LACRYMAL ORGANS. 



95 



ture will be described in speaking of the con- 

 junctiva. In birds there is no example of the 

 eyeball being covered over in a similar manner 

 by a continuation of the skin. 



In man the upper eyelid is the more de- 

 veloped and the more moveable. Descending 

 from him the lower is found gradually to 

 assume the superiority in this respect. In the 

 cetacea the upper and lower eyelids are tumid 

 folds of skin enclosing fat, but no tarsus, and 

 the Meibomian glands are entirely wanting. 

 There is no orbicularis palpebrarum muscle, 

 but muscular fibres proceed from the anterior 

 and posterior end of the orbital process of the 

 frontal bone to the eyelids in the region of the 

 outer and inner canthi. Instead of the levator 

 palpebrae superioris there is a hollow conical 

 muscle which arises from the circumference of 

 the foramen opticum and terminates in the eye- 

 lids. A similar in four divisions occurs in the 

 seal.* In the echidna there is a circular eye- 

 lid. In birds the lower eyelid is in general 

 very much larger and more moveable than the 

 upper. In the ostrich and some parrots both 

 eyelids are equally moveable. 



In birds the eyelids are closed in death. In 

 the gallinaceous birds which I have examined 

 this appears to be owing to an expansion of 

 elastic membrane attached to the margin of the 

 orbit and interlaced round the eyelids. During 

 life the lower eyelid is opened by muscular 

 action by a proper depressor muscle. The 

 upper eyelid retains its levator. There is a 

 tarsal cartilage in the lower eyelid; in the 

 upper eyelid there is a less dense fibrous struc- 

 ture. The skin of the lower eyelid is naked 

 and finer than that of the upper in accordance 

 with its greater mobility. 



In chelonian reptiles the lower eyelid is, as 

 in birds, the larger and more moveable ; and as 

 the more moveable the eyelid the finer the 

 integument, so in them the lower eyelid is naked 

 as in birds. 



In lizards the eyelids form a sort of sheet 

 stretched before the eyeball with a horizontal 

 fissure closed by a circular muscle and opened 

 by a levator and depressor. In the chameleon 

 the palpebral fissure is represented by a small 

 hole opposite the pupil. Tracing the eyelids 

 in a general way from batrachian reptiles to 

 fishes, we find a gradual depreciation of struc- 

 ture ; thus salamanders have two folds of skin, 

 upper and under, for eyelids, but not sufficient 

 to cover the eyeball, whilst the pipa has none. 

 In fishes generally, the skin before passing over 

 the front of the eye becomes finer and forms a 

 slight circular fold, often only well-marked 

 above. Sometimes there is an anterior and a 

 posterior semilunar fold, as in the herring, 

 salmon, but especially in several of the shark 

 tribe. It is the Orthagoriscus mola which has 

 circular folds for eyelids, and an orbicular mus- 

 cle for closing them. The eyelids are again 

 opened by five radiating muscles. 



* Rapp, Die Cetaceen Zoologisch-anatomisch 

 dargestellt, Stuttgardt and Tubingen, 1837, quoted 

 from Muller's Jahresbericht. p. cxx. in Archiv, 

 1838. 



Among the invertebrata some of the cepha- 

 lopoda have large palpebral folds. In the 

 octopus " the eyes are small in proportion, and 

 the skin is drawn over them so as to cover them 

 entirely at the will of the animal."* There are 

 no eyelids in the sepia officinalis, but a con- 

 tinuation of the integuments passes over the 

 eye. 



Eyebrows and eyelashes occur only in a few 

 mammalia. Eyelashes exist in the pachyder- 

 mata, ruminants, &c. but are wanting especi- 

 ally in small mammals. Meibomian glands 

 are commonly found. 



In birds the eyelids sometimes present cilia. 

 This is the case only in some birds of prey, in 

 some parrots, in the ostrich, &c. but seldom in 

 other orders. Very small Meibomian follicles 

 are said to exist in the eyelids of birds. In the 

 eyelids of a common fowl at present before me, 

 there are small transverse fissures on the mar- 

 gin of the lids filled with a sebaceous matter. 

 They have the appearance of very small Mei- 

 bomian glands, not closed as in the mammifera, 

 but open along their whole length. 



In a preceding part of this article I remarked 

 on some points of resemblance between the 

 iris and eyelids, as regards functions, and sym- 

 pathy in the performance of those functions. 

 In some of the lower animals there are certain 

 points in which they even approximate in form. 

 In connexion, therefore, with the subject of the 

 eyelids it will not be out of place to allude to 

 that flocculent growth of the uvea hanging 

 from the upper margin of the iris over the trans- 

 versely elongated pupil in the horse, &c. and 

 which appears to serve the purpose, if it may 

 be so expressed, of an internal eyelid. An 

 analogous but more curiously and highly deve- 

 loped structure a blending as it were of the 

 iris with some remains of the structure of the 

 eyelids exists in the eyes of several fishes ; 

 among others in the skate. 



The structure alluded to is a digitated exten- 

 sion of the whole substance of the upper part of 

 the iris, hanging over the pupil, which it is 

 large enough entirely to cover. There being no 

 intrinsic power of motion in the iris of fishes, 

 the mechanism by which this digitated veil is 

 drawn up from over the pupil is this : Where 

 the upper part of the ciliary margin of the iris 

 is connected with the sclerotica, the latter is 

 very flexible and is externally intimately con- 

 nected with a fold or rudimentary eyelid which 

 the integument forms before passing as con- 

 junctiva over the eye. Muscular fibres are 

 inserted into this fold at the point of connexion, 

 and draw it upwards ; the flexible part of the 

 sclerotica of course follows this movement and 

 the upper part of the iris, the sclerotica, so that 

 the digitated veil is drawn up from over the 

 pupil. In a young skate which I removed 

 from the egg and preserved alive for some 

 weeks, I observed that the digitated veil was 

 kept down during the day, but was drawn up 

 toward evening, and the large black pupil 

 exposed. 



* Cuvier, Regne animal, vol. iii. p. 12, Paris, 

 1830. 



