hour-glass-shaped parietal spot. The retina of the parietal eye was so deeply pigmented that its structure was obscured, and pigment was also present in the lens.
Agama caucasica (Owsjannikow, 1888).
The parietal eye, which was exceptionally large, lay in a parietal foramen surrounded by connective tissue containing numerous pigment cells. Both lens and retina showed cells the inner ends of which terminated in pencil-like refractile processes which projected into a vestigial vitreous body.
Grammatophora (Spencer, 1886 ; McKay, 1888).
In Spencer's specimen the parietal eye was spherical and strongly pigmented in its lower segment. Externally it was covered by a white substance. Superficial to it the tissues contained no pigment and there was a conspicuous corneal scale. McKay's case has already been alluded to (p. 273).
Moloch horridus (Spencer, 1886).
The parietal spot was visible in the middle of a small area surrounded by horny processes, which cover the skin in this region ; it appeared as a black dot surrounded by a dark circular rim. The parietal organ lay in a parietal foramen and was continuous, by means of a slender cylindrical stalk, with the epiphysis. This widened out insensibly into a conical epiphysis. The stalk was solid throughout. The interior of the parietal organ was almost completely filled with pigmented tissue, and the lens was absent.
This family, which closely resembles the preceding, comprises very various species, which are mostly found in America and in neighbouring islands such as the Galapagos. Some, such as Anolis, are arboreal ; others, such as the snub-nosed Amblyrhynchus cristatus, live to a great extent in the sea and feed on seaweed. The Californian toad Phrynosoma, which is found in the United States of America and in Mexico, lives in sandy deserts and preys on small beetles and other insects. The girdled lizards Zonuridae are remarkable for the primitive character of their skulls, the hinder and lateral parts of which are completely roofed over by dermal bones.
In contrast to the Geckonidse, the parietal organ is usually present and well developed, and the Iguanidse in this respect resemble the Agamidse and the remaining families of Lacertilia which we shall describe in the following pages. The parietal organ is usually well developed and lies beneath a transparent scale, near the centre of which is a circular dark spot surrounded by a pale rim. The epiphysis is usually subdivided into a proximal thick-walled part, a stalk, a flattened vesicle, and frequently a fibrous cord which connects the tip of the epiphysis with the parietal organ. The latter is generally regarded as the fibrous sheath of the nerve of the parietal organ, the fibres of which have disappeared. It must be remembered, however, that in Lacerta vivipara and other species the parietal nerve has been shown by NowikofT to be developed independently and in front of the epiphyseal evagination, after the parietal organ has been separated from the epiphyseal part of the primary diverticulum.
One specimen described by Spencer (1886) differs from the typical in that the long axis of the parietal organ is vertical, and that it lies in an exceptionally narrow parietal foramen which is almost completely filled by the parietal eye. It was connected with the epiphysis by a fibrous cord, which was regarded by Spencer as its nerve ; but, as pointed out by Studnicka, the cord as shown in Spencer's figure is clearly continuous with the fibrous sheath of the parietal organ, and no fibres are seen entering the retina. In another specimen of Anolis described by Spencer, the parietal organ was dorso-ventrally flattened. It is probable, therefore, that the cylindrical parietal organ of the first specimen was exceptional, and does not represent a peculiarity of this particular family. The central cells of the lens in its deepest part contained pigment in the first specimen.
Leiolcemus nitidus (Spencer, 1886).
Fig. 193. — Sagittal Section through the Parietal Eye of Leiolcemus nitidus, showing a Flattened, Undifferentiated Retina of Ependymal Type, Biconvex Lens, and a Corium with Exceptionally Tall Papilla. (After Spencer, 1886.)
lens : lens. cor. : conum. cut. : cuticle. ep. : epidermis. ret. retina.
The parietal organ (Fig. 193) is dorso-ventrally flattened ; the central part of the retina is slightly arched upwards towards the lens, so that the cavity of the organ is much reduced in its vertical diameter ; the lateral walls formed by the peripheral part of the retina are almost vertical, but slope slightly outward as they ascend to the margin of the lens. The retina is composed of a single layer of cells containing pigment in their rod-like inner extremities, while the outer ends terminate in conical expansions the bases of which are continuous with the external limiting membrane ; oval nuclei were present in the bodies of the cells near the centre. The lens was biconvex and consisted of elongated cells, the oval nuclei of which lay for the most part in a single plane. A thick layer of corium lay between the parietal organ and the epidermis. This and the epidermis with the overlying corneal scale were destitute of pigment. The corium was remarkable with regard to the exceptionally large size of the papillae which entered the epidermis. The corneal scale was slightly arched upwards, like the glass of an old type of watch. No nerve was described, and it is worth while noting that the absence of a nerve in association with such a simple structural type of retina is what might be expected, the differentiation of the cells from the primary single layer of ependymal elements having advanced very little beyond the stage of deposit of pigment in the inner ends of these cells.
