PHYSIOLOGY. 



vid impressions, which change the order 

 of their motions, accelerate or retard, 

 suspend or terminate them ; and contrac- 

 tility, by which parts, when irritated, con- 

 tract, act, or execute motions. 



By means of the senses, and of the 

 nerves which are continued from them to 

 the brain, we perceive or feel the impres- 

 sion made on our bodies by external ob- 

 jects. The brain, which is the true seat of 

 this relative sensibility, (or, as it might well 

 be called, perceptibility), being excited 

 by these impressions, influences the 

 moving powers of the muscles, and de- 

 termines the exercise of their contractili- 

 ty. This property, subjected to the com- 

 mand of the will, is manifested by the 

 sudden shortening of the muscular or- 

 gan, which swells, becomes hard, and cau- 

 ses those parts of the skeleton to which 

 it is attached to move. The nerves and 

 the brain are the essential organs of these 

 two properties ; division of the former 

 destroys sensation ; and the voluntary 

 motion of those parts to which the nerves 

 are distributed. But there is another 

 kind of sensibility, qui^j independent of 

 the presence of nerves, existing in all or- 

 gans, even where no nervous filaments 

 are distributed. Bones, cartilages, liga- 

 ments, arteries, and veins, in short, all 

 parts which are not influenced by the 

 will, posssess no nerves. Yet, though in 

 their natural state they transmit to the 

 brain no perceptible impression, though 

 they may be injured without giving the 

 animal any pain, and though the will has 

 no influence over them, yet they enjoy a 

 sensibility and contractility, by virtue of 

 which they perceive impressions, and 

 contract in their own manner, recognize 

 in the fluids which circulate through 

 them what is proper for their nutrition, 

 and, separating this part, appropriate it 

 to their own substance. 



We recognize then in the parts of our 

 body two modes of sensation, as well as 

 two species of motion : a sensibility, by 

 means of which certain parts transmit to 

 the brain impressions which they feel, 

 and of which we therefore become con- 

 scious : a second kind, pervading every 

 part without exception, and presiding 

 over the assimilating functions. We ob- 

 serve also two kinds of contractility cor- 

 responding to the differences of sensibi- 

 lity : the one by which the voluntary 

 muscles perform the contractions deter- 

 mined by the action of the will ; the other 

 manifested by actions which are equally 

 unknown as the causes which give rise 

 to them. 



When we have once clearly distin- 

 VOL. V. 



guished these two grand modifications of 

 sensibility and contractility, we shall find 

 out, without difficulty, the source of those 

 eternal disputes, raised by Haller and his 

 followers, concerning the irritable and 

 sensible nature of parts. Bones, tendons, 

 cartilages, &c. to which this great physio- 

 logist denied these two properties, enjoy 

 only that lateral sensibility and obscure 

 contractility which are common to all 

 living beings, and without which we can- 

 not conceive the existence of life. In the 

 healthy state they are completely desti- 

 tute of the power of transmitting percep- 

 tible impressions to the brain, or of being 

 influenced by that organ to any manifest 

 motion. It has also been disputed whe- 

 ther sensibility and contractility depend- 

 ed on the existence of nerves ; whether 

 these were its necessary instruments, and 

 whether their injuries were followed by 

 a loss of those vital powers in the parts 

 which have nerves. We may answer in 

 the affirmative, as far as regards percep- 

 tive sensibility, and voluntary motion, 

 which is entirely subordinate to it ; but 

 in the negative, with respect to the sen- 

 sibility and contractility which are indis- 

 pensable in the processes of assimilation. 



Sensibility may then be either pefccep- 

 tive or latent. The former is attended 

 with a conscience of the impressions or 

 perceptibility, and requires a peculiar ap- 

 paratus. The latter, unaccompanied by 

 consciousness, is common to every thing 

 that fives ; it has no particular organs, 

 but is universally expanded in all living 

 parts, whether of vegetables or animals. 

 Contractility may be either voluntary and 

 sensible (vis nervosa), which is subordi- 

 nate to perceptibility; involuntary and 

 insensible, which corresponds to latent 

 sensibility; or involuntary and sensible 

 (vis insita), as in the action of the heart, 

 stomach, &c. 



The former species of sensibility be- 

 ing that which is observed in the func- 

 tions, which connect the animal with ex- 

 ternal objects, is called by Bichat, animal 

 sensibility ; and the corresponding con- 

 tractility is distinguished by the same 

 term. The other kind of these two vital 

 powers, which are exerted in the inter- 

 nal processes of nutrition, &c common to 

 animals and plants, that is, to all organiz- 

 ed bodies, is named the organic. 



Organic sensibility is merely the facul- 

 ty of receiving an impression ; animal sen- 

 sibility is the same faculty, with the ad- 

 ditional power of conveying it to a com- 

 mon centre. In the former case the ef- 

 fect terminates in the organ. The latter 



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