The Eye Muscles in Sauropsida

The eye, rudimentary in some Ophidia and Lacertilia, is usually large; and sometimes, as in many birds and in the extinct Ichthyosauria, attains very great absolute and relative dimensions.

In the Ophidia and some Lacertilia (the Amphisboenoidea, some Scincoidea, all Ascalobota), the integument is continued over the eye, and becomes transparent. These reptiles are commonly said to possess no eyelids; but it must be remarked that this is not true of them in the sense in which it is true of most osseous fishes, as the transparent covering of the eye really represents the two eyelids of the higher Vertebrata, and is separated from the eyeball by a chamber lined by conjunctiva, which communicates with the nose by a lachrymal canal. In the other Sauropsida two lids are developed, and each generally possesses a special palpebral muscle, which acts as an elevator of the upper, and a depressor of the lower, lid. In some Scincoidea the middle of the lower lid is transparent. In many Lacertilia it contains a cartilage or an ossification.

Most lizards, all Chelonia, Crocodilia, and Aves, possess a nictitating membrane moved by special muscles, which present three different arrangements.

In the lizards a short thick muscle (bursalis) is attached to the inner and posterior wall of the orbit, and ends in a fibrous sheath. A tendon, one end of which is attached to the presphenoidal region of the inner wall of the orbit, passes backward through the sheath, and then forward to be attached to the nictitating membrane. When the muscle contracts it necessarily pulls the latter over the eye. A Harderian gland is always developed, and a lachrymal gland very generally, though not always.

In the Chelonia, muscular fibres (forming the so-called pyramidalis muscle) arise from the inner side of the eyeball, and, arching over it and the optic nerve, are inserted partly into the outer edge of the nictitating membrane, partly into the lower eyelid. The Crocodilia have a pyramidalis muscle taking the same origin and course; but it sends no fibres to the lower eyelid, its tendon being inserted altogether into the nictitating membrane.

The third arrangement, which in a manner combines together the first and the second, is that seen in birds. A pyramidalis muscle arising from the inner and under surface of the eyeball, soon ends in a tendon which sweeps round the upper and outer surfaces of the sclerotic to the nictitating membrane, as in the crocodiles. But there is also a bursalis muscle, which however arises, not, as in lizards, from the wall of the orbit, but from the upper surface of the sclerotic itself, whence it passes backward and ends in a fibrous sheath which encloses the tendon of the pyramidalis. The contraction of this muscle necessarily tends to draw the tendon of the pyramidalis away from the optic nerve. A tubercle is sometimes developed from the sclerotic above the entrance of the optic nerve, and prevents the tendon of the pyramidalis from shifting forward and inward.

The eyeball is always moved by four recti and two obliqui. The superior oblique does not pass over a pulley. The Chelonia and most Lacertilia have a more or less completely developed retractor, or choanoid, muscle.

A ring formed of bony plates is developed in the fore-part of the sclerotic in Lacertilia, Chelonia, Ichthyosauria, Dicynodontia, Pterosauria, and Aves, but not in Ophidia, Plesiosauria, or Crocodilia.

The iris and the tensor choroidel contain striated muscular fibres.

A pecten is very generally developed. It attains a large size, and becomes much plaited, in most Aves.

Only Crocodilia and Aves possess a rudiment of an external ear.

The Ophidia and the Amphisboenoidea have no tympanic cavity. In some Chelonia, in Sphenodon, and in the Chamaeleons, the tympanic membrane is covered by the integument, but a tympanic cavity exists. In Lacertilia, the tympanic cavities communicate by wide openings with the pharynx; but in Chelonia, Crocodilia, and Aves, the communicating passages, reduced in size, become Eustachian tubes. In the Chelonia, these curve backward, downward, and inward, round the quadrate bones, and open separately on the roof of the mouth. In the Crocodilia there are, as has been described above (p. 319), three Eustachian tubes-one median and two lateral. In Aves, there is but one Eustachian aperture, answering to the median of the Crocodilia; and, as in the latter group, each Eustachian tube usually traverses the osseous base of the skull, to join with its fellow in the common aperture.

The stapes is a columelliform bone, the outer end of which is attached to the tympanic membrane, where the latter is developed; but lies among the muscles when there is no tympanic cavity.

All Sauropsida possess a fenestra rotunda, as well as a fenestra ovalis, and all have a cochlea, which is never coiled spirally, and is more rudimentary in the Chelonia than in other groups. Three semicircular canals, an anterior and posterior vertical, and an external horizontal, are connected with the membranous vestibule. In Aves, the anterior vertical canal is very large in proportion to the others, and the adjacent crura of the two vertical canals overlap before they unite with one another.

Labial and buccal glands are developed in some Sauropsida, and one of them, on each side, attains a large development in the poison-glands of the venomous snakes. Well-developed sublingual, submaxillary, and parotid glands appear in Birds, and the sublingual glands attain an immense size in the Woodpecker. The tongue varies greatly, being sometimes obsolete, as in the Crocodile and some birds (e. g., the Pelicans), sometimes horny and even spinose, sometimes fleshy. In the snakes, and some lizards, the tongue is forked, and capable of retraction into a basal sheath. In the Chamaeleons, it is clubbed at its extremity, and can be retracted or protruded by the invagination or inversion of its hollow stem.