OF ANIMALS. 



have 110 chemical test that can reach its essence, no glasses that can trace 

 its mode of union with the brain, no abstract principles that can deter- 

 mine the laws of its control. We see, however, enough to convince us 

 that its powers are of a very different description from those of the body, 

 and revelation informs us that its nature is so toof Let us receive the 

 information with gratitude, and never lose sight of the duties it involves. 



But this subject would lead us astray even at our outset : it is impor- 

 tant, and it is enticing ; and the very shades in which much of it is wrapped 

 up prove an additional incitement to our curiosity. It shall form the basis 

 of some subsequent investigation,* but our present concern is with the 

 I external senses alone. 



These, for the most part, issue from the brain, which, in all the more 

 i perfect animals, is an organ approaching to an oval figure ; and consists 

 of three distinct parts : the cerebrum, or brain properly so called ; the 

 cerebel, or little brain, and the oblongated marrow. The first constitutes 

 the largest and uppermost part ; the second lies below and behind ; the 

 third, level with the second, and in front of it — it appears to issue equally 

 out of the two other parts, and gives birth to the spinal marrow, which 

 may hence be regarded as a continuation of the brain, extended through 

 the whole chain of the spine or back-bone. 



From this general organ arises a certain number of long, whitish, pulpy 

 strings or bundles of fibres, capable of being divided and subdivided into 

 minuter bundles of filaments or still smaller fibres, as far as the power of 

 glasses can carry the eye. These strings are denominated nerves ; and 

 by their different ramifications convey different kinds or modifications of 

 sensation to different parts of the body, keep up a perpetual communica- 

 tion with its remotest organs, and give activity to the muscles. They 

 have been supposed by earher physiologists to be tubular or hollow, and 

 a few experiments have been tried to estabhsh this doctrine in the present 

 day, but none that have proved satisfactory. 



As the brain consists of three general divisions, it might, at first sight, 

 be supposed that each of them is allotted to some distinct and ascertain- 

 able purpose : as, for example, that of forming the seat of intellect, or 

 thinking ; the seat of the local senses of sight, sound, taste, and smell ; 

 and the seat of general feehng or motivity. But the experiments of ana- 

 tomists upon this abstruse subject, numerous and diversified as they have 

 been of late years, and, unhappily, upon living as well as upon dead ani- 

 mals, have arrived at nothing conclusive in respect to it ; and have rather 

 given rise to contending than to concurrent opinions. So that we are 

 nearly or altogether unacquainted with the reason of this conformation, 

 and of the respective share which each division takes in producing th^ 

 general effect. 



The nerves uniformly issue in pairs, one for eaXih side of the body, and 

 the number of pairs is thirty-nine ; of which nine rise immediately from 

 the great divisions of the brain, under which we have just contemplated 

 it, and are chiefly appropriated to the four local senses ; and thirty from 

 the spinal marrow, through different apertures in the bone that incases 

 it, and are altogether distributed over the body, to produce the fifth or 

 general sense of touch and feeling, as also irritability to the muscles. 



That these nervous or pulpy fibres are the organs by which the various 

 sensations are produced or maintained is demonstrable from the following 



* Series HI. Lect. I. II. IH, 



