This classification of the motor areas is from Dana. You I will notice that the centers for the lower extremities are located highest in the cerebral cortex. Destruction by traumatism or disease of any part of this area is followed by a paralysis of that member whose nerve supply is received from the diseased area. Similarly, if there is only an irritation, convulsions of the part follow, which, if continued, lead to a paralysis.

The motor tracts shall next receive our consideration. These fibers are really a continuation of the horns or poles of the motor cells of the cortex, and we will now trace them out. We find that all converge from the cortex to the internal capsule and pass downward to the crus. The fibers which pass onward to the cord through the internal capsule are known as the knee of the posterior segment. In the knee and through the internal capsule the fibers at all times retain their relative positions. Other motor fibers pass through the anterior limb of the internal capsule. Those from the frontal lobe pass the crus on the median side of the pyramidal fibers. Fibers from the temporo-occipital lobe are behind the pyramidal fibers. The fibers or bundles of fibers enumerated above are all efferent fibers, or rather fibers of descending degeneration.

From the foregoing you will see how the motor areas have been located on the cortex, and how the motor tracts have been traced to the cord. While it is comparatively easy to take the cerebrum and locate on its surface certain areas, it is not so easy when we have before us a wound of the cranium to locate the important fissures and decide as to where we shall trephine.

The most important fissure to be located is the fissure of Rolando, for around it most of the motor areas lie. Its upper end is located 14 to 2 inches behind the bregma, or measuring from the glabella backward, 55.7 per cent. of the anterior and posterior part of the cerebrum. Now, if a line be drawn downward from this point and forward at an angle of 67°, it will give quite accurately the upper two-thirds of the fissure.

The fissure of Sylvius begins about 1 inches behind the external angular process of the frontal bone. The anterior

branch follows the line of the squamo - sphenoidal suture. The posterior branch follows the squamo-parietal suture.

Having now finished the localization of the motor areas, I shall next direct your attention to the function of the various basal ganglia and location of the sensory area. The corpus striatum is situated in front and to the outside of the optic thalami and partly without the lateral ventricle. The idea as held formerly that the corpus striatum was the great motor ganglion of the brain is not now so considered, as experimentation has not substantiated this claim. It was thought that a lesion of this ganglion, followed by a hemiplegia, was sufficient evidence that its primary and principal function was of a motor nature. Kirk explains the hemiplegia by insisting that it is not due to the lesion of the ganglion, but to the pressure exerted by the lesion on the internal capsule. So, from the data before us, we do not know as yet the real function of the corpus striatum. Some physiologists are even inclined to the belief that it is subsidiary or rudimentary.

The optic thalami are oval in shape, and lie on the inner and dorsal surface of the crura. The ganglion is connected with the following smaller ganglia: corpora mamillaria, hippocampal convolution, island of Reil, second and third convolution, central convolution of frontal lobe, operculum, supramarginal gyrus, optic tract, and inferior parietal convolution.

From the above wide distribution, you will see that the optic thalamus is connected with a large portion of the cerebrum, with both motor and sensory fibers. The idea held by many that the optic thalamus was a great sensory center, has, to a great extent, been abandoned. Kirk says that "a lesion. of the thalamus area does not cause paralysis directly, but indirectly, by pressure on the internal capsule." It is now conceded that the optic thalami preside over automatic activities during partial consciousness, as walking, writing, speaking and emotional expressions. This theory I think very tenable, judging from its wide distribution and intimate connection with the cerebrum.

The corpora quadrigemina, four in number, lie on the dorsal surface of the pons and crura, above the aqueduct of Sylvius. These ganglia are connected by their posterior bodies

with the sensory tract and auditory nerve, also by fibers to temporal region, and are closely associated with the geniculate bodies. The anterior are connected by the optic nerve and fibers to the occipital cortex. The ganglia are considered by many physiologists as homologous with the optic lobes in birds and fishes. The anterior pair are regarded as the principal centers for visual sensation, the posterior pair possibly with auditory perception.

It has been shown by experimentation that disease or removal of the corpora quadrigemina causes total blindness, while removal of one causes hemiopia in the opposite field of vision. The muscular movements are also affected, as the body is inclined to rotate. This rotation may, however, be due to giddiness and partial blindness.

Another very marked effect of irritation and removal of the glands is its effect upon the iris, which, upon irritation, is made to contract, while removal causes dilatation. It has also been demonstrated that the corpora quadrigemina preside over the coördination of the muscular movements of the eye.

