middle spongy bone is the superior meatus; the space between the middle and inferior, the middle meatus; and that between the inferior and the floor of the fossa, the inferior meatus.

[FIG. 290.

FIBRO-CARTILAGINOUS SEPTUM OF THE NASAL FOSSE, seen from the left side; after Arnold.-a. Nasal bone. b. Superior maxillary bone. c. Sphenoidal sinus. d. Central or perpendicular plate of the ethmoid bone. e. Vomer. 2. Inner part of the right alar fibro-cartilage of the nose. 4. Fibrocartilage of the septum.]

THE OUTER WALL OF THE LEFT NASAL FOSSA Covered with the pituitary membrane. 1. Frontal bone. 2. Nasal bone. 3. Superior maxillary. 4. Sphenoid. 5. The superior turbinated or spongy bone. 6. Middle turbinated bone. 7. Inferior turbinated bone. The three meatuses of the nose are seen below the three last-named bones. 8. The opening of the Eustachian tube.]

These meatuses are passages which extend from before backwards, and it is in circulating through and amongst them that the atmosphere deposits its odorant particles upon the mucous membrane. There are several openings into the nasal fossa: thus, in the superior meatus are the openings of the sphenoidal and posterior ethmoidal cells; in the middle, the anterior ethmoidal cells, frontal sinuses, and antrum maxillare; and, in the inferior meatus, the termination of the nasal duct. In the dried bones there are two additional openings, the spheno-palatine and the anterior palatine foramen; the former being situated in the superior, the latter in the inferior meatus.

The Mucous membrane of the nasal fossa is called pituitary, or Schneiderian.' The former name being derived from the nature of its secretion [pituita, phlegm, mucus,] the latter from Schneider, who was the first to show that the secretion of the nose proceeded from the mucous membrane, and not from the brain, as was previously imagined. It is closely adherent to the periosteum, constituting what is called a fibro-mucous membrane, and is continuous with the general gastro-pulmonary mucous membrane. From the nasal fossæ it may be traced through the openings in the meatuses, into the sphenoidal and ethmoidal cells; into the frontal sinuses; into the antrum maxillare; through the nasal duct to the surface of the eye, where it is continuous with the conjunctiva; along the Eustachian tubes into the tympanum and mastoid cells, to which it forms the lining membrane; and through the posterior nares into the pharynx and mouth, and thence through the lungs and alimentary canal.

1 Conrad Victor Schneider, Professor of Medicine at Wittenberg. His work, entitled "De Catarrhis," &c. was published in 1661.

The surface of the membrane is furnished with a laminated epithelium near the apertures of the nares, and in the rest of its extent with a columnar epithelium supporting vibratile cilia.

Vessels and Nerves. - The Arteries of the nasal fossæ are the anterior and posterior ethmoidal, from the ophthalmic; and spheno-palatine and pterygopalatine from the internal maxillary.

The Nerves are, olfactory, spheno-palatine and naso-palatine from Meckel's ganglion, and nasal branch of the ophthalmic. The ultimate filaments of the olfactory nerve terminate in papillæ.

ORGAN OF VISION.

THE form of the eyeball is that of a sphere, of about one inch in diameter, having the segment of a smaller sphere engrafted on its anterior surface, which increases its antero-posterior diameter. The axes of the two eyeballs are parallel with each other, but not so with the axes of the orbits, which are directed outwards. The optic nerves follow the direction of the orbits, and therefore enter the eyeballs at their nasal side.

GLOBE OF THE EYE.

The globe of the eye is composed of tunics, and of refracting media called humors. The tunics are three in number,

1. Sclerotic and Cornea,

2. Choroid, [Ciliary ligament,] Iris, and Ciliary processes,
3. Retina and Zonula ciliaris.

The humors are also three.

Aqueous,

Crystalline (lens),

Vitreous.

FIRST TUNIC.-The Sclerotic and Cornea form the external tunic of the eyeball, and give it its peculiar form. Four-fifths of the globe are invested by the sclerotic, the remaining fifth by the cornea.

Its

The Sclerotic (oxλnpòs, hard) is a dense fibrous membrane, thicker behind than in front. It is continuous, posteriorly, with the sheath of the optic nerve derived from the dura mater, and is pierced by that nerve as well as by the ciliary nerves and arteries. Anteriorly it presents a bevelled edge, which receives the cornea as a watch-glass is received by the groove of its case. anterior surface is covered by a thin tendinous layer, the tunica albuginea, derived from the expansion of the tendons of the four recti muscles; and by its posterior surface it gives attachment to the two oblique muscles. The tunica albuginea is covered, for part of its extent, by the mucous membrane of the front of the eye, the conjunctiva; and, by reason of the brilliancy of its whiteness, gives occasion to the common expression, "the white of the eye."

