These considerations afford the clew of the mechanism by which hallucinations are produced, whose cause is located in the encephalon and give rise to certain sensations attributed by the patient to the periphery.

This explains also the associated sensations; an external sensation arriving at a nerve-centre can there produce an excitation sufficient to radiate towards the neighboring centres; these will then give us sensations identical with those we should have experienced had the excitation been produced on those nerves that make communication between these centres and the periphery. In this way a foreign body introduced into the ear may produce as an associated sensation a feeling of tickling in the back part of the throat, and perhaps even coughing and vomiting. These associations are caused on account of the nearness of the central gray nucleus of the trifacial and of the nucleus of the glosso-pharyngeal and pneumogastric, from which excitations perceived by the former radiate towards the latter.

There are examples of associated sensations still more startling that seem due to the same mechanism: in certain persons an irritation on the foot between the third and fourth toe produces a sensation of tickling in the sub-umbilical region of the abdomen; an irritation on the skin of the scrotum will give rise to pains in the right hypochondriac region, etc.

Memory and Volition. Finally, the sensations present in addition to the preceding this peculiar fact, that they can be stored up in the cerebral organs; the impressions are fixed there to reappear at a later time; in this way are caused those phenomena designated under the name of memory. The sensations thus reserved in a latent condition reappear by a mechanism analogous to that of the associated sensations, and this revival of a sensation can bring on a number of others similar or analogous: as, one idea calling up another, and what is called association of ideas.1

1 The cell in the spinal cord is also susceptible of preserving up to a certain point the impression which has been produced by a centripetal nerve, though generally the former retains nothing after having brought its peculiar reflex action. Thus a certain habit of reflex actions is brought about, which terminates in happening more readily and regularly. In fact, the spinal cord can be educated; we need only cite the example of persons who play upon musical instruments, who finally attain the faculty of executing a musical piece or tune almost without any conscious volition, and

However, physiology goes no farther; it has but little means of knowing what is the internal and intimate nature of the mechanism of the seat of thoughts or ideas; as, for example, we know that softening of the brain, characterized sometimes by gay and sometimes by sad thoughts, has its seat in the gray cortical substance; little doubt can thus be had that the seat of thought is in a general way located in this substance; as, moreover, a large number of vivisections would seem to prove.

The central phenomenon of volition is equally beyond our study, at least when it forms no part of association of ideas. Still, we know at least that injuries in the brain destroy those manifestations called voluntary, and paralyze the voluntary movements of the opposite side; viz., movements of the right side of the body are abolished by a lesion having its seat in the left hemisphere of the brain, and vice versa. The centrif ugal conducting nerves of volition decussate on leaving the brain. However, this decussation must not be localized only at the lower extremity of the pyramids; it extends throughout a larger space from this point, to the most anterior portion of the protuberance. A lesion which may be seated in any part of this extent may then affect at the same time fibres which have already crossed (decussated) and those which have not; thus there may ensue those peculiar alternating paralyses, which for example may be located in the right side of the face and on the left side of the remainder of the body (see physiology of the spinal cord, pp. 43 and 45).

We find equally in the case of phenomena of motility as in the volitionary phenomena associations analogous to those which we have explained in regard to sensation or sensibility. Thus a centre becoming the seat of a lively action can do so to such a degree that its activity may extend even to the neighboring centres.

This is the mechanism of all the little convulsive movements and also of involuntary associated movements. This also explains why it is that during a very intense and general muscular exertion, as for instance when lifting a heavy weight, a person involuntarily contracts the frontal muscles; as, also when sneezing, the eyes are involuntarily closed, etc.

Thus we might as a general rule state that all our voluntary movements are associated movements, because we cannot

without the intervention of the brain. The cerebral memory is simply in a higher degree a sort of medullary memory.

contract one muscle apart from others, but contraction of muscles usually occurs in groups; this association is wholly performed in the spinal cord by certain groupings of globules and fibres, and the brain serves only to excite this group of globules; this association is found in those movements which are purely of a reflex character, as those of defence which are observed in experiments on decapitated animals (physiology of the spinal-cord, p. 53).

Special functions of certain cerebral centres, or what is called encephalic centres.

We shall not enter into the detail of the numerous hypotheses which, even in the experimental investigations of the modern school, have founded the physiology of the encephalic organs. The system founded upon the union of specified faculties of the mind to particular circumscribed portions of the brain is regarded as an illusion (Phrenology, system of Gall), especially when an attempt is made to define these faculties, otherwise arbitrarily classified, according to the development of certain external portions of the skull (Craniology).

However, very recently it has been believed according to certain pathological observations that the centre of faculty of language or at least the memory for words is seated in the third convolution of the left (Broca) or right (Bouillaud) hemisphere. It is evident that perception and thought form an undefined phenomenon, which depends upon a peculiar activity of the cerebral cells and of an association of these cells connected by numerous commissures; yet our knowledge is too uncertain to localize these functions.

