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[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.) 


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 


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 

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 : 


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 



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 
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. 


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

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 


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 

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. 



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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