A nearly allied species, Leiolcemus tenuis, also described by Spencer, showed very similar conditions to those described above.
Plica umbra (Spencer)
The pineal system of this animal is remarkable for the extraordinary length of the epiphysis, which, arising from the roof of the diencephalon in the usual situation, is seen to consist of a proximal nearly vertical part and a horizontal segment which comprises the greatly elongated stalk, at the distal end of which is a small pigmented vesicle of doubtful nature, which has been regarded by different authors as the end vesicle of the pineal organ ; as a part of, or offshoot of the parietal organ ; or an accessory organ. Both the upper and lower walls are pigmented. The true parietal organ is a small flattened vesicle lying far forward and in an exceptionally large parietal foramen. The superficial wall, corresponding to the lens in other specimens, is unpigmented ; the lower or retinal segment is deeply pigmented. There was a specially large corneal scale in the middle of which was a small, slightly arched parietal speck.
Iguana tuber culata
The pineal system of Iguana has been specially studied by Spencer (1886), Leydig (1896), and Klinckowstroem (1895). The latter author also followed out in detail the developmental history of Iguana. His conclusions from the general standpoint are so important that we shall give them in full, in place of detailed descriptions of individual specimens.
I. The Parietal Eye
1. The parietal eye of Iguana, arising from the constriction at the distal part of the primary epiphyseal evagination, appears on the 9th day as an oval vesicle which is more or less constricted off from the proximal epiphysis.
2. The originally markedly biconvex lens in the course of development takes on a flat, slightly biconvex form or even a plano-convex form.
3. From the 14th to 18th days the entire inner side of the eye cavity is covered with cilia. A strong nerve unites the under side of the eye vesicle with the roof of the diencephalon. The retina shows two zones, one inner without cells and an outer cellular zone. Black pigment begins to appear on the retinal cells, and from the surrounding mesoderm a connective tissue capsule is formed which envelops the nerve and eye.
4. In 24 to 26 days the parietal nerve has reached its highest degree of development. Through the inward growth of the nerve-fibres the retina is divided into an outer and inner cell layer, between which the subdivisions of the nerve can be seen as a nerve-fibre layer. In the inner zone an abundance of pigment has been formed.
5. In 35 to 40 days the nerve and nerve-fibre layer shows significant signs of degeneration. The amount of pigment has greatly increased.
6. In the adult Iguana the pineal eye shows the varied features which are characteristic of rudimentary organs.
7. The nervous elements seem to have entirely disappeared in the retina, and the formation of pigment has become so excessive that a recognition of the structural relations is often impossible.
II. The Parietal Nerve.
1. The nerve which enters the pineal eye is dissimilar in its development to that of the optic nerves of the paired eyes.
2. At the end of the 9th day there is still no trace of a nerve to be seen entering the vesicle, which has already separated from the pineal.
3. By the 14th day the nerve is formed, and passes from the floor of the eye vesicle to a cell accumulation lying in the roof of the diencephalon, called the centrum parietale.
4. The parietal centre lies asymmetrically to the right of the middle line, immediately in front of the pineal evagination.
5. In 24 to 27 days the nerve is surrounded by a connective-tissue sheath (perineurium), and the parietal centre now lies in a direct line with the right ganglion habenulae.
6. In 35 to 40 days the nerve shows undoubted signs of degeneration that already appear to have produced atrophy of its central part.
7. In adults the nerve elements are completely atrophied and only the thickened perineurium remains.
8. In one embryo of Iguana there are two parietal nerves, one from each ganglion habenulee.
III. The Proximal Pineal
1. At the end of the 9th day the pineal has the form of a vesicle communicating with the third ventricle ; this shows a structure comparable with the parietal eye vesicle.
2. In the course of development the pineal becomes covered with cilia internally and is gradually transformed into a long funnel.
