Optic thalanzi.—In the internal concave sur face of each corpus striatum, the optic thala mus is placed. The latter body is therefore posterior and internal to the former. 'The lighter colour of the optic thalamus distin guishes it at once from the corpus striatum. The optic thalami come into close relation to each other by their inner surfaces, which form the lateral boundaries of the third ventricle.
Each optic thalamus, like the corpus stria tum, presents a free and an attached portion. The former projects into the ventricle--the intra-ventricular portion ; the latter adheres to the inner side of the corpus striatum and to the mass of the hemisphere, and posteriorly, to the olivary columns, the quadrigeminal tuber cles, and the processus cerebelli. The supe rior surface is free and forms part of the floor of the lateral ventricle ; the internal surface is likewise free and forms the lateral wall of the third ventricle, being, however, interrupted in a very small space by the adhesion of the soft commissure. A portion of its external and posterior surface is also free, and projects back svards and outwards into the inferior horn of the lateml ventricle, presenting a pointed ex tremity in that situation. These free surfaces are smooth and moist, being covered by the membrane of the ventricles. The velum in terpositum, which again is overlapped by the fornix, rests upon the superior surface of the optic thalamus.
The optic thalami are placed obliquely, so that they are nearer each other at their anterior than at their posterior extremities. Each mea sures about an inch and a half in length, nine to ten lines in height, and about eight lines in breadth. In colour they are very much lighter than the striated bodies, and they appear to be covered with a delicate layer of fibrous matter. A band of fibrous matter passes along the inner surface of each from behind forwards, which posteriorly is connected to the pineal gland, and forms, with its fellow, the peduncles of that body.
Beneath the posterior free extremity of the thalamus, situated in the angle between that body and the superior surface of the crus, we find a small rounded eminence of a darkish grey colour perforated by very numerous fora mina for the transmission of bloodvessels. This is the corpus geniculatum internum. Lower down and more external and anterior, there is another similar body, somewhat smaller and less dark, the corpus geniculatum ex ternum. Both of these bodies are connected with the quadrigeminal tubercles. A band of fibrous
matter passes from the testes to the external geniculate body, and from the nates to the in ternal one.
In point of structure the thalamus resembles a ganglion much more closely than does the corpus striatum. A light reddish grey is the colour of the surface when cut into ; it has been not inappropriately compared to that of coffee mixed with a good deal of milk (cafe au lait). When thin sections are examined, they are found to consist of very numerous fibres interlacing freely, svith nerve vesicles occupying their in tervals. The fibres are not collected into bun dles as in the corpus striatum, nor do they take a radiating course in the thalamus. The reti culation which they form is very like that in the ganglia on the posterior spinal roots.
The fibres of the optic thalami, inasmuch as they are very numerous, have extensive. con nections. Along its ventricular surface they are evidently continuous with those of the he misphere, which appears to radiate from it to the grey matter of the convolutions. Posteriorly the fibres of the processus cerebelli ad testes and those of the olivary columns pass into it. The anterior pillars of the fornix are connected with it in front, and derive from it some nervous fibres ; and below and within, a cylinder of fibres emerge from it to the mamillary bodies. Thus the optic thalami are connected with the hemispheres on one hand, with the olivary co lumns and with the cerebellum on the other hand. The quadrigeminal tubercles placed upon the processus cerebelli may have some connection with them through the latter bundles of fibres. Although these bodies have been viewed as having a special connection with the optic nerves, it does not appear that those nerves have any relation to them but through the geniculate bodies or the quadrigeminal tubercles. It is important to bear in mind respecting the optic thalami that they are di rectly continuous with the superior portion of the crus cerebri, so that in viewing a vertical j section of the encephalon we see no line of demarcation between. The thalamus grows , as it were, from the superior extremity of th crus ; it is recognised from the latter by i swelling into an ovoidal mas§. It is emplia tically, as Willis long ago expressed it, a epiphysis upon the crus cerebri ; and in this sense it may be classed with the striated bodies and the quadrigeminal tubercles, which, with the thalami, form a series of gangliform masses, disposed in pairs, one beyond the other.