Cut the corpus callosum transversely through near its posterior edge and bend the anterior portion of it forwards and sideways. The rear edge (splenium) left in situ bends round and downwards and becomes

FIG. 36.-Horizontal section of human brain just above the thalami.-Ccl, corpus callosum in section; Cs, corpus striatum; Sl, septum lucidum; Cf, columns of the fornix; Tho, optic thalami; Cn, pineal gland. (After Henle.)

continuous with the fornix. The anterior part is also continuous with the fornix, but more along the median line, where a thinnish membrane, the septum lucidum, triangular in shape, reaching from the one body to the other, practically forms a sort of partition between the

contiguous portion of the lateral ventricles on the two sides. Break through the septum if need be and expose the upper surface of the fornix, broad behind and narrow in front where its anterior pillars plunge down in front of the third ventricle (from a thickening in whose anterior walls they were originally formed), and finally penetrate the corpus albicans. Cut these pillars through and fold them back, exposing the thalamic portion of the brain, and noting the under surface of the fornix. Its diverging posterior pillars run backwards, downwards, and then forwards again, forming with their sharp edges the corpora fimbriata, which bound the cleft by which the ventricle lies open. The semi-cylindrical welts behind the corpora fimbriata and parallel thereto in the wall of the ventricle are the hippocampi. Imagine the fornix and corpus callosum shortened in the fore-and-aft direction to a transverse cord; imagine the hemispheres not having grown backwards and downwards round the thalamus; and the corpus fimbriatum on either side would then be the upper or anterior margin of a split in the wall of the hemispheric ventricle of which the lower and posterior margin would be the posterior border of the corpus striatum where it grows out of the thalamus.

The little notches just behind the anterior pillar of the fornix and between them and the thalami are the so-called foramina of Monro through which the plexus of vessels, etc., passes from the median to the lateral ventricles.

See the thick middle commissure joining the two thalami, just as the corpus callosum and fornix join the hemispheres. These are all embryological aftergrowths. Seek also the anterior commissure crossing just in front of the anterior pillars of the fornix, as well as the posterior commissure with its lateral prolongations along the thalami, just below the pineal gland.

On a median section, note the thinnish anterior wall of the third ventricle and its prolongation downwards into the infundibulum.

Turn up or cut off the rear end of one hemisphere so as to see clearly the optic tracts turning upwards towards the rear corner of the thalamus. The corpora geniculata to which they also go, distinct in man, are less so in the sheep. The lower ones are visible between the optic-tract band and the 'testes,' however.

The brain's principal parts are thus passed in review. A longitudinal section of the whole organ through the median line will be found most instructive (Fig. 37). The student should also (on a fresh brain, or one hardened in bichromate of potash or ammonia to save the contrast of color between white and gray matter) make transverse sections through the nates and crura, and through the

FIG. 37.-Median section of human brain below the hemispheres. Th, thalamus; Cg, corpora quadrigemina; VI, third ventricle; Com, middle commissure; F, columns of fornix; Inf, infundibulum; Op.n, optic nerve; Pit, pituitary body; 4v, arbor vitæ. (After Obersteiner).

hemispheres just in front of the corpus albicans. The latter section shows on each side the nucleus lenticularis of the corpus striatum, and also the inner capsule (see Fig. 38, Nl, and Ic).

FIG. 38.-Transverse section through right hemisphere (after Gegenbaur). Cc. corpus callosum; Pf, pillars of fornix; Ic, internal capsule; V, third ventricle; Nl, nucleus lenticularis.

When all is said and done, the fact remains that, for the beginner, the understanding of the brain's structure is not an easy thing. It must be gone over and forgotten and learned again many times before it is definitively assimilated by the mind. But patience and repetition, here as elsewhere, will bear their perfect fruit.

General Idea of Nervous Function.-If I begin chopping the foot of a tree, its branches are unmoved by my act, and its leaves murmur as peacefully as ever in the wind. If, on the contrary, I do violence to the foot of a fellow-man, the rest of his body instantly responds to the aggression by movements of alarm or defence. The reason of this difference is that the man has a nervous system, whilst the tree has none; and the function of the nervous system is to bring each part into harmonious coöperation with every other. The afferent nerves, when excited by some physical irritant, be this as gross in its mode of operation as a chopping axe or as subtle as the waves of light, conveys the excitement to the nervous centres. The commotion set up in the centres does not stop there, but discharges through the efferent nerves, exciting movements which vary with the animal and with the irritant applied. These acts of response have usually the common character of being of service. They ward off the noxious stimulus and support the beneficial one; whilst if, in itself indifferent, the stimulus be a sign of some distant circumstance of practical importance, the animal's acts are addressed to this circumstance so as to avoid its perils or secure its benefits, as the case may be. To take a common example, if I hear the conductor calling All aboard!' as I enter the station, my heart first stops, then palpitates, and my legs respond to the air-waves falling on my tympanum by quickening their movements. If I stumble as I run, the sensation of falling provokes a movement of the hands towards the direction of the fall, the effect of which is to shield the