At the entrance of the optic nerve, the sclerotic forms a thin cribriform lamella (lamina cribrosa), which is pierced by a number of minute openings for the passage of the nervous filaments. One of these openings, larger than the rest, and situated in the centre of the lamella, is the porus opticus, through which the arteria centralis retinæ enters the eyeball.

The Cornea (corneus, horny) is the transparent prominent layer which constitutes the anterior fifth of the globe of the eye. In its form it is circular, concavo-convex, and resembles a watch-glass. It is received by its edge, which

is sharp and thin, within the bevelled border of the sclerotic, to which it is very firmly attached, and it is somewhat thicker than the anterior portion of that

tunic. When examined from the

exterior, its vertical diameter is seen to be about one-sixteenth shorter than the transverse, in consequence of the overlapping above and below of the margin of the sclerotica; on the interior, however, its outline is perfectly circular.

FIG. 291.

LONGITUDINAL SECTION OF THE GLOBE OF THE EYE. 1. Sclerotic, thicker behind than in front. 2. Cornea,

The cornea is composed of four layers namely, the conjunctiva; the cornea propria, which consists of several thin lamellæ connected together by an extremely fine areolar tissue; the cornea elastica, a "fine, elastic, and exquisitely transparent membrane, exactly applied to the inner surface of the cornea proper;" and the lining membrane of the anterior chamber of the eyeball. The cornea elastica is remarkable for its perfect transparency, even when submitted for many days to the action of water or alcohol; while the cornea propria is rendered opaque by the same immersion. To expose this membrane, Dr. Jacob suggests that the eye should be placed in water for six or eight days, and then, that all the opaque cornea should be removed layer by layer. Another character of the cornea elastica is its great elasticity, which causes it to roll up when divided or torn, in the same manner as the capsule of the lens. The use of this layer, according to Dr. Jacob, is to "preserve the requisite permanent correct curvature of the flaccid cornea proper."

received within the anterior margin of the sclerotic,
and connected with it by means of a bevelled edge.
3. Choroid, connected anteriorly with (4) ciliary liga-
ment, and (5) ciliary processes. 6. Iris. 7. Pupil.
8. Third layer of the eye, the retina, terminating an-
teriorly by an abrupt border at the commencement of
circles the lens (12); the thin layer in front of this
canal is the zonula ciliaris, a prolongation of the vas-
cular layer of the retina to the lens. 10. Anterior
chamber of the eye, containing the aqueous humor;
the lining membrane by which the humor is secreted
is represented in the diagram. 11. Posterior chamber.
12. The lens, more convex behind than before, and
inclosed in its proper capsule.
inclosed in the hyaloid membrane, and in cells formed
of the hyaloid membrane, for the passage of the
artery of the capsule of the lens. 15. Neurilemma
of the optic nerve. 16. Arteria centralis retinæ, im-
bedded in the centre of the optic nerve.

the ciliary processes. 9. Canal of Petit, which en

13. Vitreous humor

The opacity of the cornea, produced by pressure on the globe, results from the infiltration of fluid into the areolar tissue connecting its layers. This appearance cannot be produced in a sound living eye, although a small quantity of serous fluid (liquor cornea) is said to occupy the spaces of the areolar tissue.

Dissection. The sclerotic and cornea are now to be dissected away from the second tunic; this, with care, may be easily performed, the only firm connexions subsisting between them being at the circumference of the iris, the entrance of the optic nerve, and the perforation of the ciliary nerves and arteries. Pinch up a fold of the sclerotic near its anterior circumference, make a small opening through it, then raise the edge of the tunic, and with a pair of fine blunt-pointed scissors, divide the entire circumference of the sclerotic, and cut it away bit by bit. Then separate it from its attachment around the circumference of the iris

by a gentle pressure with the edge of the knife. The dissection of the eye must be conducted under water.

In the course of this dissection, the ciliary nerves and long ciliary arteries will be seen passing forwards between the sclerotic and choroid, to be distributed to the iris.

SECOND TUNIC. -The second tunic of the eyeball is formed by the choroid, ciliary ligament, and iris, the ciliary processes being an appendage developed from its inner surface.

The Choroid' is a vascular membrane of a rich chocolate-brown color upon its external surface, and of a deep black color within. It is connected to the sclerotic externally, by an extremely fine areolar tissue (membrana fusca), and by nerves and vessels. Internally it is in simple contact with the third tunic of the eye, the retina. It is pierced posteriorly for the passage of the optic nerve, and is connected anteriorly with the iris, ciliary processes, and the line of junction of the cornea and sclerotic, by a dense white structure, the ciliary ligament, which surrounds the circumference of the iris like a ring.

CILIARY NERVES.-a. Sclerotica. b. Venæ vorticosæ, and outer surface of choroid. c, c. Two of the chief trunks of the venæ vorticosæ as they leave the choroid. d, d. Ciliary nerves. e. Annulus albidus, or ciliary ligament. f. Iris. g. Pupil.]

