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Harderian gland and the retractor oculi co-exist, as usual, with the nictitating eyelid. This is largely developed, and the conjunctiva covering its free margin is stained black. Beneath the upper eyelid, in the Kangaroo, there is a cartilaginous ridge having the conjunctiva reflected over it. There are no palpebral cilia in Didelphis.

206

The Harderian gland subserves the movements of the third or nictitating lid, and with the choanoid muscle, are present in all quadrupeds up to the Quadrumana. In these, as in Man, the third lid is reduced to a small fold, fig. 206, g, at the inner canthus, within and projecting a little beyond the vascular protuberance called 'caruncula lacrymalis,' ib. f: the Harderian gland ceases to be developed the true lacrymal gland at the upper and outer part of the orbit, fig. 209, k, l, is large. In fig. 206 the orifices of the tar

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sal' or 'meibomian' glands

The eyelids of the left side opened. XCVIII".

207

are shown at a, a. In Man and Quadrumana the upper of the two horizontal lids is the largest and most movable, contrary to the case in most lower Mammals. The fibrous tissue within that fold of skin is now condensed to form a tarsal cartilage,' largest and most conspicuous in the upper lid, of which it forms the basis: its straight and thick border constitutes the ciliary margin. In the lower lid the so-called cartilage is hardly more developed than it is in both lids of quadrupeds. The meibomian follicles extend into the fibrous (lower lid) or fibrocartilaginous (upper lid) tissue. The muscle closing the lids is the orbicularis palpebrarum,' fig. 29, o. The upper lid is raised by a special muscle, levator palpebræ superioris,' which extends from the upper border of the optic foramen, to the tarsal fibro-cartilage. The lower lid on the relaxation of the orbicularis' which draws it up, falls down by its own elasticity: rarely in Mammals has it à proper

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Section of eyelids showing extent of conjunctive membrane and ducts of lacrymal gland. CX".

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depressor. The outer border of the ciliary margin of both lids is provided, in Man, with eye-lashes, fig. 207, the orifices of which, when plucked out, are shown at h, fig. 206. In this figure b is the outer canthus,' c the inner canthus,' d lacrymal papilla or 'punctum' of the upper lid; e, the same of the lower lid; f, the lacrymal caruncle; g, the semilunar fold representing the third eyelid,' and now forming the bottom of the lacus lacrymalis' within the fissure of the inner canthus; i, the eyebrow. In the section of the outer parts of the eyelids, in fig.

208

207, is shown the line of reflection of the con-
junctive membrane upon the eyeball, g, at the
upper and outer part of which line open the 9 to
12 orifices of the ducts of the lacrymal gland, into
which bristles have been inserted.

The gland, fig. 208, consists of an upper portion a, a, which is lodged in the shallow depres sion at the outer side of the roof of the orbit, and a lower thinner portion, b, b, which is a looser Lacrymal gland, left aggregate of lobules extending into the substance of the upper eyelid. The fluid contributed by the lacrymal and meibomian glands to the conjunctival cavity, after being spread by the winking movements of the lids over

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side. CX".

209

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the front of the eyeball, is carried
along the groove formed by the
margins of the closed lids to the
inner canthus, and is there im-
bibed by the puncta lacrymalia,'
fig. 209, a, a.

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From each of these orifices a canal is continued, ascending in the upper, descending in the lower lid; in both, then, bending at an acute angle and converging to a long dilated receptacle, f, g, called lacrymal sac.' The large blind end, e, is directed upward; the sac gradually contracts, h, to the nasal duct,' i, which opens into the inferior meatus, fig. 152, k, of the nose. In all Mammals with divided or horizontal eyelids there is a similar provision for carrying off the waste lubricating fluid of the eyeball. In Man, in whom the true lacrymal gland is relatively largest, its peculiar secretion - the tears when emotionally secreted in excess, overflows the palpebral groove.

Lacrymal apparatus, Human. cx".

