BRAIN 191 pear-shaped, tapering gradually backward in a long process which winds down into the an- terior extremity of the descending horn of the ventricle, and striated when cut in an oblique Fio. 5. Human Brain cut open horizontally, to show Its inter- nal parts, a. Corpus striatum of the right side. 6 Optio thalamus of the right side. Both these bodies are seen on the floor of the lateral ventricle, which is represented as cut open. c. Anterior pillars of the fornix, cut off at the level of the corpus striatum. d. The middle transverse commissure, passing across from side to side, between the two optic thalami. e. The pineal body. /,/. The tuber- cula quadrigemina. 4. The fourth ventricle, situated just above the medulla oblongata. 5. The fifth ventricle, situ- ated in the substance of the septum lucidum. direction upward and outward, on account of the passage of the fibres of the crura cerebri into the vesicular matter; through these bodies, by three sets of fibres, communications are es- tablished between the tuber annulare, medulla oblongata, and cerebral convolutions ; they are generally considered the more essential part of the nervous system which controls voluntary movements. The optic thalami are of a lighter color, of the same texture and appearance as the olivary columns, of which they are the con- tinuations ; a portion projects into the ventricles, and the rest adheres to the striated bodies, the hemispheres, olivary columns, and quadrigom- inal tubercles ; the fibres no doubt are contin- uous with those of the white substance of the hemispheres, and with those of the striated bodies; between them is the third ventricle, the roof of which is formed by the velum interposi- tum, a process of the pia mater. The corpora geniculata, externum and internum, are small gangliform masses, projecting from the posterior part of the optic thalami. Behind the third ven- tricle is a conical, dark gray body, enclosed by a process of the pia mater, the " pineal body ; " it rests in a groove between the nates, and is connected with the thalami by fibres, called peduncles ; it consists chiefly of large nucleated vesicles, with a few fibres, and in a cavity near the base contains a sandy substance composed of phosphate and carbonate of lime ; its use in the economy is unknown. The optic thalami 116 VOL. in. 13 have been considered but the principal sensitive centres, without which the sensorium could not perceive the physical change resulting from a sensitive impression ; but this is by no means certain. The cerebral hemispheres constitute the great mass of the brain, and their horizon- tal section presents an oval, of which the smaller extremity is directly forward ; the ex- ternal surface is smooth, being covered by the arachnoid membrane ; they are divided longi- tudinally along the middle line by the deep fis- sure which receives the falx cerebri, and at the bottom of which in the middle portion is the great commissure, the corpus eallotum; the in- ferior surface, or base of the brain, is divided into anterior, middle, and posterior lobes, cor- responding with the fossa in the cranial bones ; the anterior lobe rests chiefly on the roof of the orbits, and on its inferior surface presents the nerve of smell ; between it and the middle lobe is the " fissure of Sylvius," through which runs the middle artery of the brain; the middle lobes are gradually lost in the posterior, which are separated from the cerebellum by the ten- torium. The space between the middle lobes in the centre is occupied by the pituitary body, crossing of the optic nerves, and the mammillary bodies. The pituitary body is lodged in the sella turcica of the sphenoid bone, and is a glandi- form mass, surrounded by the coronary sinus, and connected with the brain by the infundibu- lar process; it has two lobes, and somewhat resembles the vesicular substance of the brain ; its use is unknown. Between the crura of the cerebrum the third pair of nerves emerge. The usual way of examining the hemispheres is to make a horizontal section at about one third from the summit ; this section, denominated the cen- trum ovale majus, presents a centre of white sub- stance, surrounded by a narrow border of gray, showing the zigzag outlines of the convplu- Fio. C. Cerebral Hemispheres, viewed from above.
- ions, and spotted by numerous small red points
caused by the escape of blood from the cut ends of minute vessels. In the central line is a broad >and of white substance, uniting the hemi-
