motor influence, which is generated in respondence to the stimulus thus conveyed, appears to act chiefly through the branches of the Par Vagum, which are distributed to most of the muscles concerned in swallowing; but the Facial, the Hypoglossal, the motor portion of the Fifth, and perhaps also the motor portions of some of the Cervical nerves, are also concerned in the movement, and may effect it, though with difficulty, after the pharyngeal branches of the Par Vagum have been divided.

898. In the propulsion of the food down the Esophagus, to which the glosso-pharyngeal nerve does not extend, the muscular contraction, so far as it is of a reflex nature (§ 455), must depend upon the œsophageal branches of the Par Vagum alone; their afferent portion being the excitor, and their motor portion giving the requisite stimulus to the muscles. The same must be the case in regard to the muscular contractions of the cardiac and pyloric sphincters, and of the walls of the stomach, so far as regards their dependence upon the nervous system at all; but the degree of this is doubtful.

899. There are other reflex actions of the Medulla Oblongata, connected with the regulation of the aperture of the Glottis; these, which are effected through the superior and inferior laryngeal branches of the Par Vagum, will be better noticed, when the actions of the Larynx are under consideration (§ 976).-In like manner the reflex action concerned in the regulation of the aperture of the Pupil, will be more conveniently noticed in the sketch to be hereafter given of the Physiology of Vision (§ 969).

5. Functions of the Sensory Ganglia.

900. All the nerves of Sensation, both general and special, may be traced into a series of ganglionic masses lying at the base of the brain; which seem to constitute their own particular centres. Thus we have seen in Fishes, the Olfactive, Optic, and Auditory ganglia, marked out as such, by the termination of the nerves proceeding from the organs of smell, sight, and hearing, in these masses respectively. These ganglia bear an evident correspondence with the cephalic ganglia of the Invertebrata; which must chiefly, however, be regarded as optic ganglia, since the development of the eyes far surpasses that of the other organs of special sense. On the other hand, they find their representatives in certain organs at the base of the brain, in Man and the higher Mammalia; which, though small in proportion to the whole Encephalon, are capable of being clearly marked out as the ganglionic centres of the several nerves of sense. Thus, anteriorly, we have the Olfactive ganglia, in what are commonly termed the bulbous expansions of the Olfac tive nerve; which, however, are real ganglia, containing gray or vesicular substance; and their separation from the general mass of the Encephalon, by the peduncles or footstalks commonly termed the trunks of the olfactory nerves, finds its analogy in many species of Fish (§ 869). The ganglionic nature of these masses is more evident in many of the lower Mammalia, in which the organ of smell is highly developed, than it is in Man, whose olfactive powers are comparatively moderate.-At some distance behind these, we have the representatives of the Optic Ganglia,

in the Tubercula Quadrigemina, to which the principal part of the roots of the Optic nerve may be traced. Although these bodies are so small in Man, as to be apparently insignificant, yet they are relatively larger, and form a more evidently-important part of the encephalon, in many of the lower Mammalia; though still presenting the same general aspect. The Auditory ganglia seldom form distinct lobes or projections; but are usually lodged in the substance of the Medulla Oblongata. Their real character is most evident in certain Fishes, as the Carp; in which we find the Auditory Nerve having as distinct a ganglionic centre as the Optic. In higher animals, however, we are able to trace the Auditory nerve into a small mass of gray matter, which lies on each side of the Fourth Ventricle; and although this is lodged in the midst of parts whose function is altogether different, yet there seems no reason for doubting that it has a character of its own, and that it is really the ganglion of the auditory nerve.-We are not able to fix upon any such mass of gray matter, as the distinct Gustatory ganglion; nor

Diagram of the relation of the Sensori-motor tract at the base of the Brain, to the Cerebrum, as seen in horizontal section:-olf, olfactive ganglia; opt, optic ganglia; aud, auditory ganglia; cs, corpora striata; thal, thalami optici; a, a, olfactive nerves; b, b, optic nerves; c, c, auditory nerves.

is it necessary to attempt to do so; for, as we shall see hereafter, there is strong reason to regard the sense of Taste as only a refined kind of Touch, combined with the sense of Smell.

