On the sclerotic ring of the eyes of birds and reptiles

IRLF.

948

Ass ' B 3 8"

ON THE

SCLEROTIC RING

OF

THE EYES

f

OF

BIRDS AND REPTILES,

BY

THOMAS IALLIS, ESQ.

YORK :

H. SOTHERAN, BOOKSELLER, CONEYSTREET.
1855.

THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

M366951

A58

[Extract from the "Proceedings" of the Yorkshire
Philosophical Society^

On the Sclerotic Ring of the Eyes of Birds and Reptiles. —
By THOMAS ALLIS, ESQ. Read April, 1849.*

When I began the preparation of Bird's Skeletons, I was not
aware that their eyes were furnished with this bony apparatus,
and when first informed of the fact, it was mentioned only with
reference to rapacious birds.

By accident the sclerotic bones of an Eagle Owl became
detached from each other in consequence of over boiling ; I was
induced to articulate them together and to count them ; and on
finding it stated in Cuvier's Comp. Anatom. that the usual
number of these bones was twenty, whereas in the bird in ques-
tion they only amounted to fifteen, I was led to proceed further,
and after dissecting minutely the sclerotic ring of upwards
of seventy birds, taken from every great division, the greatest
number I have found in any instance is seventeen ; and the
smallest number eleven ; except in a single instance, in which
the ring is composed of but one single bone. I have sixty-seven
species on these tablets : of these, the ring consists in one
instance of only a single bone ; three have eleven bones ; eight
have twelve ; twelve have thirteen ; twenty have fourteen ;
nineteen have fifteen ; two have sixteen ; and two have seven-
teen. I have consulted several authors, but have met with little
respecting either the bones themselves or their functions ; the
little I have found is contained in the following extracts ; after
giving which, I shall mention concisely those particulars in
which the results of my own researches differ from the state-
ments made by these authors.

Blumenbach Comp. Anat., 296, says, " the eyes of birds of

* This paper was composed in 1837, and in that year was read to the British
Association assembled in Liverpool. At the request of the Yorkshire Philosophical
Society, in whose Museum the Allisian Collection of Comparative Osteology is
preserved, it was read to that Society in 1849. In the interval between these dates,
Mr. Allis had the satisfaction of shewing to the authors of that splendid publication,
4 The Dodo and its Kindred,' that their inference of the place of Dodo among tho
Columbidse was entirely confirmed by the independent evidence of its Sclerotic
Bones.— (Note by Editors.)

2

prey have a peculiar form ; which is similar to that of the
chalice or cup used in the communion service ; the cornea which
is very convex forms the bottom of the cup, and the posterior
segment of the sclerotica resembles the cover; this peculiar
form arises from the curvature and length of the bony plates,
which, as in all other birds, occupy the front of the sclerotica,
lying close together and overlapping each other; these bony
plates form in general a slighty convex ring ; being long and
curved in the Accipitres, they form a concave ring, which gives
the whole eye-ball the above-mentioned form."

Dr. Albers observes, " that the orbit is very imperfect in
birds, and that this bony ring may supply the deficiency."

Carus says, " the firm and elastic sclerotica of birds, the
structure of which has been very accurately examined by
Albers, consists of three laminae, between the outer and middle
ones of which the osseous circle is inserted anteriorly. This
structure, which already exists in some fishes and amphibia, is
common to all the species of birds ; it is composed of from
fifteen to seventeen oblong quadrangular laminae of bone, with
the corners rounded off; forming in some cases simply a smooth
circle, in others a more or less prominent cylinder ; in Owls this
cylinder is particularly long."

Cuvier says, {f the sclerotic of birds is divided into laminae,
the interval of which receives a circle of small, thin, hard,
oblong bones, which lie over each other like tiles, and which
give to the anterior part of the eye a great degree of firmness
and a fixed form. These ossicula are almost flat in the greater
number of birds ; in which they form only an annular disk of
little convexity ; they are slightly arched and concave externally
in the Horned Owl, in which they form a short tube in the form
of a truncated cone ; they are usually twenty in number."
Cuvier further states, that " the sclerotica determines the shape
of the eye ; it can therefore be really soft and flexible only in
animals that have the eye nearly globular ; that is to say in
man and quadrupeds ; because their sclerotic assumes of itself
that shape, in consequence of the nearly uniform resistance
made by the fluids contained in the eye to the pressure of its

coats ; but in all animals that have the eye more removed from
a spherical form, as the Cetacea, Fishes and Birds, that mem-
brane is supported by hard accessory parts ; or by a greater
solidity of texture and a more considerable thickness."

