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to the grosser kind of elements with which they come in contact. On whatever principle vegetable motions depend, elasticity or irritability, it is generally allowed that the motions of animals arise from the irritability of a muscular fibre. This irritability, however, though admitted as a fact, is obscure or inexplicable in its cause and mode of operation. It is no less mysterious how a simple muscular fibre should possess the inherent power of contraction and relaxation, should become weak or strong by exhaustion or tone; than it is how the simplest animal structure should be able to commence, continue, and suspend its spontaneous motions.
Of Animal Motions.
Beyond the sphere of vegetation, the system of vital irritability is presumed to commence; but the barrier is so ill defined, that we can scarcely tell where the first actually terminates, and where the latter begins.
Notwithstanding the subject of muscular motion has been investigated by some of the most learned and ingenious men, it still remains involved in the greatest obscurity. A medical writer well observes, that "although many curious observations have been made, and as far as the laws of dead mechanism can be ap
plied to a living machine, the investigators may have been successful: yet still there has been a ne plus ultra, a certain barrier by which their investigations have been limited, which no person has hitherto been able to pass, and which it is very improbable ever will be passed."
That unknown property by which a muscle when wounded, touched or irritated, contracts, indepen dently of the will of the animal that is the object of the experiment, and without its feeling pain, is called by Haller, its vis insita, or inherent power.
Now it is to be observed that the vis nervea, or nervous power, which Dr. Monro, in accounting for muscular motion, contrasts with the vis insita, comes to the muscle from without, that is, from the influence of a brain and nervous system; whereas the vis insita resides constantly in the muscle itself. The nervous power ceases when life is destroyed; the other appears, from experiments, to remain for some time after death; the nervous power is also suppressed by tying a ligature upon the nerve, by hurting the brain, or by taking opium. The vis insita suffers nothing from all these circumstances; it remains after the nerve going to the muscle is tied; it con. tinues in the intestines, though they be taken out of the body and cut in pieces; it appears with great strength in such animals as are destitute of brain : that part of the body is moved which has no feeling; and the parts of the body feel which are without
motion. The will excites and removes the nervous power, but has no power over the vis insita.*
Sir Gilbert Blane in his Croonian Lecture, delivered in the year 1788, considers, in the course of his reasoning, that the nervous system is not only a mere appendage to life, or muscular irritability, but that it tends to impede its operation, and to shorten its existence. Hence he maintains that muscular irritability does not depend upon a sentient principle. Many animals, it is well known, exist without brain or nerves. This was first observed by Haller, and was confirmed by Hunter: who maintains farther that the stomach is the centre or seat of life, and more essential to it than the brain. That the stomach should be an organ of so much consequence, seems natural enough, from the importance of its function, which is that of assimilation or nutrition; and life can be more immediately and completely extinguished by an injury done to it, such as a blow, than by the same violence to any other part of the body. It is also well known, as before observed, that the muscular fibres of animals endowed with a nervous system, will retain their irritability for some time after their separation from the brain and nerves. And from the phenomena of vegetation, he thinks irritability may exist in nature, without sensation, consciousness, or any suspicion of the existence of a nervous system. Besides, those animals which are
* Edinburgh Medical and Surgical Dict.
destitute of brain and nerves, are of the class vermes, the most simple in nature, having only one function, viz. that of assimilation; and therefore not requiring that variety of action, and those perceptions, which are pecular to more complex animals.
Lastly, the state of an egg before incubation, and the condition of those animals which become torpid from cold, and afterwards revive, afford facts which favour this opinion; as they show that there is a certain principle of self-preservation, independent not only of the operation of the nervous system, but also of the circulation: for, in this quiescent state, those portions of animal matter are preserved, for a great length of time, from that corruption to which they would otherwise be liable; and their fluids are prevented from freezing in a degree of cold which would congeal them, were they destitute of every principle of life.
Sir Gilbert Blane adds, that simple life will not only survive sensation, but will survive it longer, if the animal is killed by destroying the nervous system, than if it had been killed by hæmorrhage, suffocation, or other violence. If a fish, immediately on being taken out of the water, be stunned by a violent blow on the head, the irritability and sweetness of the muscles will be preserved much longer than if it had been allowed to die with the organs of sense entire. This is so well known to fishermen, that they put it in practice, in order to make them longer susceptible of the operation called crimping. A salmon is one of
the fishes least tenacious of life, insomuch that it will lose all signs of life in less than half an hour after it is taken out of the water, if suffered to die without any farther injury; but if, immediately after being caught, it receives a violent blow on the head, the muscles will show visible irritability for more than twelve hours afterwards.
The same author observes, that in warm-blooded animals, an excessive exertion of voluntary motion, immediately before death, prevents the muscles from being rigid when cold, and renders them more prone to putrefaction. Thus, if an ox is killed soon after, being over-driven, the carcase will not become stiff when it grows cold, nor is it capable of being preserved by means of salt. Hence also, in some disorders of the brain, as hydrocephalus, and apoplectic palsy, in which the functions of the brain are suspended, the office of digestion is sometimes better performed than in health.
Sir Gilbert Blane concludes with Hunter, that the exercise of sensation is inimical to mere animal life, and that a sort of fatigue is induced by this, as well as by voluntary motion; so that all that intercourse carried on through the nerves, whether towards the brain in the case of sensation, or from the brain in acts of volition, tends to wear out the animal powers, in other words, to exhaust the irritability. And, as intense and long continued thought, though not terminating in any outward action, tends also to produce an inability for farther exertions, it would