passing upwards from the crus cerebri, forming a white lamina termed the internal capsule. Above the level of these nuclei the internal capsule expands into the medullary centre of the hemisphere (see also figs. 244 and 245, with their description).

FROZEN.

c.c. corpus callosum; below its middle part the septum lucidum, and below that again the fornix; LV, lateral ventricle; th, thalamus; between the two thalami the third ventricle is seen; below the thalamus is the subthalamic prolongation of the tegmental region, and below this the prolongation of the crusta; str, lenticular nucleus of the corpus striatum; c, caudate nucleus of the same; between c, th, and str, the internal capsule is seen; outside str is the thin grey band of the claustrum, and outside this again the island of Reil at the bottom of the Sylvian fissure; n.a. nucleus amygdalæ; immediately within this is the optic tract seen in section; p, pituitary body; B, body of the sphenoid bone; sa, subarachnoid space; v, villi of the arachnoid.

The nucleus caudatus of the corpus striatum is composed of a reddish-grey neuroglia containing both moderately large and small multipolar nerve-cells. It receives fibres from the part of the internal capsule which separates it from the nucleus lenticularis, and next the lateral ventricle it is covered by a thin layer of neuroglia, and over this by the epithelium of the cavity.

The nucleus lenticularis, which corresponds in position internally with the island of Reil externally, is divided by two white laminæ into three zones. It is separated from the nucleus caudatus and optic thalamus by the internal capsule (figs. 244, 245, ic), which consists of the bundles of medullary fibres which are passing between the white centre of the hemisphere and the crus cerebri; it receives on its inner side many white fibres from the capsule, and these impart to it a radially striated aspect. Many of the nerve-cells of the nucleus lenticularis contain yellow pigment.

FIG. 244.-SECTION ACROSS THE OPTIC THALAMUS AND CORPUS STRIATUM IN THE REGION OF THE MIDDLE COMMISSURE. (Natural size.)

th, thalamus; a, e, i, its anterior, external, and internal nuclei respectively; w, its external white layer; m. c. middle commissure; v. 3, third ventricle; a small part is also seen below the middle commissure; c. c. corpus callosum; f, fornix, separated from the third ventricle and thalamus by the velum interpositum. In the middle of this are seen the two veins of Galen and the choroid plexuses of the third ventricle; and at its edges the choroid plexuses of the lateral ventricles, v. l. ; t. s. tænia semicircularis; cr, forward prolongation of the crusta passing laterally into the internal capsule, i. c.; s. t. r. subthalamic prolongation of the tegmentum, consisting of (1) the dorsal layer, (2) the zona incerta, and (3) the corpus subthalamicum; s. n. substantia nigra; n. c. nucleus caudatus of the corpus striatum; n. l. nucleus lenticularis; e. c. external capsule; cl. claustrum; I, island of Reil; h, hippocampus; d, fascia dentata.

FIG. 245.-HORIZONTAL
SECTION THROUGH THE
MIDDLE OF THE RIGHT
CEREBRAL HEMISPHERE.

(Natural size.)

v.l. lateral ventricle, anterior
cornu; c. c. corpus callo-
sum; s. 7. septum lucidum;
a. f. anterior pillars of the
fornix ; v. 3, third ven-
tricle; th, thalamus opti-
cus; s.t. stria terminalis;
n. c. nucleus caudatus, and
n. l. nucleus lenticularis of
the corpus striatum; i. c.
internal capsule;
g, its
angle or genu; n. c. tail of
the nucleus caudatus ap-
pearing in the descending
cornu of the lateral ven-
tricle; cl, claustrum; I,
island of Reil.

The optic thalamus, which lies at the side of the third ventricle and forms part of the floor of the lateral ventricle, is covered externally by a layer of white fibres, most marked next to the internal capsule, fibres from which pass into the thalamus and serve to connect it with the hemisphere.

The grey matter of the thalamus (fig. 244) is partially subdivided by an oblique white lamina into a smaller, inner (i), and a larger, outer, nucleus (e); these contain a number of small scattered nervecells. Anteriorly another portion of grey matter (a) is divided off in a similar way; this contains comparatively large nerve-cells.

Attached to the optic thalamus below and externally are the two geniculate bodies which are connected with the optic tract. The outer geniculate body has a lamellated structure consisting of alternating layers of grey and white matter.

