When we have once clearly distin 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 petrep 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 lives ; 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 belongs only to animals, whose perfec tion is in a direct ratio to the quantity of this sensibility. There is some reason for supposing that these two are not differ ent powers, but that they differ only in • quantity. For inflammation, which is an
increased action of parts, raises organic into animal sensibility in diseases of bones, &c.
Different stimuli, applied to the same organ, determine the des-elopement of one or other of these powers : thus no sensation is transmitted to the brain from the passage of blood in the arteries, but when an extraneous fluid is injected, the animal's cries shew that he feels it. The coats of the stomach experience in the healthy state no perceptible impression from the food, but very distinct and even painful sensations are transmitted to the brain when a few grains of poisonous mat ter are mingled with the aliment. The animal sensibility excited on mucous mem branes by foreign bodies (as bougies in the urethra, &c.) is quickly lost, and sub • sides into organic.
Each atrgan seems to have, indepen dently odiltccidental variations, an original quantity or dose of sensibility, to which it returns after any deviation. In this con sists the peculiar life of each organ, and from this arise the relations which it has to extraneous substances. Hence excre tory ducts, opening on mucous mem branes, refuse admission to the sub stances passing along those canals. Hence the lacteals absorb the chyle only. These particular relations may also take place with matters foreign to the body, as well as with animal fluids, as we see in the case of medicines acting on particular organs, as cantharides, mercury, &c.
Contraction is the common, but not the universal mode of animal motions. For the iris, corpus cavernosum, &c. dilate when they move. Organic contractility • is always and immediately connected with organic sensibility, for there is no inter mediate function between these ; the or gan itself is the point in which the sen sation ends, and from which the principle of contraction begins. The animal sensi bility and contractility are not so closely united ; we may feel without moving : here the nerves and brain perform their functions between the action of the two powers.
Sensible organic, or, in other words, involuntary and sensible contractility, cor responds very nearly to irritability ; while the insensible seems more like tenacity. To consider irritability as the exclusive endowment of muscles, is taking a very contracted view of the subject. These organs have indeed the largest portion, but every part possessing life reacts, al though less manifestly, on the applica tion of certain stimuli. No rule is more fallacious than that of estimating the mus cularity of a part by the action of arti ficial irritants. The organic and animal contractilities cannot be converted into each other as the corresponding sensibili ties can.