SENSATION IN GENERAL. 335 



present, but has escaped detection. Lower than this, we cannot imagine, on 

 physiological grounds, the presence of a nervous system, which controls move- 

 ment ; for, without one afferent, and one efferent, fibre, and a connecting 

 nerve-cell, we cannot well understand how nerve-substance can exercise any 

 governing influence over muscular fibres, contractile cells, or sarcodous sub- 

 stance. Without these three elements at least, the muscular substance prob- 

 ably contracts by direct stimulation of its fibres, and the resulting movements 

 would be irregular and indeterminate. The supposed admixture of nerve-cells, 

 with the other tissue elements, or of nervous granules with the cell-contents 

 of the uni-cellular animals, might still, however, have for its office, some feeble 

 sensorial function, or the regulation of the nutrition of the tissues, in some 

 manner unknown to us, but analogous to that of a sympathetic system. 



Protozoa. 



In this, the lowest subdivision of the animal kingdom, so far as microscopic 

 research has extended, and so far as physiological inference may be our guide, 

 a nervous system, consisting of nerve-cells and nerve-fibres, cannot exist ; and 

 the idea, already alluded to, of the presence of nervous granules in their sub- 

 stance, is, of course, hypothetical. The Infusoria, which are uni-cellular ani- 

 mals, that is, composed of a single organic cell, and which are moved entirely 

 by the action of cilia on their surface, may well be understood to require no 

 motorial nervous apparatus to govern their movements, seeing that the cili- 

 ary motion, even in the higher animals, is independent of nervous influence. 

 It is certain, however, that Infusorial animalcules, kept in a vessel of water, 

 will congregate towards the light ; and in some of these minute organisms, 

 colored spots present themselves, which, if they be ocular spots, would imply 

 some feeble form of sensation. A granule, or particle of nerve-substance, 

 situated at some particular point, within this single-celled animal, might form 

 an excitable sensorial nerve-centre, without exercising any controlling motor 

 influence. In the Rhizopoda, or Foraminifera, and in the Spongida and Gre- 

 garinida, all of which consist of a fleshy or contractile sarcodous mass, not 

 enveloped by any distinct cell-wall, and the shape of which may, in the two 

 former groups, change most irregularly, it would seem as if the contraction 

 of the sarcode, on which the movements of these primitive animals depend, 

 were excited directly by external stimuli, acting without the intervention of 

 nervous substance. Their motions can scarcely be said to possess the spon- 

 taneity and regularity which are characteristic of those dictated .by or excited 

 through, a nervous system, and which therefore are characteristic of animal 

 life. They resemble more, those performed by the parts of plants, or by some 

 of the lower plants, in which the co-operation of nervous substance is not 

 even suspected. 



If these lowest forms of Protozoa possess any nervous granules within them, 

 they are probably not even sensory, for no trace of ocular pigment is found 

 in them ; such granules might, however, exercise a control over the nutrition 

 of the cell, and in this way, indeed, might influence ciliary motion, not only 

 in the Spongida, but in the Infusoria, in the Coelenterata, and in all the higher 

 animals, even in the Vertebrata ; for that motion may be intimately connected 

 with, if not wholly dependent on, nutritive molecular changes. 



THE SENSES. 



Sensation in general. 



Sensation consists essentially in a certain change in the sensorium, 

 or the sensorial portion of the cerebrum, accompanied by the mental 

 state known as consciousness. By this we become aware of external 

 or internal impressions made upon the sensory nerves, or rather of 

 certain conditions of those nerves produced by those impressions. The 