901. At the base of the Cerebral Hemispheres, we find two ganglionic masses on either side; through which all the fibres pass that connect the Hemispheres with the Medulla Oblongata. These are the Corpora Striata, and Thalami Optici. Upon tracing forwards the tract of motor fibres that ascend from the Anterior Pyramids, we find it passing chiefly into the Corpora Striata; whilst if we follow the

Sensory Column that ascends from the Posterior Pyramids, we shall find it to enter the Thalami Optici. These bodies have been usually considered as mere appendages to the Cerebrum; but the fact that they are independent centres of action is fully established by the presence of a large quantity of vesicular matter in their substance; and there is now a sufficiently large amount of evidence, both anatomical and physiological, to render it probable that the fibres which seem to pass through them from the Crura Cerebri, and then to radiate towards the periphery of the Cerebral Hemispheres, do not do so in reality, but that these ganglionic masses receive, on the one hand, the fibres that ascend to them from the Medulla Oblongata, and, on the other, are the point of departure of a new set, passing to the proper Cerebrum. Looking to the connexion of the Thalami Optici with the sensory tract, it may be regarded as not improbable that we may consider them as the ganglionie centres of common sensation; standing in the same relation to the sensory nerves, that converge from various parts of the body towards the Encephalon, as do the Optic and other ganglia to their nerves of special sensation. And as these last give origin to motor fibres, so may we regard the ganglionic matter of the Corpora Striata (which are in close connexion with the Thalami) as probably sharing in the same function; giving origin to the motor fibres, which produce the respondent consensual movements; just as the anterior peak of gray matter in the Spinal Cord gives exit to the motor filaments, which effect the reflex movements excited through the afferent fibres forming part of the posterior roots.

902. The functions of this series of ganglia may be more certainly determined by the aid of Comparative Anatomy, than by experimental mutilations. Reverting to the class of Fishes, we find that it there constitutes, with the Cerebellum, nearly the entire Encephalon; scarcely a rudiment of the true Cerebellum being discoverable in that group.* And when we descend to the Invertebrata, we find the cephalic masses entirely to consist of the ganglionic centres of the nerves of sense and motion. There can scarcely be a reasonable doubt, that these Cephalic ganglia are the seat of consciousness and the sources of those movements which are directed by sensation, in such animals as present this low type of nervous organization; and there is no adequate reason for the belief that the superaddition of the Cerebral Hemispheres in the Vertebrated series alters the endowments of the Sensory Ganglia on which they are superimposed; on the contrary, we everywhere see that the addition of new ganglionic centres, as instruments of new functions, leaves those which were previously existing in the discharge of their original duties. Hence we should be led to regard them as the centres of consciousness, even in Man, each pair of ganglionic centres ministering to that peculiar kind of sensation for which its nerves and the organs they supply are set apart; thus we should consider the Optic ganglia to be the seat of Visual sensations, the Auditory to be the seat of the sense of hearing, and so on. And we should also consider them as the instru

*The ganglionic masses, commonly designated as the Cerebral labes or hemispheres, must be really likened in great part (as already stated & 869) to the Corpora Striata.

ments whereby sensations, of whatever kind, either originate or direct Automatic movements.