3. The proximal part of the pineal retains a structure which is suggestive of that of the medullary tube ; the development of the distal part resembles that of the retina of the parietal eye.
4. The conical pineal of adult animals retains at its distal end the funnel-shaped form of the embryo, while its proximal part undergoes a follicular transformation during the growth of its wall.
5. The epiphyseal evagination in the tegu shows in the early stages of development, just as in Iguana, a constriction causing a division into a distal part (pineal eye vesicle) and a proximal part (pineal).
6. Later this constriction disappears and the entire primary epiphyseal evagination is developed into a pineal.
7. The pineal of the tegu, in a stage corresponding to 24-26 days of Iguana, shows traces of pigmentation that appear to vanish afterwards.
8. In an embryo of Iguana nerve bundles from the diencephalic roof, entirely similar to the parietal nerve, enter the posterior part of the pineal (Fig. 187, B, p. 264).
9. On the end of the proximal pineal of one Iguana embryo and in an adult a secondary pigmented pineal eye was developed.
Among the special points which Klinckowstroem emphasizes in the summary of his observations on Iguana one may note : (1) the separation of the parietal vesicle, by means of a constriction from the anterior part of the primary evagination ; (2) the independent formation of the parietal nerve, which takes place after the separation of the eye vesicle from the epiphysis ; (3) the connection of the parietal nerve with the right habenular ganglion in some cases, and the demonstration in one embryo of Iguana of two parietal nerves, one ending in the right habenular ganglion and the other in the left habenular ganglion ; (4) the degenerative changes in the retina and parietal nerve which occur in the late stages of development, which eventually lead to a total disappearance of the nerve-fibres ; (5) the distinction between (a) the proximal part of the epiphysis, which undergoes a follicular transformation and an accompanying thickening of its wall, and (b) a distal part, the structure of which resembles and suggests that of the retina of the parietal eye.
The different species of Lacerta have been studied by Spencer, Owsjannikow, Strahl and Martin, Leydig and Nowikoff. All the typical parts of the pineal system are represented in the ordinary lizards, and the variations in form and structure which are met with are largely due to age and the concomitant degeneration found in old specimens. The different stages of development were described by Nowikoff in 1910, and have already been alluded to (p. 242). They confirm Klinckowstroem's observations on Iguana with respect to the separation of the parietal vesicle by constriction and the independent origin of the parietal nerve subsequent to the separation of the parietal vesicle.
The pineal system of the adult lizard comprises the following parts, which, passing from the superficial surface of the head towards the brain, are :
A pentagonal parietal scale, near the centre of which is a slightly raised dark spot, the " parietal spot " (Fig. 192, C, p. 274).
A pigment-free, transparent parietal area, which is formed of a mucoid connective tissue, lying between the superficial closing membrane of the parietal foramen and the epidermis. The parietal eye and parietal nerve. The terminal or habenular ganglion.
A membranous fibrous capsule which encloses the retinal segment of the parietal eye, and the parietal nerve ; in old specimens in which the nerve-fibres have degenerated the fibrous sheath of the nerve may persist as a cord uniting the parietal eye with the brain roof or with the tip of the epiphysis.
A pineal organ which consists of : a proximal part, springing from the diencephalic roof between the habenular and posterior commissures ; a tubular body or stalk : an end vesicle, which is sometimes prolonged forward as a fibrous cord which ends in the vicinity of the parietal organ or is united with its fibrous sheath. The end vesicle of the pineal organ is surrounded by blood-vessels which are contained in a fibrous connective tissue which is sometimes deeply pigmented. This is continuous with a similar vascular and pigment-bearing connective tissue around the parietal organ.
Striations have been observed in the peduncle of the proximal part of the pineal organ, which Leydig considered were nerve-fibres, but no separate pineal tract, such as that figured by Klinckowstroem in Iguana tuberculata, appears to have been described.
In some cases the walls of the epiphysis may be folded ; thus Studnicka describes in a specimen of L. agilis a series of lateral buds, right and left, on each side of an elongated sac-like epiphysis ; and Leydig found a similar condition in L. ocellata. In this specimen the epithelial cells at the distal end of the epiphysis were pigmented, as in a case of Anguis fragilis also described by Leydig.
