elements, but merely an interlacement of ramified fibrils (fig. 301). In birds, reptiles, and amphibia, a small oil-globule, often brightly coloured red, yellow, or green, is found in the inner segment of each cone, and other variations of structure are met with in animals. The cones are most numerous at the back of the retina; they are fewer in number, and the rods are proportionally more numerous, towards the anterior part. The pigmentary layer forms the most external part of the retina. It consists of hexagonal epithelium-cells (fig. 303), which are smooth externally where they rest against the choroid, but are prolonged internally into fine filaments which extend between the rods. The pigmentgranules, many of which are in the form of minute crystals, lie in the

FIG. 305.-VERTICAL SECTION THROUGH THE MACULA LUTEA AND FOVEA CENTRALIS; DIAGRAMMATIC. (After M. Schultze.)

1, nerve-layer; 2, ganglionic layer; 3, inner molecular; 4, inner nuclear; and 5, outer molecular layers; 6, outer nuclear layer, the inner part with only cone-fibres forming the so-called external fibrous layer; 7, cones and rods.

inner part of the cell, and after prolonged exposure to the light they are found extending along the cell-processes between the rods (Kühne), their function being probably connected with the restoration of the purple colouring matter which has been bleached by the light. This extension of the pigment is accompanied by a shortening of the cones (Engelmann).

Fibres of Müller.-The fibres of Müller (fig. 297, fig. 301, J, and fig. 304) are long stiff cells which pass through several of the retinal layers. Commencing at the inner surface of the retina by expanded bases which unite with one another to form the so-called internal limiting membrane (fig. 297), they pass through all the layers in succession, until they reach the outer nuclear layer. Here they branch and expand into a sort of honeycomb tissue which serves to support the

fibres and nuclei of the rod and cone-elements. At the bases of the rods and cones, this sustentacular tissue ceases, being here bounded by a distinct margin which has been called the external limiting membrane (fig. 304, m.e.l.), but delicate sheaths pass from it around the bases of the rods and cones. Each Müllerian fibre, as it passes through the inner nuclear layer, has a nucleated enlargement (n), indicating the cell-nature of the fibre.

There are two parts of the retina which call for special description. The macula lutea (yellow spot, fig. 305), with its central fovea, lies in the visual axis, and is the part of the retina which is most immediately concerned in direct vision. It is characterised firstly by its

greater thickness (except at the middle of the fovea), secondly by the large number of ganglion-cells, which are rounded or conical, not oval and oblique as represented in the diagram, and thirdly by the large number of cones it contains as compared with the rods. In the central fovea itself there are no rods, and the cones are very long and slender; all the other layers become gradually thinned down almost to complete disappearance, so that the middle of the central fovea is the thinnest part of the retina. Since there are few rods, the outer nuclear layer (6) loses in great measure its appearance of being composed of closely packed nuclei, and the cone-fibres are very distinct. The direction of

these fibres is very oblique in this part of the retina.

The pars ciliaris retinæ, which commences at the ora serrata, where the retina proper abruptly ends, is composed of two epithelial layers (fig. 306), and has no nervous structures. Of the two layers, the external is a thick stratum of pigmented epithelium formed of rounded cells and continuous with the pigmentary layer of the retina on the one hand, and with the uvea of the iris on the other; the inner is a layer of columnar cells, each containing an oval nucleus.

The retina contains but few blood-vessels. The artery enters and the vein leaves it in the middle of the optic nerve. The larger vessels ramify in the nerve-fibre layer, and there are capillary networks ir this layer and in the inner nuclear layer. There are perivascular lymphatic spaces around the veins and capillaries. The sensory epithélium receives no blood-vessels, but is nourished from the vessels of the choroid.

S

FIG. 308.-FIBRES OF THE CRYSTALLINE LENS. (350 diameters.) A, longitudinal view of the fibres of the lens from the ox, showing the serrated edges. B, transverse section of the fibres of the lens from the human eye. C, longitudinal view of a few of the fibres from the equatorial region of the human lens. Most of the fibres in C are seen edgeways, and, towards 1, present the swellings and nuclei of the 'nuclear zone'; at 2, the flattened sides of two fibres are seen. (A and B from Kölliker; C from Henle.)

Structure of the lens.-The lens is a laminated fibrous body inclosed by a transparent elastic capsule to which, around the circumference, the fibres of the suspensory ligament are attached. Immediately within the capsule, in front and at the sides, there is a layer of cubical epithelium termed the epithelium of the capsule, but at the margin of the lens the cells become longer and pass by a gradual transition into the lens-fibres (fig. 307). The fibres which compose the lens are long and riband-shaped, with finely serrated edges (fig. 308, A); in transverse section they appear prismatic (B). Many of the superficial fibres are nucleated (c), the lens-fibres having originally been developed by the elongation of epithelium-cells.

The vitreous humour is composed of soft gelatinous tissue, apparently structureless when examined in the fresh condition, but containing fibres and a few scattered amoeboid cells, the processes of which are often long and varicose, and the cell-bodies distended by large vacuoles. The hyaloid membrane, which invests the vitreous humour, is homogeneous and structureless except in the region of the ciliary processes, where it is fibrous in structure, forming the zonule of Zinn and spreading out into the suspensory ligament of the lens. This part of the hyaloid membrane is connected with a circular fibrous portion of the vitreous humour which serves to give additional firmness to the attachment of the fibres of the suspensory ligament of the lens (A. Stuart).

LESSON XLIV.

STRUCTURE OF THE OLFACTORY MUCOUS MEMBRANE AND OF THE EXTERNAL AND MIDDLE EAR.

1. VERTICAL sections of the olfactory mucous membrane. The sections may be carried either across the upper turbinate bone, after decalcification in 0.2 per cent. chromic acid, or across the upper part of the nasal septum. Make a sketch under the low power. Notice the difference in the character of the epithelium in the olfactory and respiratory parts of the membrane.

2. Teased preparation of the epithelium of the olfactory mucous membrane. A piece of the membrane is placed quite fresh in osmic acid (1 per cent.) for a few hours, and is then macerated for two days or more in water. The epithelium is broken up in dilute glycerine; the cells easily separate from one another on tapping the cover-glass. Notice the two kinds of cells. Sketch some of the cells under a high power.1

3. Sections of the external ear (these have been already studied for the cartilage, Lesson XII.).

4. Sections across the cartilaginous part of the Eustachian tube. Sketch under the low power.

5. Preparation of the membrana tympani. A piece of the membrane, stained with hematoxylin, and mounted flat in Canada balsam.

Determine the composition of the membrane-i.e. the several layers composing it by focussing carefully with the high power.

STRUCTURE OF THE OLFACTORY MUCOUS MEMBRANE.

The olfactory region of the nasal fossæ includes the upper and middle turbinate processes and the upper third of the septum. It is covered by a soft vascular mucous membrane of a yellow colour in man.

The epithelium of the olfactory mucous membrane (figs. 309, 310) is very thick and is composed of long tapering cells, set closely side by side and bounded superficially by a cuticular lamina, through which the free ends of the cells project. The cells are of two kinds: 1. Long narrow spindle-shaped or bipolar cells consisting of a larger part or body (b), containing the nucleus, and of two processes or poles, one (c) straight and cylindrical and extending to the free surface, the other (d)

1 For investigating the connection of the olfactory cells with the olfactory nerve-fibres, the method of Golgi must be employed.