In a Paper in the third vol. of the Zoological Journal,
Wm. Yarrell says, " the eyes of birds are much larger in pro-
portion than those of quadrupeds, and exhibit also two other
peculiarities." One of these peculiarities is " a ring of thin
bony plates enveloped in the sclerotic coat. Comparative
Anatomists do not seem to be agreed as to the means by which
birds obtain their powers of vision ; whether by alteration in
the form or situation of the crystalline lens, or by both ; either
or both of which, the greater quantity of aqueous humour
which birds are known to possess, would seem to facilitate, and
the existence of a muscle attached to the inner surface of the
bony hoop of the sclerotica and inserted by a tendinous ring into
the internal surface of the Cornea, as shewn by Mr. Cramptoii
in the Annals of Philosophy for 1813, by which the convexity
of the Cornea may be altered, gives a still greater scope of
action." He afterwards says, " the external convex form of
the Golden Eagle will be found to extend through all the
species of every genus of British birds, except the Owls, in all
of which it is concave."

In speaking of the sclerotic bones generally, Dr. Buckland,
page 174, of his Bridgewater Treatise, says, " In living animals
these bony plates are fixed in the exterior or sclerotic coat of
the eye, and vary its scope of action by altering the convexity
of the cornea ; by their retraction they press forward the front
of the eye and convert it into a microscope ; in resuming their
position when the eye is at rest, they convert it into a telescope."

As regards the form of the bony ring, Blumenbach certainly
is not correct when he says, that " in the accipitres it is con-
cave externally ;" that is only true of the nocturnal accipitres,
as stated by my friend Wm. Yarrell ; nor is the latter correct in
saying that every species of British birds has it convex like the
Golden Eagle. The Woodcock, Spoonbill, Caprimulgus, and
some others have merely a flat and narrow ring towards the

external edge of the eye, and certainly no degree of convexity
at all approaching to that of the Golden Eagle. The shape of
the individual bones is so various that it cannot he given in any
general term ; the external edge of the bones is in most
instances beautifully serrated ; but the serration is not visible
in the bony ring ; but in the separate bones which were boiled
until the extraneous matter would wipe off easily with a cloth,
it is very perceptible ; the rings would separate if boiled to the
same extent, and in cleaning them with a knife I have not
been able to preserve the serrations.

As regards the structure of the rings, the bones generally
overlap each other, there being a depression on the under side
of one bone, and a precisely corresponding one on the upper
side of its neighbour, so that when overlapping each other, they
present nearly an even surface, both exteriorly and interiorly ;
having one bone with both depressions on its inner surface, and
forming an exterior key to the arch, and one having both
depressions on its outer surface, and forming an internal key ;
in some instances there are two external keys ; and in several
instances, instead of one bone overlapping the other, they inter-
lock into each other in a curious and beautiful manner.

I take it for granted that the principal function of these
sclerotic rings is, as stated by Cuvier, to preserve that peculiar
form of the eye which is adapted to the nature and wants of the
animals. But in nature's laboratory there is no prodigality or
waste of power ; and these rings having, in the first place,
fulfilled the duties for which they were primarily created, still
subserve the wants of the animal in other ways, and under cir-
cumstances and in situations to which those classes of animals
which have not such rings are not exposed.

I allude to their use as a defence and protection to the eye.
On examining these specimens it will be found that those birds
which are peculiarly pugnacious ; those which have a peculiarly
rapid flight ; and those which, from the extended variation of
altitude at which they fly, are exposed to great or very unequal
degrees of atmospheric pressure, have the sclerotic rings of
larger size, of more convex form, and the individual bones of

greater strength than the weak-billed or low-flying birds ; and
the same remark holds good with the water birds, on comparing
those that take their food on or near the surface of the water,
with those that dive ; among which class are to be found the
strongest bones.

Another subsidiary use, is that of altering the convexity of
the Cornea as mentioned by Dr. Buckland.

I have eight specimens of diurnal Rapacidse ; the Golden
Eagle affords an example of a bony ring of greatly increased
strength in a bird that takes a lofty flight and follows its prey
with great velocity : it is a smaller bird than the white tailed
Eagle, yet the bony ring is larger, more convex, and the indivi-
dual bones much stronger. The King Vulture and Lammer-
geyer have the rings stronger than their congeners ; and the
Secretary also has it very strong, but with its habits of flight I
am unacquainted ; of this series, three have fourteen bones,
four has fifteen, and one has sixteen.

