PHYSIOLOGY. 



jecture, by analogy, that the same mode 

 may obtain in others. There are some 

 obvious differences, which may account 

 for the different mode of affection in 

 the various senses. The retina, which is 

 the expanded end of the optic nerve, is of 

 so delicate pulpy a nature, as to approach 

 to fluidity, and it is acted on only by the 

 rays of light, which are not perceived by 

 any other sense : the auditory nerve, 

 which alone perceives sounds, is rather 

 firmer, &c. 



That the mind is very immediately and 

 essentially connected with the brain, and 

 that the animal functions of sensation and 

 voluntary motion, are no less intimately 

 dependant on the same organ, may be 

 proved by such an abundance of physio- 

 logical and pathological phenomena, that 

 no doubt can be entertained of the fact. 

 An injury of this organ suspends or an- 

 nihilates the whole, or a part, of the men- 

 tal operations, and puts an end to all feel- 

 ing and motion : the organs of the body 

 remain entire, the nerves connecting these 

 with the sensorium are uninjured; but 

 the perceptive faculty is lost. Again, an 

 injury of one side of the brain often causes 

 a loss of feeling and motion in one side 

 "ttnly of the body ; which, in consequence 

 of principles inexplicable by us, always 

 affect the opposite half to the injury. 

 That the sensible impressions made on 

 our organs are conveyed by the nerves to 

 the brain ; and that the latter part is the 

 seat of the sensation, although it is refer- 

 red by the mind to the part itself; is 

 proved by cutting or tying a nerve : in 

 which case, the usual impression causes 

 no perception. The truth of this asser- 

 tion, which will hardly meet with credit 

 among the uninformed, is illustrated by 

 what happens to persons whose limbs 

 have been amputated : they are constant- 

 ly complaining of pains in the toes or fin- 

 gers of the limbs they have lost. Here 

 the middle of the nerve is irritated, but 

 the pain is referred by the mind to its ex- 

 tremities. 



Yet, although the influence of the brain 

 be thus essential, in the business of sen- 

 sation and voluntary motion, and an un- 

 impaired state of the nerves passing be- 

 tween the organ and the sensorium, be 

 consequently an indispensable condition in 

 those functions, other departments of the 

 animal economy are not so immediately 

 subject to the power of the brain. The 

 processes of digestion, absorption, circu- 

 lation, secretion, and nutrition, those, in 

 short, which constitute the internal lite, 

 still go on, when injuries of the brain have 



suspended the animal functions: nay, 

 they may survive for months or even years. 

 The ligature of the nerves of a part does 

 not destroy its circulation or nutrition ; 

 although these processes may perhaps be 

 impaired. How, then, will it be said, 

 does an injury of the brain so often prove 

 fatal ? The individual ought still to live 

 internally, although his external life has 

 been annihilated. But here we notice a 

 function that partakes of both : namely, 

 respiration. The dilatation of the chest 

 can only be performed by means of mus- 

 cles, whose principle of action comes from 

 the brain : as the injury of the latter organ 

 has paralysed these, the blood can no lon- 

 ger receive those changes which it under- 

 goes in respiration, and thereby becomes 

 unfit to stimulate the heart to action, or to 

 keep up the powers of life in any of the 

 organs of the body. 



That the nerves are, as we have describ- 

 ed, the medium of connection between 

 the mind and its organs, is clear ; but how 

 their offices are performed, is a much 

 more obscure question. It has occupied 

 the attention, and engaged the experi- 

 ments of physiologists, in all ages ; but 

 nature has not hitherto lifted the veil, and 

 the subject remains nearly in its original 

 obscurity. An oscillatory or vibratory 

 motion of the nerves, or a nervous fluid 

 contained in or adhering to these organs, 

 have been assumed in explanation of the 

 facts. According to some, the latter is a 

 liquid contained in tubes ; while others 

 liken it to caloric, light, oxygen, the elec- 

 tric, or magnetic fluids. The partisans 

 of the latter opinions consider, that the 

 recent discoveries of galvanism add much 

 weight to their arguments. See GALVA- 

 NISM. 



A supposed central point, to which all 

 sensations are carried, and from which all 

 motions emanate, is called the sensorium 

 commune ; and is considered as tht- seat 

 of the soul Des Cartes placed this in the 

 pineal gland, others in the corpus callo- 

 sum, pons Varolii, corpora striata, &c. 

 The learned Sormmerring has lately en- 

 deavoured to show, that the seat of the 

 soul must be in the water of the ventricles, 

 as he has succeeded in tracing the origins 

 of all the nerves from the sides of these 

 cavities. The records of morbid ana- 

 tomy refute many of these opinions, 

 as they show the parts considered as 

 sensoria to have been diseased and de- 

 stroyed without any impairment of the 

 menial faculties. 



The curious and complicated structure 