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c. Parallel between eye and ear.-The author of the excellent articles, XCVII" and cx" has drawn a parallel between the eye and ear which, in the main, appears to me to express justly the serial homologies' of the parts of those sense-organs. I include, however, the consideration of the cavities in which they are respectively lodged. The 'otocrane' parallels the orbit.' The homology is masked by the deeper situation of the former, its communication rather with the interior than with the exterior of the cranium, and its more frequent coalescence with the fixed bony sense-capsule which it includes. In some Mammals, however, that capsule retains its primitive and typical distinctness, and can be removed from the otocrane.1 This is, then, seen to be formed by the exoccipital and alisphenoid, the mastoid, the tympanic, and, in Mammals, the expanded and intercalated squamosal. The primitive bony nuclei of the capsule which appear round the fenestra rotunda, on the outer end of the upper vertical semicircular canal, and on the middle of the hinder vertical semicircular canal, extend to form the bony labyrinth, and are wholly independent of the centres from which the ossification of the mastoid or other otocranial bones begins. The addition of bony matter envelopes in various degrees the first formed part of the capsule, called bony labyrinth,' and constitutes, therewith, the 'petrosal.' This capsule of the ear corresponds with the sclerotic in the eye; which, in many Vertebrates, becomes the seat of ossification, and in some (Cetacea, e. g., fig. 195, a) is thickened as much out of proportion to the nervous and vascular parts of the essential organ it contains, as is the petrosal. The orifice by which the optic nerve enters the eye-bulb answers to the foramen auditorium internum. The membranous labyrinth answers to the parts of the eyeball within the sclerotic. The delicate vascular external tissue of the labyrinth, frequently exhibiting pigmentspecks, answers to the choroid, the expansions of the acoustic nerves to the retina, the endolymph to the vitreous humour. The fluid in the space between the sclerotic and choroid, including the aqueous humour, represents the perilymph. Wharton Jones compares the lens' to the otolites."

If we compare the conjunctival space in front of the eyeball with the tympanic cavity, and the duct therefrom leading to the nose with the eustachian tube, then the anterior opening of the sclerotic will answer to the fenestra vestibuli, and the membrane closing it, or cornea, to that which closes the fenestra. In mammals the open movable eyelids seem very remote analogues to the 1 XLIV. p. 557. 2 xcvii". p. 562.

external membrane closing the tympanum: but they are superadded developments to the true serial homologue of the tympanic membrane, shown in Reptilia, vol. i. p. 338, 339, fig. 220; and which disappears or blends with the later added developments of integument with special cartilages, muscles, and glandules, and which truly parallel the pinna' of the ear. In the eyelids, the meibomian follicles repeat the ceruminous ones, and the eyelashes, the cilia which guard the entry to the meatus auditorius. Wharton Jones compares the muscles of the eyeball to those of the otosteals, and I concur, with him, in accepting the opinion of Weber as to the special relation of both to their respective Organs of Sense, and as to their being parts superadded to the elements of the vertebral skeleton. But I believe that the divergence of functions so governs the development of special motive organs and ossicles as to remove the ground for safely or usefully homologising such parts, and I refrain from going beyond the serial repetitions in the eye and ear which are above indicated.

265

CHAPTER XXIX.

DENTAL SYSTEM OF MAMMALIA.

§ 218. General characters of the Teeth.-The present class includes a few genera and species that are devoid of teeth; the true ant-eaters (Myrmecophaga), the scaly ant-eaters (Manis), and the spiny monotrematous ant-eater (Echidna), are examples of strictly edentulous Mammals: Ornithorhynchus has horny teeth; the whales (Balana, Balanoptera) have transitory embryonic calcified teeth, fig. 219, succeeded by whalebone substitutes, fig. 217, in the upper jaw. The female Narwhal seems to be edentulous, but has the germs of two tusks in the substance of the upper jaw-bones: one of these so remains; the other becomes developed into a large horn-like weapon in the male Narwhal, fig. 220, A, and suggested to Linnæus the name, for its genus, of Monodon but the tusk is never median, like the truly single tooth on the palate of the Myxine; and occasionally both tusks are developed. In Hyperoodon the teeth are reduced in the adult to two in number, whence the specific name, H. bidens; but they are very small and confined to the lower jaw. Ziphius has two teeth of functional size and shape, one in each ramus of the lower jaw; and this is perhaps a sexual character. The Delphinus griseus has five teeth on each side of the lower jaw : but they soon become reduced to two. The Marsupial genus Tarsipes is remarkable for the paucity as well as minuteness of its teeth. The Elephant has never more than one entire molar, or parts of two, in use on each side of the upper and lower jaws, to which are added two tusks, more or less developed, in the upper jaw. Some Rodents, Hydromys, e. g., have two grinders on each side of both jaws, which, added to the four cutting teeth in front, make twelve in all; the common number of teeth in this order is twenty; but the hares and rabbits have twenty-eight teeth. The sloth has eighteen teeth. The number of teeth, thirty-two, which characterises man, the apes of the Old World, and the true Ruminants, is the average one of the class Mammalia; but the typical number is forty-four. The examples of excessive number of teeth are presented, in the order Bruta, by the Priodont

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