Internally, the mucous membrane of the la crymal sac and nasal duct forms various small phew or rugs. Red and villous, it is quite different from the white and smooth mucous membrane of the canalicules. Like the pitui tary membrane of the nose, it secretes, in the healthy state, a clear, mild, fluid mucus.
Lacrymal or tensor tarsi muscle.—Ilere perhaps the proper place to notice a muse which was discovered many years ago by Duverney,' delineated and described by Rose miillert rn 1805, and more recently re-descri by Dr. Ilorner,t an American anatomist, wit whose name it is now commonly associated.
To get a view of this muscle, Professor Ho ner directs us to cut through the eyelids an separate them from the ball, except at the inn canthus ; then turn the lids over the nose, r move the semilunar fold and the conjunctiva i the neighbourhood, with the fatty matter, when the muscle, such as it is represented in the fol lowing description, will be seen.
"The tensor tarsi arises from the posteri superior part of the os unguis, just in advan. of the vertical suture between the os planum a the os unguis. having advanced three lines, bifurcates ; one bifurcation is inserted alon the upper lacrymal canalicule, and terminat at its punctum, or near it ; and the lower bifu cation has the same relation to the lower lacr mal canalicule. The base of the lacrym caruncle is placed in the angle of the bifurcatioi The superior and the inferior margins of t muscle touch the corresponding fibres of orbicularis palpebrarum, where the latter connected with the margin of the internal ca thus of the eye, but may be readily disti guished by their horizontal course. The face of this muscle adheres very closely to th portion of the sac which it covers, and also t the lacrymal canalicules. The lacrymal sa rises about a line above its superior margin, an extends in the orbit four lines below its inferi margin. The orbital face of the muscle covered by a lamina of cellular membrane, a between this lamina and the ball of the eye a placed the semilunar fold of the conjunctiv and a considerable quantity of adipose matt: As the bifurcated extremities of the muscle f• low the course of the canalicules, they are c. vered by the conjunctiva. The muscle is oblong body, half an inch in length, and abo one quarter wide, bifurcated at one end ; and arises much deeper from the orbit than any knowledged origin of the orbicularis. The s
perior fork, however, has a few of its fibres blended with the ciliaris." The action of the muscle appears to be to direct the lacrymal papillae and points in to wards the lacus lacrymalis, and to assist in keeping the edges of the eyelids properly ad justed to the eyeball.
Nerves.—The parts of the organ of vision which have been just described receive their nerves from the fifth and seventh pairs ; the former communicating sensibility, the latter the power to move. See articles, FIFTH PAIR OF NERVES, and SEVENTII PAIR OF NERVES.
The first division of the fifth pair gives nerves not only to the accessory parts of the eye, but supplies also the eyeball ; hence it is called ophthalmic. The second division of the fifth sends filaments to the lower eyelids.
Nerves from the first division of the fifth distributed to the accessory parts of the eye.— The first division of the fifth pair or the oph thalmic divides into three nerves, thefron- tat, the nasal, the lacrymal.
1. Frontal nerve. The supra-trochlear branch of this nerve gives filaments to the upper eyelid and inner canthus. The continu ation of the frontal nerve sends filaments to the upper eyelid and external canthus.
2. Nasal nerve. The infra-trochlear branch of this nerve supplies the parts at the inner canthus, the conjunctiva, the lacrymal caruncle and lacrymal sac ; it also gives filaments to the orbicularis palpebrarum. The tensor tarsi* receives two twigs from it. The infra-trochlear ends branches upwards, which anastomose those of the supra-trochlear.
3. Lacrymal nerve.—After supplying the lacrymal gland the branches of this nerve emerge from it, and ramify in the conjunctiva, orbicularis muscle, and skin of the eyelids. The lacrymal nerve forms anastomoses with other branches of the fifth.
Nervesfrom the second division of the fifth pair distributed to the accessory parts of the eye.—The principal of these is the inferior palpebral branch of the infra-orbital. The in ferior palpebral nerve divides into two branches, an external and an internal, which indeed may be separate from the first.