We shall next consider the geniculate bodies, two in number, outer or lateral and inner or median, formed respectively from the swelling on the lateral division of the optic tract and from the termination of the median division of the optic tract. The lateral geniculate body is associated with the anterior corpora quadrigemina in the sense of sight. The median is associated with the posterior corpora quadrigemina in the sense of hearing. This is readily shown, for when the internal ear is destroyed atrophy of these centers follows.

We shall now take up the special sensory centers. The visual or optic center can be located by following the optic nerve and tract. The optic nerve fibers converge in the nerve to the chiasma. The tracts are formed or pass backward from the chiasma, and terminate in three distinct nuclei: the pulvinar of the optic thalamus, the median corpus quadrigeminum, and the lateral corpus geniculatum. The three nuclei just mentioned receive fibers from that part of the cortical area known as the optic radiation, or that portion posterior to the parieto-occipital fissure. If a portion of the cortical

surface or the optic radiation is removed, there follows atrophy of the above nuclei if in an adult animal; if in a young animal the nuclei never develop.

From the foregoing we consider the occipital lobe and particularly the cuneus as the visual center of the cortex.

The olfactory area or center is now thought to be in the nucleus amygdale, and the hippocampal gyrus in the temporal lobe. This has been proven by lesion of this area, loss of the sense of smell following.

The auditory center is situated in the superior temporal convolution. This area is also intimately connected with the posterior corpus quadrigeminum and median corpus geniculatum, and has much the same relation as the optic radiation has to its corresponding ganglion.

Experimentation places the areas of cutaneous sensation, touch, pain, and temperature essentially identical with the cortical motor area, but owing to the wide distribution of afferent impulse the sensory areas are not so definitely located within limited areas.

From the foregoing we can definitely localize a gumma, cyst, hematoma, purulent exudate, or any pressure from within, for so soon as the pressure is sufficient to irritate we have in the case of the special senses disordered function, and in the motor area there is the convulsive movement corresponding to the distribution of the nerve filaments. If by traumatism or disease the cerebral area is destroyed, there is paralysis and loss of the special senses.

The cerebellum is intimately connected with the cerebrum and medulla, and is composed of two hemispheres and an elongated portion termed the vermiform process. With the exception of the middle lobe the cerebellum is insensitive to irritation and may be cut away without eliciting any pain, and removal leaves the animal normal as regards sight, smell, hearing and pain. Following this, however, if its crura be irritated, severe pain is felt. It cannot then be regarded as a sensory center.

If

By experimentation of Longet and others it was found that irritation of the cerebellum does not cause movement. portion after portion of this is removed there results a grad

ual feebleness and want of harmony of muscular movement, which continues until the animal is utterly helpless in performing any muscular movement.

From these experiments we must conclude that the cerebellum belongs neither to the sensory or intellectual apparatus, that it is not the source of voluntary movement, but that it is the center for the coördination of muscular movement. In order to substantiate more fully our conclusion we have only to note its nervous connections. It is connected with the posterior column of the cord and the direct cerebellar tract which convey muscular sensation, with the auditory nerve and bulb by the internal and external arcuate fibers, and with the tegmentum through the red nuclei.

The medulla is connected with the cerebrum by the pyramidal tracts, internal and external arcuate fibers, to the cerebellum by the direct cerebellar and antero-lateral tracts, fibers from the olivary bodies, and lastly, the arciform fibers. At the entrance of the cord to the cranium it becomes enlarged, posteriorly the posterolateral and median tracts diverge, forming the floor of the fourth ventricle. The functions of the bulb are as follows:

1.

Conduction. All impressions of whatever source which pass through the cord pass the medulla.

2. Reflex action. As as center of reflex action it is analogous to the cord except that the act is more complex.

If portions of the brain and cord are gradually removed from without inward the animal may retain the forces of life, but so soon as the medulla is wounded death is instantaneous. Many cases of instant death are due to injury of the medulla or of the cord above the origin of the phrenic nerve, produced by fracture or displacement of the upper cervical vertebræ. The medulla also contains some special centers which we wish to mention, namely:

1. Simple reflex centers, as mastication, deglutition, sucking, secretion of saliva, vomiting, coughing, sneezing, and dilatation of the pupils. These are simple reflex acts caused by the centers being stimulated by voluntary or efferent impulses.