FUNCTIONS OP ANIMAL LIFE. 373 



impression, and the excitement of a visual perception, becomes evident. The 

 former may take place through the intervention of nerves, whose sensory ex- 

 tremities offer no extraordinary peculiarities : the latter can only be received 

 through the medium of an instrument, which shall, from the mixture of rays 

 falling equally upon every part of its surface, produce an optical image, and 

 then impress it upon the expanded surface of the nerve; so that, each fibril 

 receiving a distinct impression, the mind may form its picture by the combina- 

 tion of the whole. That this is, in fact, the share which the organs of Special 

 Sense bear in the general endowments of the whole apparatus, may be inferred 

 especially from the conformation of the Eye; which is in every respect a merely 

 optical instrument, of the greatest beauty and perfection, adapted to form upon 

 the retina, in the most advantageous manner, the images of surrounding objects 

 in all their variations. There can be little doubt that the structure of the Ear 

 is arranged to do the same for the sonorous vibrations which the eye does for 

 the rays of light; that, is, through its means, the undulations which strike upon 

 the external surface of the organ are separated and distinguished, those of a 

 like kind being brought together upon one division of the nerve, and those of 

 another order upon a different set of fibres; so that the different kinds of sound, 

 and the peculiar quality and direction of each, may be discriminated; whilst, by 

 the concentration of all the impressions of the same character, a higher amount 

 of force is given to them. The apparatus which ministers, however, to the 

 sense of Smell, is far less complete in its endowments; for it serves only, in 

 Man at least, for the discrimination of odorous emanations, and affords no 

 guidance with regard either to their direction or their source. In fact, the kind 

 of information which Man receives through this sense, seems very much akin 

 to that which the lowest animals possessing visual organs can derive from their 

 employment. Still, a special organ of sense is required, to enable the olfactive 

 nerve to be impressed by the peculiar agency of odorous emanations ; which, 

 whatever be its nature, has no operation upon ordinary sensory surfaces. It is 

 not a little remarkable, that the specialty of organization of the nerves of Sight, 

 Hearing, and Smell, renders them incapable of receiving ordinary mechanical 

 impressions; so that the contact of solid substances with the sensory surfaces 

 which they supply is not felt, except through the instrumentality of other 

 nerves; and no irritation of their trunks, mechanical or otherwise, gives rise to 

 feelings of pain. The sense of Taste, however, though special in regard to the 

 peculiarity of the impressions which its organ is adapted to receive, is closely 

 akin to that of Touch in the conditions under which it is exercised; the abso- 

 lute contact of the sapid substance with the sensory surface being requisite ; and 

 the papillary organs in which the gustative nerves may be said to originate, 

 being essentially the same in structure with those of ordinary tactile surfaces. 



392. The Brain and Spinal Cord of Man, in which by far the greater part 

 of the afferent nerves terminate, and from which nearly all the motor nerves 

 arise, may be considered as made up of an aggregation of a number of distinct 

 ganglionic centres, each of which has its own special endowments, and is con- 

 nected with nervous trunks of its own. Commencing with the Spinal Cord, 

 we find, on comparing it with the gangliated column of Articulated animals, 

 that it really consists of a series of ganglia disposed in a longitudinal line, which 

 have coalesced with each other; each ganglion being the centre of the "nervous 

 circle" proper to one vertebral segment of the trunk. Throughout the entire 

 series, we find no other endowment than that of reacting upon an excitant ; this 

 excitant being either conveyed by the afferent nerve-trunks, or transmitted 

 downwards from the higher parts of the nervous system. No impression which 

 is limited to this series of ganglia, excites any sensorial change; so that we 

 may consider the Spinal Cord as the special instrument of the " excito-motor" 

 division of the functions of the nervous system. The ordinary Spinal Nerves 