The skink family comprises a number of different species, mostly found in sandy deserts and often showing retrogressive features, such as the union of the premaxillas and diminution in the size and number of the limbs, the fore-limbs or both fore and hind-limbs being absent in some species. In Chalets flavescens, a long, worm-like animal, both pairs of limbs are present, but are less than 2 mm. in length. One interesting adaptation to a dust-blown sandy environment is the existence in certain skinks of a transparent window in the lower eyelid of each lateral eye, which allows the eye to be used when closed, while at the same time it serves as a protective covering for the eye.
A description of the pineal system of this animal was given by Spencer in 1886. A corneal scale, parietal fleck, and parietal foramen were all present. The pineal organ took origin from the roof of the diencephalon in the usual situation, and coursing forward with an arch-like bend, ended in a vesicular expansion above the anterior part of the hemispheres and in the region of the parietal foramen, which it partly filled. Spencer regarded this expansion as a parietal eye which had remained in an undeveloped and rudimentary condition. Studnicka, however, states that there can be no doubt that the structure described by Spencer as a parietal eye was in reality the well-developed end vesicle of a completely formed pineal organ, and that the parietal organ had not been developed. Both the upper and the lower wall of the end vesicle was composed of long, cylindrical cells from which cilium-like processes projected into the lumen of the vesicle.
Legge (1897) described the pineal diverticulum of an embryo 10 mm. long, which was divided into a distal part, which he considered to be the rudiment of the parietal eye, and a proximal part, the epiphysis. The distal segment, namely, the parietal eye (or possibly the end vesicle of the epiphysis), had disappeared in those specimens which had passed the embryonal period, and there remained only the true epiphysis which had become greatly enlarged. This consisted of a proximal vertical segment and a horizontal part which was directed forward ; it showed saccular outgrowths and ended in a pointed extremity, the walls of which were of equal thickness.
The parietal eye, which is only present in the embryonic stage of development, has a distinctly biconvex lens consisting of elongated cells and a retina containing brown pigment. The retinal cells ended in long rod-like processes, which projected into the lumen of the vesicle ; and between the bases of these small round cells could be distinguished. Neither nerve-fibres nor parietal nerve could be recognized. No parietal foramen was present, nor was there any specially modified corneal scale, the scales in the pineal and the surrounding region being small and of approximately equal size.
The published description of other species of the Skink family, such as Scincus officinalis, Hinulia, and Chalcides tridactylus, do not differ essentially from that of Cyclodus and Gongylus, and it will be unnecessary to give a special account of each of these. It will suffice to give a general idea of the structure of the pineal organ by the drawing (Fig. 194) reproduced, with the kind permission of the author, from a photograph of a section through the parietal eye of Scincus officinalis and published by Jean Calvet in his work L'Epiphyse, 1934. This shows the lens, retina, and parietal nerve, with the general relations of the organ. Calvet also gives photographs showing the position of the parietal scale in Varanus and Iguana.
This family, which is represented by the chamaeleons, is from the standpoint of the pineal system as remarkable as it is in other respects. The pineal system in general resembles that of Lacerta, but although all parts of the complete system are represented in the embryonic condition, the parietal organ in some examples has disappeared in the adult animal. A detailed account of the pineal system of chamaeleons was given by Spencer in 1886 and one two years later by Owsjannikow. In both of these examples and in another mentioned by Studnicka, which was examined by himself, there is substantial agreement with reference to the presence of a parietal foramen. In Studnicka's case the contained parietal organ was surrounded on all sides by a dense plexus of anastomosing vessels, and in all, the tissue overlying the organ was devoid of pigment and its position on the head was indicated by a small, slightly raised transparent scale or nodule. According to Spencer's description, the pineal organ (epiphysis) was a hollow, tubular structure, which was directed at first dorsally and was then bent sharply forward. In its distal part the wall was folded, and from the extremity of this portion a cord, which he regarded as the nerve, coursed forward to the parietal eye. Owsjannikow, however, considered that this cord consisted of connective tissue rather than of nerve-fibres. The parietal organ in Spencer's specimen was a slightly flattened hollow vesicle which was elongated in the anteroposterior direction. No distinction in the structure of different parts of its wall could be recognized, neither lens nor retina being distin
Fig. 194. — Vertical Section through the Pineal Eye of a Skink, showing the Lens, Retina, Nerve, and Pigment Cells.
It will be noted that there is a considerable amount of pigment in the epidermis
and subepidermal tissue lying over the pineal organ.
