160 



ON THE EXTERNAL SENSES 



the sensory, which usually remains for a long time after the exciting cause 

 has ceased to operate. This effect, for want of a better term, we call impres- 

 sions; and the particular facts, or thing's impressed, and of which the impres- 

 sions retain, as it were, the print or picture, ideas. 



The sensory has the power of suffering this effect or these ideas to remain 

 latent or unobserved, and of calling them into observation at its option : it is 

 the active exercise of this power that constitutes thought. 



The same constitution, moreover, by which the mind is enabled to take a 

 review of any introduced impression, or to exercise its thought upon any in- 

 troduced idea, empowers it to combine such impressions or ideas into every 

 possible modification and variety. And hence arises an entirely new source 

 of knowledge, far more exalted in its nature, and infinitely more extensive 

 initsTange: hence memory and the mental passions; hence reason, judg- 

 ment, consciousness, and imagination, which have been correctly and ele- 

 gantly termed the internal senses, in contradistinction to those by which we 

 obtain a knowledge of things exterior to the sensorial region. 



Thus far we can proceed safely, and feel our way before us ; but clouds 

 and darkness hang over all beyond, and a gulf unfathomable to the plummet 

 of mortals. Of the sensory, or mind itself, we know nothing ; we have no 

 chemical test that can reach its essence, no glasses that can trace its mode 

 of union with the brain, no abstract principles that can determine 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 too. 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 important, 

 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 external senses 

 alone. 



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

 fect 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 sensa- 

 tion to different parts of the body, keep up a perpetual communication with 

 its remotest organs, and give activity to the muscles. They have been sup- 

 posed by earlier physiologists to be tubular or hollow, and a few experiments 

 have been tried to establish 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 ascertainable pur- 

 pose: 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 feeling or motivit5^ But the experiments of anatomists upon this 

 abstruse subject, numerous and diversified as they have been of late years, 

 and, unhappily, upon living as well as upon dead animals, 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 unac- 



* Series iii. Lectures i. ii. Ui. iv. 