The tegmentum of the crus cerebri is prolonged below the thalamus opticus into a mass of grey substance, with longitudinally and obliquely crossing white bundles, which is known under the name of subthalamic region. In it at least three parts differing from one another in structure may be distinguished (see fig. 244, 1, 2, 3).

The pineal gland, which is developed in the roof of the third ventricle, is composed of a number of tubes and saccules lined and sometimes almost filled with epithelium, and containing deposits of earthy salts (brain sand). These may, however, occur in other parts

FIG. 246.-SECTION OF THE UPPER PART OF THE BRAIN AND MENINGES TO SHOW THE RELATIONS OF THE ARACHNOIDAL VILLI. (Magnified.)

c.c. corpus callosum; f, falx cerebri; s.a. subarachnoid space, pervaded by a network of fine trabecula; from it the fungiform villi are seen projecting into the dura mater. Some are projecting into the superior longitudinal sinus, s.

of the brain as well. The follicles are separated from one another by vascular connective tissue derived from the pia mater.

The pituitary body is a small reddish mass which lies in the sella

P

turcica, and is connected with the third ventricle by the infundibulum. It consists of two lobes, a larger anterior, and a smaller posterior. The anterior lobe is originally developed as a hollow protrusion of the buccal epithelium. It consists of a number of tubules, which are lined by epithelium, and united by connective tissue. In some of the tubes the epithelium is ciliated, and sometimes a colloid substance like that occurring in the vesicles of the thyroid has been found in them.

The posterior lobe of the pituitary body, although developed from the floor of the third ventricle, contains scarcely any perceptible nervous elements in the adult. It consists chiefly of vascular connective tissue.

The membranes of the brain are similar in general structure to those of the spinal cord, p. 185. The dura mater is, however, more closely adherent to the under surface of the bony cavity than is the case in the vertebral canal. The arachnoid is in many places close to the dura mater, and separated by a wide subarachnoid space, which is bridged across by finely reticulating bands of areolar tissue (subarachnoid trabeculæ, fig. 246, s.a.) from the pia mater. In the vicinity of the longitudinal sinus, small rounded elevations (arachnoidal villi, Pacchionian glands) project into the dura mater, and even become embedded in the skull itself.

LESSON XXXIX. AND XL.

STRUCTURE OF THE EYELIDS AND OF THE PARTS OF

THE EYEBALL.

LESSON XXXIX.

1. SECTIONS of the eyelid vertical to its surfaces and transverse to its long axis. The lid should be hardened in alcohol, and the sections may be stained with hematoxylin and mounted in the usual manner.

Notice the long sacculated Meibomian glands lying in dense connective tissue close to the conjunctival surface, and their ducts opening at the margin of the lid. External to these the small fibres of the orbicularis palpebrarum cut across; a few of the fibres of the muscle lie on the conjunctival side of the duct. A short distance from the Meibomian gland may be observed another tolerably large sebaceous gland; outside this again are the eyelashes. In the skin covering the outer surface of the eyelid a few small hairs may be seen. At the attached part of the eyelid are some bundles of involuntary muscular fibres cut longitudinally in the section, and in the upper eyelid the fibrous attachment of the elevator muscle may be observed attached to the dense connective tissue.

Make a general sketch under a low power.

2. Sections through the posterior part of an eyeball that has been hardened in Müller's fluid. The sections are stained and mounted in the usual way. These sections will show the relative thickness of the several coats and the layers of which each coat is formed. Sections which pass through the point of entrance of the optic nerve will also exhibit the manner in which the nerve-fibres pierce the several coats to reach the inner surface of the retina. The modifications which are found in the neighbourhood of the yellow spot may also be made out if the sections have been taken from the human eye.

3. Sections of the anterior half of an eyeball which has been hardened in Müller's fluid. These sections should pass through the middle of the cornea. The lens may be left in situ, but this renders the preparation of the sections and the mounting of them much more difficult.

In these sections make a general sketch under a low power, showing the relations of the several parts one with another; and study carefully, and sketch in detail, the layers of the cornea, the place of junction of the cornea and sclerotic, the ciliary muscle, the muscular tissue of the iris, the mode of suspension of the lens, and the pars ciliaris retinæ.

4. Mount in glycerine thin tangential sections of a cornea stained with chloride of gold by Cohnheim's method. Sketch three or four of the connective-tissue cells (corneal corpuscles). The arrangement and distribution of the nerve-fibres and their termination amongst the epithelium-cells as shown in chloride of gold preparations have been already studied (Lesson XIX.)