(Drawn from a photograph — J. Calvet.)
guishable. The walls throughout were composed of cells of approximately equal length, and the inner ends of these cells bore long cilia which projected into the cavity of the vesicle. No pigment was present. The lower wall of the vesicle, moreover, showed on its inner aspect a small invagination which Spencer took to be a remnant of the primary connection of the lumen of the parietal organ with that of the epiphysis.
Owsjannikow's specimen differed from Spencer's in the more complete differentiation of the parietal eye, a definite lens and retina being distinguishable. He described two layers in the retina : an inner composed of deeply pigmented, long, rod-like cells bearing cilia, and an outer layer which was devoid of pigment and contained fibres and cells with round nuclei. The lens was biconvex, contained no pigment and was formed of elongated cells. In the cavity of the vesicle there was a fine granular mass, which represented a vestige of the vitreous body. The conclusion to be drawn from these two widely different examples is that a variable degree of differentiation of the parietal organ may occur in different specimens, the development proceeding farther in some cases than in others, in which it becomes arrested at an early stage — the arrest being followed by degenerative changes. Another interpretation of the differences between these two specimens is the possibility of the hollow structure devoid of pigment, which was described by Spencer as the parietal eye, being the end vesicle of the pineal organ, the parietal eye having failed to develop or having been present only for a short period during embryonic development, as in the examples of disappearance of the organ described by Legge in Gongylus (p. 281).
The Pineal System of Ophidia
The pineal system of the snakes is represented only by the epiphysis, the parietal eye being absent. In one case, however, published by Hanitsch in an embryo of the common viper (Pelias berus) a well-differentiated parietal organ with a lens, which contained much pigment in its interior, was described. The epiphysis had the usual form, and had the appearance of twisted tubes between which connective tissue was interposed, giving it a glandular appearance. The case seems, however, to have been exceptional and the parietal organ has not been seen in other species. In view of the temporary embryonic existence of a parietal organ in Gongylus (described by Legge, p. 281), the correctness of this single observation cannot be denied, and the very occasional occurrence of a parietal eye in snakes must be regarded as awaiting further confirmation.
The skull in living species of snake has no parietal foramen, and in the python the parietal bones are fused in the median plane so as to form a prominent keel-like ridge in which it is unlikely that any remnant of a parietal canal would persist. A foramen is, however, present in the skull of the extinct Pythonomorpha, in which the parietal bones, although united, are not produced upwards as an intermuscular ridge, but form a flat, triangular plate with the parietal foramen near or at its anterior border. Examples are met with in Clidastes propython (Cope), Upper Cretaceous, Alabama ; and Platecarpus coryphceus (Cope), Upper Cretaceous, Kansas. These were marine, lizard-like animals of large size (5 metres), having anterior and posterior paddles adapted for swimming. The parietal foramen of Platecarpus was oval in outline, the longest diameter being about 2 cm., it was in the median plane and situated at the anterior border of the triangular parietal bone, immediately behind the frontal bone. Although possessing certain characters which differentiate them from the Lacertilia, the Pythonomorpha or Mosasaurida: are more nearly allied to lizards than the snakes, and fossil skulls of the true Ophidia which are of comparatively recent date do not appear to differ markedly from the skulls of living species.
The pineal system of living examples of Ophidia has been studied by Sorensen, 1894 5 Leydig, 1897 'Studnicka, 1893 5 an ^ Rabl-Riickhard, 1894. The epiphysis of the common ringed snake, Tropidonotus, a nonpoisonous British species, is developed as a simple hollow evagination from the roof of the diencephalon in the usual situation between the habenular and posterior commissures, and there is usually a well-developed paraphysis and dorsal sac. Later it becomes pear-shaped, and eventually the cavity disappears in the distal part, which becomes a solid rounded mass, and at the base, where the neck becomes constricted and cord-like. What appears to be a remnant of the original cavity may, however, persist at the junction of the stalk with the body of the organ (Fig. 179, Chap. 20, p. 251).
In the adult the body is surrounded by blood-vessels which penetrate into its substance and produce a lobulated appearance. The whole is enclosed in a sheath consisting of an inner and outer layer of connective tissue, which is continuous with the pia mater and encloses a space. Leydig described a striated appearance in the pedicle, which he thought was produced by nerve-fibres connecting the organ with the posterior commissure. In one case the distal end of the organ was surrounded by deeply pigmented cells. Studnicka considered that the lobulation and penetration of the organ by connective tissue septa containing capillary blood-vessels might be an indication of its transformation into a gland of internal secretion. It is quite common, however, to find degenerating nervous or neuroglial tissue invaded by blood-vessels, and the appearance is very different from that of a typical endocrine gland.
