THE VITAL FUNCTIONS OP ANIMALS. 23 



With respect to the animal life a perfect sensation appears to 

 be possible only through a Nervous System. This nervous system, 

 in the higher, or more perfect animals, consists principally of the 

 brain and the spinal cord. The larger the mass of the brain is in 

 proportion to the nerves, the more perfect appears to be the develop- 

 ment of the intelligence and mental faculties of the animal a law 

 that was first discovered by the celebrated SCEMMEEING. In pro- 

 portion as we descend to the lower animals, the nervous masses are 

 more dispersed and removed from one another, and in the lowest 

 families of the animal kingdom no traces of a special nervous 

 system remain. 



The Head is that part of the body which includes the brain and 

 the chief organs of sense. There are five senses, of which Touch 

 (tactus) appears to be the most widely diffused through the whole 

 animal kingdom. The seat of touch is the skin, the general 

 covering of the body, which is everywhere interwoven with nerves. 

 The nerves of the skin are lost, with their little twigs, in its 

 middlemost and very dense layer. The ends of the. cuticular nerves 

 are covered and protected by the cuticle, and in many places by 

 other external insensible parts, as scales, hair, &c. In the organ of 

 Taste, the twigs of the nerves of taste pass into the soft papillse of 

 the tongue, and end there. The twigs of the Olfactory Nerve are 

 spread out upon a mucous membrane (the membrana Schneideriana] : 

 the continuation of the medulla of the Optic Nerve forms the Retina, 

 which Physiologists determine to be the seat of vision. Lastly, the 

 most simple form of the Auditory organ is that of a sac filled with 

 fluid, in which there float, as it were, the soft and delicate termina- 

 tions of the auditory nerve. From all this it appears, that the 

 general form (typus] of an organ of sense is to be sought for in a 

 nerve whose terminations form a delicate mass suited to the recep- 

 tion of external impressions. But in each particular organ of sense 

 the proper nerve of sense is only capable of a determinate action. 

 The auditory nerve is only susceptible of sound, or rather, every 

 stimulus which affects it is perceived only as sound: the optic 

 nerve recognises no other impressions than those of light. Such, 

 at least, is the case with man and the higher animals: and one 

 organ of sense can never supply the proper office of another. Im- 

 pressions are conveyed, by means of the nerves, to the brain or any 

 other nervous center. 



