centres in the rat immediately above the medulla oblongata, and pinch its foot severely: it will utter a short sharp cry of pain, which is reflex or sensori-motor. Now destroy the medulla oblongata, and again pinch the foot: there will be reflex movements, but no cry. The rat, by reason perhaps of having been hunted through so many generations, is a very fearful animal, very susceptible, scampering away at the least unusual sound. If its cerebral hemispheres, its corpora striata, and optic thalami be removed, it remains quiet; but if a sharp noise be made, such as a cat makes sometimes, the animal makes a bound away, and repeats the jump each time that the noise is made.*

Examples of sensori-motor movements are to be found in the involuntary closure of the eyelids when the conjunctiva is touched, or when a strong light falls upon the eye; in the distortion of the face on account of a sour taste; in the quick withdrawal of the hand when it is touched by something hot; in the cry which excessive pain calls forth; in the motions of sucking which take place when the nipple is put between the infant's lips; in coughing and sneezing; and in yawning on seeing some one else yawn. Illustrations of acquired movements of this class are seen in the adaptation of the walk to the music of a military land, in dancing, in the articulation of words on seeing their appropriate signs, and in many other of the common actions of life of which we are not conscious at the time, but of the necessity of which, were there no power of automatically performing them, we should soon become actively conscious. The instinctive actions of animals fall under the category of consensual acts: without the intervention of any conception, the sensation at once excites the appropriate movement, and the animal is as skilful on its first trial as it is after a life experience. It is true that the instinctive life is extremely limited in man, but sensorimotor action plays a large part in such manifestations of it as

action," p. 258. After instancing, as examples, sneezing, coughing, the contraction of the pupils, and the movements of the eyelids, he says: “We seem authorized, there! re, by the fullest evidence, to assume that sensation is the mental cause, wi ever the physical links, of a great proportion of the mus cular contractions of our frame; and that among those s produced are found some of the most constant, the most remarkable, and the most important of that great class of corp real phenomena.”—Analysis of the Huma‹ Mind, p. 265. • Vulpian, op. p. 548.

are witnessed; in the taking of food the movements of mastication and deglutition, like the earlier ones of sucking, are in answer to sensations, as also are some of the co-ordinated movements necessary to the gratification of the instinct of procreation. The adjustment of the human eye to distances, which takes place with such marvellous quickness and accuracy, is effected, according to the best authorities, by a change in the convexity of the lens or the cornea, and an alteration in the direction of the axes of the eyes. It is not a voluntary, not even a conscious act, but a consensual act in respondence to a visual sensation, and it is well suited to convey a notion of what an instinctive act in an animal is.*

It was said, when treating of the spinal cord, that its faculties were, for the most part, not innate but acquired by education; and the same thing may be said of the sensory centres. Sensation is not, as the common use of the word might seem to imply, a certain inborn faculty of constant quantity, but in reality a general term embracing a multitude of particular phenomena that exhibit every degree of variation both in quantity and quality. The sensation of each sense is a gradually organized result or faculty that is matured through experience; the visual sensation of the adult is a very different matter from that of the child whose eyes have recently opened upon the world; Mr. Nunneley's patient, whose sight was restored by operation, held his hands before his face to prevent objects touching his eyes; the wine-taster's cultivated sense is nowise comparable with that of a man who knows nothing of wine; the tactile sensation of the blind man differs toto cælo from that of the man who has always had the full use of his eyes. The complete and definite sensation is slowly built up in the proper nervous centres from the residua or traces which previous sensations of a like kind have left behind them; and the sensation of the cultivated sense thus sums up, as it were, a thousand experiences, as one word often contains the accumulated acquisitions of genera

Theorie

• For the best summary and discussion of the theories of vision, des Schens und raümlichen Vorstellens vom physikalischen, physiologischen und psychologischen Standpunkte mus betrachtet. Halle, 1861. By C. S. Cornelius. Also by the same author, Zur Theorie des Schens mit Rücksicht auf di neuesten Arbeiten in diesem Gebiete, 1×64.

tions. Simple as a sensation appears, it is in reality infinitely compound. (2) We do not see, hear, or otherwise perceive by sense the exact impression made on the organ, but the effect excited by the impression in the nerve centre; in other words, we perceive the interpretation of the impression which our previous experience has made familiar to us. Vision, Bishop Berkeley aptly says, is a language speaking to the eye, which we are not conscious to have learned because we have been learning it ever since we were born. All that is innate in the different ganglionic centres is a specific power of reaction to certain impressions made upon organs specially adapted to receive them; but as the waste following activity is restored by nutrition, and a trace or residuum is thus embodied in the constitution of the nervous centre, becoming more complete and distinct with each succeeding repetition of the impression, it comes to pass that an acquired nature is ultimately grafted by education on the original nature of the cell. In the common metaphysical conception of sensation as a certain constant faculty, what happens is this: the abstraction from the particular is converted into an objective entity which thenceforth leals captive the understanding.