DISSECTION OF THE EYEBALL, SHOWING ITS SECOND TUNIC, AND THE MODE OF DISTRIBUTION OF THE VENE VORTI

COSE OF THE CHOROID; after Arnold. 1. Part of the sclerotic coat. 2. Optic nerve. 3, 3. Choroid coat. 4. Ciliary ligament. 5. Iris. 6, 6. Venæ vorticos. 7, 7. Trunks of the venæ vorticosæ at the point where they have pierced the sclerotica. 8, 8. Posterior ciliary veins, which enters the eyeball in company with the posterior ciliary arteries, by piercing the sclerotic at 9. 10. One of the long ciliary nerves, accompanied by a long ciliary vein.

The choroid membrane is composed of three layers :-an external or venous layer, which consists principally of veins arranged in a peculiar manner: hence they have been named venæ vorticosæ. The marking on the surface of the membrane produced by these veins resembles so many centres to which a number of curved lines converge. It is this layer which is connected with 1 The word choroid has been very much abused in anatomical language; it was originally applied to the membrane of the foetus called chorion, from the Greek word Xópio domicilium, that membrane being, as it were, the abode or receptacle of the foetus. Xópiov comes from xwpów, to take or receive. Now, the chorion in the ovum is a vascular membrane of peculiar structure. Hence the term choroid, xóptov aidos like the chorion, has been used to signify vascular structures, as the choroid membrane of the eye, the choroid plexus, &c., and we find Cruveilhier, in his work on Anatomy, vol. iii. p. 463, saying in a note, Choroïde est synonyme de vasculeuse."

66

the ciliary ligament. The middle or arterial layer (tunica Ruyschiana)' is. formed principally by the ramifications of minute arteries. It is reflected inwards at its junction with the ciliary ligament, so as to form the ciliary processes. The internal layer is a delicate membrane (membrana pigmenti) composed of several laminæ of nucleated hexagonal cells, which contain the granules of pigmentum nigrum, and are arranged like the tiles of a tesselated pavement. În animals, the pigmentum nigrum of the posterior wall of the eyeball, is replaced by a layer of considerable extent, and of metallic brilliancy, called the tapetum.

The Ciliary ligament, or circle (annulus albidus), is the bond of union between the external and middle tunic of the eyeball, and serves to connect the cornea and sclerotic, at their line of junction, with the iris and external layer of the choroid. It is also the point to which the ciliary nerves and vessels proceed previously to their distribution, and it receives the anterior ciliary arteries through the anterior margin of the sclerotic. A minute vascular canal is situated within the ciliary ligament, called the ciliary canal, or canal of Fontana, from its discoverer.

The Iris (iris, a rainbow) is so named from its variety of color in different individuals; it forms a septum between the anterior and posterior chamber of the eye, and is pierced somewhat to the nasal side of its centre by a circular opening, which is called the pupil. By its periphery it is connected with the ciliary ligament, and by its inner circumference forms the margin of the pupil; its anterior surface looks towards the cornea, the posterior towards the ciliary processes and lens.

The iris is composed of two layers, an anterior or muscular layer, consisting of radiating fibres which converge from the circumference towards the centre, and have the

power of dilating the pupil; and circular fibres which surround the pupil like a sphincter, and by their action produce contraction of its area. The posterior layer is of a deep purple tint, and is thence named uvea, from its resemblance in color to a ripe grape.

FIG. 294

ANTERIOR SEGMENT OF A TRANS

VERSE SECTION OF THE GLOBE OF THE

EYE, SEEN FROM WITHIN. 1. Divided

edge of the three tunics; sclerotic, cho2. Pupil. 3. Iris, the surface presented

roid (the dark layer), and retina.

The Ciliary processes (corpus ciliare) may be seen in two ways, either by removing the iris from its attachment to the ciliary ligament, when a front view of the processes will be obtained, or by making a transverse section through the globe of the eye, when they may be examined from behind, as in Fig. 294.

They consist of a number of triangular folds, formed apparently by the plaiting of the middle and internal layer of the choroid. According to Zinn, they are about sixty in number, and may be divided into large and small, the latter being situated in the spaces between the former. Their periphery is connected with the ciliary ligament, and is continuous with the middle and internal layer of the choroid. The central border is free, and rests against the circumference

1 Ruysch was born at the Hague in 1638, and was appointed Professor of Anatomy at Amsterdam in 1-665. His whole life was employed in making injected preparations, for which he is justly celebrated, and he died at the advanced age of ninety-three years; having come to the conclusion that the body was entirely made up of vessels.

2 Felix Fontana, an anatomist of Tuscany. His "Description of a New Canal in the Eye" was published in 1778, in a Letter to the Professor of Anatomy in Upsal.