Here are five specimens of Owls ; two belong to the great
Horned Owl : one set of the detached bones shews the upper,
the other set the under surface of these bones ; the principal
use of the elongated tube appears to be, to bring the eye beyond
the loose feathers of the head; if the bones were no longer
than in the generality of birds, the eyes would be so buried in
the feathers that the bird would only be able to see objects
straight before it. The bones of the Barn Owl are not larger
than those of the little Scops, though the bird itself is nearly
three times as large ; they are not required to be so long in the
Barn Owl on account of the large circular disk of close and
short feathers that surrounds each eye. The bones of the Owls,
instead of being, as stated in general terms by Cuvier, hard,
flat and thin, are very soft and porous ; as is also the case in
those of the Caprimulgidse, and those of the great Horned Owl
are also of considerable thickness.

I have seven specimens of Gallinidse ; one of these has
thirteen bones ; four have fourteen ; one has fifteen ; and one
has seventeen ; the ring in most of these is strong.

Of Columbidec, I have three specimens, each of which has

G

eleven bones : the bones are strongest in the rapid flying
Carrier Pigeon, though the Crowned Pigeon is three times its
size. The ring of the Dove is small and the bones feeble.

I have a specimen of each of the Struthious birds ; the
Ostriches have each fifteen bones ; the Cassowary and Emew
thirteen each ; the latter affords a beautiful example of the
interlocking of the bones into each other.

I have nine specimens of Grallse : of these, one is imperfect ;
one has thirteen bones ; five have fourteen ; and two have
fifteen. In the soft-billed birds of this class the rings are small
and feeble, and larger in the sharp-billed specimens, though not
so strong in these as in some other classes.

Of Scansorial birds, I have seven specimens : of these, five
have twelve bones, and two have thirteen. The Parrot, Macaw
and Cockatoo have the rings particularly small and feeble ;
owing, I presume, to the skulls of the Psittacidse being provided
with perfect bony orbits, which, I believe, no other class of birds
possess. The Woodpecker has the ring as strikingly large ;
indeed it has, with only one exception, a larger portion of the
eye protected by the sclerotic ring than any other bird I have
met with in proportion to its size, and as I find nothing in its
mode of flight, or in the altitude at which it flies to make it
require this etxra protection, I am induced to think that it is
intended to protect the eye from injury from the small chips of
wood which it scatters in all directions when searching for its
insect prey, or boring the tree for the construction of its nest.

Here are seven specimens of swimming birds : one has
twelve bones ; one fourteen ; four have fifteen ; and one has
sixteen. The Ducks, Geese and Swans which seek their food
at or near the surface of the water, have the rings remarkably
weak and small ; while the Gulls, which descend into the water
with some degree of force, have them considerably stronger.

I have ten specimens of diving birds, including the Pelican,
which though not strictly a diving bird, I have included in this
group on account of its affinity with the Gannet ; of these, two
have twelve bones ; three have thirteen ; three have fourteen ;
and two have fifteen. In the Pelican, which takes its food

near the surface, we find merely a feeble annular disk ; while
the other birds of the group have the ring very strong and
convex : the Gannet, which takes its prey by descending per-
pendicularly from a considerable height, with great force into
the sea, has the individual bones stronger than any other bird I
have met with : — the true divers, (which dive from the surface
to a considerable depth after their prey) have their ring very
strong and convex, though the individual bones are much
weaker than in the Gannet. The Guillemot, Razor-bill and
Sea Parrot, also have the ring very convex, especially the latter.
Here are ten specimens of Passerine birds : two have thirteen
bones ; five have fourteen ; one fifteen ; one uncertain ; and one
(the Podargus Humeralis) differs from all other instances I
have met with ; the bony ring being composed of one single
bone, instead of a series of plates. The European Night Jar
has a very weak ring without any convexity ; while its Austra-
lian congener has the ring of considerable size ; though the bone
itself is of a soft and porous texture like the Owls. The Swift
has the ring larger, stronger and much more convex than any
other bird at all approaching its own size. The Kingfisher's is
also large for the size of the bird ; and the bones of the Hum-
ming-bird are also strong.

Of Reptiles, Cuvier says, (Comp. Anatom. translation, page
396,) ee There are similar laminae in the sclerotic of the
Chameleon and several other Lizards ; but they do not form
the anterior disk of the eye ; but merely surround the lateral
part." Here are bones of two species of Turtle and three
species of Lizards ; Serpents and Frogs appear destitute of
them. The bones of the Turtle are less symmetrical than those
of birds or Lizards, and form a nearly flat disk, having but little
convexity.