The Pineal System of the Chelonia
The parietal organ and parietal foramen are lacking and the pineal organ is represented merely by a rudimentary epiphysis.
The development of the parietal region of the brain has been studied by Voeltzkow (1903) in Chelone, and in Chelydra serpentina, the snapping turtle, by Humphrey (1894). Voeltzkow found that in the early embryonal stages the epiphysis had the form of a simple tubular evagination of the roof of the thalamencephalon, which in later stages, becoming thinner at its basal or proximal part, developed a secondary stalk. Still later, by a further attenuation of the stalk, the epiphysis became completely separated from the roof of the brain. A detailed description of the parietal region in an embryo of Chelydra (Fig. 195) shows that this region of the roof of the brain differs little from that of other reptiles. The tubular epiphysis springs from the roof of the third ventricle in the usual situation in front of the posterior commissure and passes upwards on the posterior wall of the dorsal sac, and then forwards over its superficial surface as far as the velum and tip of the paraphysis. Its lumen widens as it passes forwards and the wall of the epiphysis, which in the basal part consists of a single layer of epithelial cells, shows two layers of nuclei at its rounded distal end. as it passes forwards and the wall of the epiphysis, which in the basal part consists of a single layer of epithelial cells, shows two layers of nuclei at its rounded distal end.
Fig. 195. — Parietal Region of the Brain of an Embryo of a Snapping Turtle (Chelydra serpentina), showing the Relation of the Epiphysis to the Dorsal Sac and Paraphysis. (After Humphrey, 1894.) Cp. : posterior commissure. MB. : midbrain.
Ds. : dorsal sac. Par. : paraphysis.
Ep. : epiphysis. PL Ch. : plexus choroideus.
V. : velum.
In the adult animal the lumen in some cases is found to have disappeared, in the stalk and also in the body of the epiphysis ; in other cases it may persist, and in one specimen of Cistudo europcea described by Faivre it contained small granules of calcium phosphate. The shape of the organ varies, being described as ovoid, conical, or tuberous ; it is surrounded by blood-vessels and ingrowths of fibrous tissue may penetrate the wall, giving a lobulated appearance to its surface, but there is no outgrowth of epithelial buds or true lobulation. The structure of the epiphysial wall of an adult specimen of Cistudo europcea was described by Studnicka (1905) ; the wall was thick and penetrated here and there by connective tissue bundles derived from the capsule. The lumen was lined by a single layer of cylindrical ependymal cells, which were everywhere of the same size and ended at the same level on the inner surface of the wall. The wall was composed of elongated ependymal cells which were attached by their outer ends to the external limiting membrane. The ependymal fibres tended to unite in bundles, which gained attachment to the ingrowing fibres from the connective tissue capsule. The tissue consists of peculiar stellate cells which send out slender protoplasmic processes and probably correspond to neuroglia cells, although no true neuroglial fibres are present. The cells are widely separated from each other and the tissue thus has a somewhat spongy appearance. There are no true ganglion cells, nor are any nerve-fibres present. In many places the inner surface of the wall shows pits which are due to small diverticula of the single central lumen ; external to these are small closed cavities lying in the substance of the wall. Sometimes the diverticula and cavities are lined by cylindrical ependymal cells, similar to those bounding the main cavity ; at others by the ordinary cells of the surrounding tissue. There were no indications of a secretory process in the walls of the epiphysis. In the central cavity some isolated cells were found, the protoplasm of which seems to have been slightly changed, and in the diverticula and spaces a few separate cells or small amoeba-like syncytia were present. The stalk of the epiphysis had no lumen, but small cavities similar to those in the body of the epiphysis were present, and also spaces lying between the stellate cells which form the bulk of the tissue of the stalk. No nervous cord could be seen in the interior of the stalk. The epiphysis of the Chelonia thus appears to occupy an intermediate position between that of the saurian reptiles and that of birds.
The pineal system of Crocodilia has already been alluded to (p. 47), and it will only be necessary to note that the absence of the parietal organ in all species examined is contrary to expectation in these animals which live in tropical climates, should the theory of the function of the parietal eye being an organ for the estimation of heat be correct.
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