and mammals) controls the complex associations of coördinated movements necessary for the perfect performance of such acts as standing and walking.

The enormous number of muscles simultaneously used in some of our commonest daily actions, concerning which we have but little thought, and take no voluntary trouble, shows the great importance of this part of the brain. If we take a simple example, that of standing in the upright position (equilibration) (see page 481), we find that a great number of muscles have to act together with the most exact nicety to accomplish what, even in man, is an unconscious, if not quite involuntary, action. In the frog, as has been seen, equilibration is performed by reflex action alone. In man the nervous mechanisms are probably more complicated by his erect attitude and the addition of the cerebellum, etc., but they are nevertheless comparable with those of the frog. It may, therefore, be instructive to examine the details of the mechanisms in a frog deprived of its cerebral hemispheres.

The optic lobes of the frog's brain (which correspond to the corpora quadrigemina, and also take the place of the cerebellum of the higher animals) form the great centres of equilibration, locomotion, etc. If these lobes be destroyed, the animal can no longer sit upright, jump or swim. The first point to determine is, whence do the impulses arrive which bring about these complex coordinations. The first set is that coming from the tactile sense of the skin of those parts touching the ground. A second set arrives from the acting muscles indicating to the centres the amount of work done (muscular sense). A third set comes from the eyes, by which the position of the surrounding objects is gauged. Finally comes the fourth set from the semicircular canals of the internal ear, which communicate to the equilibrating centres the position of the head.

By depriving a frog of these several portals by which incoming stimuli direct the balancing centres, it can be rendered incapable of any of the acts requiring equilibration, even when the regulating centres are intact. In our own bodies we can convince ourselves of the importance of these afferent regulating

impulses arriving from the eye, ear, skin and muscles. When the eyes are shut and heels together we cannot stand as steadily as when we keep our eyes fixed on something; even with care not to move, we sway slowly to and fro. If, having bent our head to the handle of a walking stick, the end of which is fixed on the ground, we run three or four times around this axis so as to disturb the fluid in the semicircular canals, and then attempt to walk straight, we find how helpless our volition becomes when deprived of the aid naturally coming from special mechanisms in the internal ear.

When the feet are "asleep" or benumbed, standing or walking can only be performed in a most awkward manner, showing the necessity of tactile sensation for perfect equilibration. In a disease known as locomotor ataxy the muscular sense is lost, and the power of standing or walking correspondingly impaired.

CRURA CEREBRI.

Passing above the Pons Varolii, we come to an isthmus, composed of two thick strands of nerve substance connecting the pons Varolii with the cerebral hemispheres. These are called the crura cerebri. They diverge slightly in their upward course toward the hemispheres, and lie just below the corpora quadrigemina, already referred to. Minute examination of these crura brings to light an anatomical difference which corresponds with a physiological separation between the paths taken by the sensory and motor impulses in each crus. The lower and more anterior part, which can be seen on the base of the brain, is called the base or crusta. This is made up of efferent nerve channels only. The posterior or upper part, which lies next to and is connected with the corpora quadrigemina, is called the tegmentum, and is composed of afferent fibres. Anatomically, the separation between the two is indicated by some scattered nerve cells (locus niger). The base, or crusta, which is the great bond of union between the spinal cord and the cerebral motor centres, passes into the corpus striatum; and the tegmentum, or great sensory tract, is directly connected with the optic thalamus.

BASAL GANGLIA.

653

FIG. 256.

On the floor of the lateral ventricles are the corpora striata and optic thalami, which together are spoken of as the basal ganglia. The exact relationship borne by their functions to those of the mesencephalon and cerebral cortex is not perfectly understood. The following are some of the more important points in the evidence on the subject:CORPORA STRIATA.The motor tracts, coming from below, lie in the lower part of the crus cerebri, and thence one on each side passes into the corresponding corpus striatum. Anatomically, this part may be regarded as the ganglion of the motor tract.

Destructive lesion of one corpus striatum is followed by loss of motion of the other side. This is equally true of lesions artificially produced in animals, and those resulting from disease in man. When the crura on both sides are destroyed, the

e,

it the valve of Vieussens partially divided; c,
Corpora quadrigemina; d, d, Optic thalami:
pineal body; f,f, Corpora striata; g, g, Cerebral
hemispheres in section; h, Corpus callosum; i,
Fornix;,, Lateral ventricles; 3, Third ventricle;
4, Fourth ventricle; 5, Fifth ventricle, bounded on
each side by septumi lucidum.

animal remains motionless and prostrate.

Electrical stimulation of one corpus striatum causes movement of the other side. This fact, however, does not teach us much concerning the functions of the particular cells of its gray

matter, since the stimulus cannot be kept from affecting the fibres passing through the corpus striatum and forming the direct motor tract.

In dogs, and still more in rabbits, the corpora striata seem to be able to carry out some complex motions which in man are believed to require the coöperation of the higher cerebral centres. It has been stated that a dog whose cerebral cortex is completely destroyed can perform movements that in man can only be evoked by the cortex of the hemispheres.

It would appear that the gray matter of the corpus striatum is motor, being nearly related in function to the cerebral cortex. The cells of this ganglion are probably agents working under the direction of the cortical centres, organizing and distributing certain motor impulses. In animals whose hemispheres are less complexly developed, such as the dog or rabbit, the "basal agent" seems capable of carrying on more elaborate work, independent of the guidance of the higher motor centres in the gray matter of the brain.

OPTIC THALAMI.-The evidence concerning the function of these ganglionic masses is far from being even as satisfactory or conclusive as that relating to the corpora striata.

Anatomically, their relationship is equally clear; they are the ganglia of the sensory tracts, since the tegmentum or sensory parts of the crura pass directly into them. They form the chief routes by which impulses, giving rise to different sensory impressions, arrive at the cerebral cortex. But the evidence we obtain by the physiological examination of sensory impressions is indistinct compared with the results when motor tracts are excited. In the complete absence of all motion, it is impossible to know whether an animal feels or not, as we have no other signs of the stimulus taking effect. It is difficult, as has been already seen, to stimulate any sensory tract without the impulse being reflected to its motor neighbors, so a muscular movement often results from stimulation of a group of cells purely sensory in function, and yet is not decisive evidence of conscious sensation. When we take into consideration the foregoing points, and the fact that it is difficult, if not impossible, to destroy a portion of