Out of three species of Lizard, which are all I have examined,
and of which the Chameleon is one, in two cases the ring, in
opposition to the statement of Cuvier, does form the anterior
disk of the eye, and that as completely as in any class of birds :

8

in the Iguana, these bones (from which the figure in Dr. Buck-
land's Bridge water treatise was taken) are remarkably broad at
their inner edge, and overlap each other to a greater extent than
I have observed in any birds, thereby greatly increasing the
strength of the convex ring ; the external edge forms a kind of
pedestal or foot; in birds the external edge is generally the
broadest. The Chameleon has the bones of the same form as
the Iguana, but the pedestal is less produced ; the ring is very
strong; and to increase still further the strength of the eye's
defence, the bones are covered with the external skin of the
animal, leaving only the pupil exposed. The Gecko has the
ring of very slight texture, and it merely surrounds the lateral
part of the eye, as described by Cuvier ; the single bones (of
which I have only preserved one specimen) are very slight, and
from back to front exhibit a considerable curve. In fossil
Saurians, these bones are of a very different and less complex
form than those of the Iguana or Chameleon ; they appear to be
merely oblong bones of uniform shape, forming a protection to
the front of the eye ; they are beautifully figured as placed in
the head of the Ichthyosaurus in Dr. Buckland's treatise ; and
I have here, from our Geological collection, as beautiful an
example of these bones in situ.

Note. — The Paper was illustrated by anatomical preparations of the sclerotic
rings. Figures of many examples may be seen in the plates which accompany this
volume.

EXPLANATION OF THE PLATES

PLATE 4.

Figure 1. A sclerotic ring of the Cassowary.

la. The detached bony plate forming the ring ; being
one of the best examples of the manner in which
these bones interlock into each other.

£. Ring of the African Ostrich.

3. Ring of the Golden Eagle, which is proportionally

larger and the bones thicker and stronger than
the author has found in any other bird of this
family.
3a. Detached bones of the same.

4. Ring of the White Tailed Eagle.

5. A front view of the ring of the Great Horned Owl ;

in which the bones are of considerable thickness,

but light and porous.
5a. A side view of the same, shewing a more perfect

tube than is found in any other bird.
5b. Three detached bones of the same.

PLATE 5.

Figure 1. Sclerotic ring of the Norfolk Plover; which is
proportionally larger than in any other known
instance.

2. Ring of the Spoonbill ; very small and feeble.

3. Ring of the Crane.

4. Ring of the Wild Goose ; which is proportionally

one of the smallest.

Figure 5. Ring of the Herring Gull, considerably larger than
in the Wild Goose, though the bird is but little
more than a third its size and weight.

6. Ring of the Gannet.

6a. Two detached bones of the same.

6£. A section of one of the bones to shew its thickness ;
in this bird the bones are thicker, more compact,
and stronger than in an any other known instance.

7. Ring of the Pelican, a much larger bird than the

preceding.

8. Ring of the Black-throated Diver, which is large

and strong.
8a. Detached bones of the same.

9. Ring of the Red-throated Diver, which though a

larger bird than the preceding, has the ring more
feeble and smaller; indicating that it takes its
food nearer the surface of the water.
9#. Detached bones of the same.

10. Ring of the Sea Parrot ; which covers a larger por-

tion of the eye than in any other aquatic bird ;
and acquires greatly increased strength from the
extent to which the bones overlap each other.
100. Detached bones of the same.

11. Ring of the Domestic Fowl.

12. Ring of the Collared Dove, which like all the other

Columbidae which have come under the author's
notice, consists of 11 bony plates ; the smallest
number met with in any class of birds ; the
extinct Dodo being the only bird with the like
number.

13. Ring of the Wood Grouse.

14. Ring of the Green Woodpecker, which is larger

and covers a much greater portion of the eye,
than in any other scansorial bird.

15. Ring of the Blue Macaw, a bird more than twelve

times the size of the Woodpecker, though with a
much smaller ring.

Figure 16. Ring of the Lemon-crested Cockatoo, which is like
the Macaws, very small.

17. Ring of the Toucan, a hird not a third part the

size of the preceding, but which has a much
larger ring.

18. Ring of the Touraco or Plaintain Eater.

19. Ring of the European Night Jar ; the bones are

soft and the ring is very feeble.

20. Ring of the Swift, proportionally one of the largest

and strongest.

PLATE 4.

REPTILES.

Figure 6. Sclerotic ring of the Iguana.

6a. Two detached bones ; the outer edge of which
forms a kind of pedestal, contrary to what is
found in birds, where the outer edge is generally
the broadest ; in the present case the ring is
much strengthened from the extent to which the
bones overlap each other in consequence of this
peculiarity in their form.

7. A front view of the ring of the Gecko, which in

this surrounds the eye, as described by Cuvier,
and covers scarcely any part of the anterior por-
tion of the eye.
7a, A side view of the same ring.

8. Ring of the Chameleon ; which covers the whole

surface of the front of the eye except the pupil,
and shews the smallest aperture of any known
sclerotic ring.

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