903. So far as the results of experiments can be relied on, they afford a confirmation of these views, by showing that sensory impressions can be felt, and that automatic movements of a higher kind than the simply reflex can be called into play after the removal of the Cerebral Hemispheres, provided that these ganglia be left intact. Thus, if a Bird be thus mutilated, it maintains its equilibrium, and recovers it when it has been disturbed; if pushed, it walks; if thrown into the air, it flies. A pigeon deprived of its cerebrum has been observed to seek out the light parts of a partially-illuminated room in which it was confined, and to avoid objects that lay in its way; and at night, when sleeping with closed eyes and its head under its wing, it raised its head and opened its eyes upon the slightest noise. So, again, the removal or destruction of one pair of these Sensory centres appears to involve the loss of the particular sense to which it ministers; and frequently, also, to occasion such a disturbance in the ordinary movements of the animal, as to show the importance of these centres in regulating them. Such experiments have been chiefly made upon the Optic ganglia, or Corpora Quadrigemina, the partial loss of which on one side produces temporary blindness in the eye of the opposite side, and partial loss of muscular power on the opposite side of the body; and the removal of a larger portion, or the complete extirpation of it, occasions permanent blindness and immobility of the pupil, and temporary muscular weakness, on the opposite side. This temporary disorder of the muscular system sometimes manifests itself in a tendency to move on the axis, as if the animal were giddy; and sometimes in irregular convulsive movements. Here, then, we have proof of the necessity of the integrity of this ganglionic centre for the possession of the sense of vision; and we have further proof that the ganglion is connected with the muscular apparatus, by motor nerves issuing from it. The reason why the eye of the opposite side is affected is to be found in the decussation of the optic nerves, a point to be immediately adverted to (§ 907). The influence of the operation on the muscles of the opposite side of the body is at once understood from the fact of the decussation of the motor fibres in the anterior pyramids (§ 890). Similar disturbances of movement have been produced by injuries to the organs of sense themselves, or to the nerves connecting them with the sensorial centres. Thus, if one of the eyes of a pigeon be blindfolded, or its humors be evacuated, vertiginous motions ensue; and section of one of the semicircular canals of the ear in pigeons and rabbits has been found to occasion constant efforts to move in the plane of that canal, thus confirming the belief that the function of these canals is to indicate the direction of sounds. (§ 952).

904. Notwithstanding that, in Man, the high development of Intelligence and the exercise of the Will, supersede in great degree the operations of Instinct, we still find that there are in ourselves certain movements which can be distinguished as neither voluntary nor simply reflex, and which are examples of the method of operation that seems to be the chief source of the actions of the lower Vertebrata, as of the

As

Invertebrated classes in general. These movements are as automatic and involuntary as are the ordinary reflex actions, but differ from them in requiring that the impressions which originate them should be felt as sensations; and hence they are conveniently designated as consensual. As examples of this group, we may advert to the start upon a loud and unexpected sound; the sudden closure of the eyes to a dazzling light, or on the approach of bodies that might injure them, which has been observed to take place even in cases in which the eyelids could not be voluntarily closed; the act of sneezing excited by an irritation of the nostril, and sometimes also by a dazzling light; the semi-convulsive movements and the laughter called forth by tickling; and the vomiting occasioned by the sight or the smell of a loathsome object. So, again, the act of yawning is ordinarily called forth by certain uneasy sensations within ourselves, but also by the sight or hearing of the act as performed by another. Various phenomena of disease exhibit the powerful influence of sensations in producing automatic motions. instances of this kind, we may refer to the effects of the sight or the sound of liquids, or of the slightest currents of air, in exciting the Hydrophobic paroxysm; whilst in many Hysteric subjects the sight of a paroxysm in another individual is the most certain means of its induction in themselves. The most remarkable examples, however, of automatic movements depending upon sensations, are those which we come to perform habitually, and, as we commonly say, mechanically, when the attention and the voluntary effort are directed in quite a different channel. Thus the man who is walking through the streets in a complete revery, unravelling some knotty subject, or working out a mathematical problem, not only performs the movements of progression, which may be simply reflex, with great regularity, but also directs these in a manner which plainly indicates the guidance of sensations. Thus, he will avoid obstacles in the line of his path, and he will follow the course which he has been accustomed to take, although he may have intended to pass along some very different route; and it is not until his attention is recalled to his situation, that his train of thought suffers the least intermission, or that his will is brought to bear upon his motions.

905. We may trace the agency of the Sensory Ganglia, however, in the Human subject, not merely in their direct and independent operation upon the muscular system, but also in the manner in which they participate in all Voluntary actions. The existence of a Sensation of some kind, in connexion with a Muscular exertion, seems essential to the continuance of the latter. Our ordinary movements are guided by what is termed the Muscular Sense; that is, by a feeling of the condition of the muscle, that comes to us through its own sensory nerves. How necessary this is to the exercise of muscular power may be best judged of from cases in which it has been lost. Thus, a woman who had suffered complete loss of sensation in one arm, but who retained its motor power, found that she could not support her infant upon it, without constantly looking at the child; and that, if she were to remove her eyes for a moment, the child would fall in spite of her knowledge that her infant was resting upon her arm, and of her desire to sustain it. Here, the muscular sense being entirely deficient, the sense of