The tubercular quadrigemina (corpora quadrigemina) are the centre of visual perceptions, and of reflex movements which cause the dilatation or contraction of the iris (Herbert, Mayo, and Flourens); yet when the cerebral hemispheres are removed, luminous impressions, though perfectly perceived (the animal follows with his eyes and head the movements of a lighted taper), are not preserved and cannot give rise to an intellectual effort; in this aspect of the case the sensation must be imperfect, the animal looks but does not see. The tubercular quadrigemina are to visual sensations what the protuberance generally is to sensations of touch, pain, etc.

It is probable that these tubercles, moreover, preside over other functions, not now known, since they appear considerably developed in animals completely deprived of the power

of sight (Talpa Asiatica, some of the Ophidio-Batrachians, Myxine); Serres also considered these as centres for co-ordination of movements; the excuse for this seems to be due to the fact that these tubercles have some relation to excitomotory impulsions, which would authorize their classification with the cerebral centres, as a medium between the protuberance and the clusters of cerebral and cerebellar cells (see p. 33, fig. 12).

The functions of the cerebellum are a problem difficult of solution; experimentation and observation from pathological conditions give us but negative and contradictory results; ablation of the cerebellum shows that this large portion of the encephalon takes no part in the intellectual functions, strictly speaking, nor in the manifestations of sensation, memory, instinct, or volition. Its peculiar functions are so difficult to define that almost all possible opinions have been proposed. Leaving out of consideration the opinion of Willis, who, at the time when the theory of the animal spirits held sway, made it the point of departure for the innervation of organic functions, we notice that some physiologists (Lapeyronie, Pourfour du Petit, Dugès), basing their opinion upon the apparent continuity of the inferior cerebellar peduncles with the posterior columns of the spinal cord (conductors of sensation or sensibility), considered the cerebellum as the central organ of sensibility, the sensorium commune to which all the peripheral sensations pass for elaboration and arrangement, especially including the auditory (Foville) and visual sensations (Lussana). We have already noticed that this rôle of the centre of sensation belongs in part to the protuberance and in part to the tubercular quadrigemina. With less reason, but perhaps more fortunate in his hypothesis, Gall considered the cerebellum as a centre of animal love, or erotic passion; indeed, in spite of the experiments and contradictory observations by Leuret, Ségalas, Combette, and Vulpian, we notice several reasons brought out by experimentation and clinical observation by Budge, Valentin, Wagner, Lussana, which would seem to give some appearance of reality to the hypothesis of Gall, and to assign an important function to the middle lobe in the manifestations of genital instinct.

The cerebellum, however, seems to have an important part in the apparatus for the co-ordination of movement as appears from the experiments of Rolando, and especially from the more recent and numerous experiments of Flourens; in the

animals (birds) from which this latter physiologist removed the cerebellum, "flying sensations and perceptions remained; the possibility of executing combined movements likewise persisted, but the co-ordination of movements into movements of regulated and definite locomotion was lost." This has been the view adopted by the majority of physiologists; and Lussana has even gone further, and attributes to the cerebellum the function of the centre of muscular sensibility. However these functions of locomotion are manifested only when the deeper portions of the cerebellum are injured, whilst superficial injuries do not give any result, and leave us without any indications of the functions of the cortical layers of the cerebellum. Moreover Vulpian and Philippeaux have caused no disturbance of locomotion in fishes after ablation of the cerebellum; let us recall to mind also that physiology of the spinal cord has furnished almost all the elements necessary to explain the reflex mechanism of locomotion; moreover, Küss saw a rabbit whose head he had amputated with dull scissors, thereby hacking the head so as to prevent hemorrhage, jump from the table and run the whole length of the room with a perfectly defined movement of locomotion, and we shall arrive at the conclusion that, in spite of the very numerous solutions that have been offered to explain the physiology of the cerebellum, we still possess no precise notion of the functions of this nervous centre.

The corpora striata and optic thalami are inexcitable, as well as the gray axis of the spinal cord, and all the gray centres; we cannot then arrive at a knowledge of their functions except by their destruction in animals, or by the study of the clinical phenomena which follow their alteration, either by the presence of a tumor or by a hemorrhage. In these cases no lesions of general sensibility have been proved, nor of any special sensibility, and we must admit that the optic thalami, in spite of their name, have no more concern with vision than the corpora striata have with the sense of olfaction. Lesions of these centres produce only paralyses (paralysis on the opposite side, as we have seen à propos to the conductors in the spinal cord), and we may consider the corpora striata and optic thalami as grand excito-motory centres, but without assigning to the former the movements of the posterior extremities, and to the latter those of the anterior extremities.

The corpus callosum (trabs cerebri), and the different commissures between the cerebral hemispheres taught in anatomy,