Whether, as some hold, our perception of the form and distance of external objects be due to our muscular experience, or whether, as others inaintain, our visual sensation by itself may give the notion of extension and distance, it is certain that our ordinary estimates of distance are very gradually acquired. But it is not so in many animals: the young swallow can seize its small prey with as accurate a skill as the old one can after a lifeexprience; and there is a fish that spurts a drop of water at the little insect moving above the surface, and fails not to bring it down. The intuition of distance is obviously in such cases complete and distinct from the first. It is, however, conformable to the law of development from the general to the special in the

• In regard to this question, an experiment by Volkmann, quoted by Fick, is interesting and instruct se. When the finger, or any limited pation of skin on one side of the body, frequently experimented upon with the compasses, in order to test the degree of sensibility, and its tactile sensibility thereby increased, as it notably is, above the level of that of neighbouring parts, the symmetrical part of the skin on the oppte side of the body will be found to be almost as acute, an experimental proot of the same kind as that which the stereope furnishes,

crganic world, that what is innate in some of the lower animals should be acquired by man: the absence of such limitation in his original nature marks his higher freedom. Still it is most interesting to observe how much even he is indebted to original endowment in this very matter of estimating distance. For what is the immediate cause that determines the muscular adjustment of the eye to distance? The act is consensual, or, using the vaguer term, instinctive, in respondence to a visual sensation or picture—an act of which there is no direct consciousness, and over which the will has no direct control. Though the process is confused and uncertain at first, unlike in that regard the process in the lower animals, yet it is not long before the proper muscular adaptations are acquired and definite muscular intuitions organized. Plainly, then, very much is due to the pre-arranged constitution of the nervous centres even in man. And while we assert that sensation is not an inborn faculty of constant value in man, it behoves us not to forget the fact that there are implanted in the constitution of his nervous centres the capabilities of certain definite associated movements answering to certain sensations.

The idea to be formed and fixed in the mind from a consideration of the phenomena of the development of sensation, and necessary to its proper interpretation, as indeed to the interpretation of every manifestation of life, is the idea of organization. The mind is not like a sheet of white paper which receives just what is written upon it, nor like a mirror which simply reflects more or less faithfully every object, but by it is connoted a plastic power ministering to a complex process of organization, in which what is suitable to development is assimilated, what is unsuitable is rejected. By the appropriation of the like in impressions made upon the senses we acquire a sensation, of which we might speak, as we do when speaking of idea, as general or abstract; it henceforth exists, latent or potential, as a faculty of the sensory centres, and on the occasion of the appropriate impression relers the sensation clear and definitein other words, gives the interpretation. It is exactly like what happens in the spinal centres, and exactly like what happens, as we shall hereafter see, in the ideational centres. Coincidently with the assimilation of the like in impressions, there is neces

sarily a rejection of the unlike, which, being then appropriated by other cells, becomes the foundation, or lays the basis, of the faculty of another sensation, just as nutrient material which is not taken up by one kind of tissue element is assimilated by another kind. In the education of the senses, then, there takes place a differentiation of cells; in other words, a discernment, as well as an improvement of the faculty of each kind of sensation by the blending of similar residua. There is an analysis separating the unlike, a synthesis blending the like; and by the two processes of differentiation and integration are our sensations gradually formed and developed. The process illustrates the increasing speciality of individual adaptation to external nature; and the length of childhood in man is in relation to the formation of his complex sensations.

The organization of our sensations is not, however, limited simply to the formation of the particular sensation; by it is effected also the association or catenation of sensations. In animals there can be no doubt that one sensation frequently calls another into activity, in accordance with the order established among them, without the intervention of idea; they are much more dependent on sensation than man is, and therefore the association of sensations in the causation of movements is more marked. Hence it is that blinding of one eye produces vertiginous movements in pigeons, as Flourens and Longet have shown, and that section of the semicircular canals of the ear also produces various disturbances of movements. The trouble, inconvenience, and occasional vertiginous feelings produced for a time in man when he suddenly loses his hearing in one ear, probably spring from the interruption to the complex association of sensations habitual to him in the daily movements of life. He only learns how much he depends on such associations when disorder or loss of them occurs. It is certainly difficult in him to eliminate the influence of the higher cerebral centres, yet in those functions in which consensual action has most part-in the taking of food, for example, where succeeding sensations bring into successive action different complex muscular movements and again in the sexual act-there is abundant evidence of an association of sensations.

Thus much concerning sensation, viewed on its passive or