128 



M A M M A L I A. 



no precise information, and therefore the theoretical f 

 explanations which have been from time to time 

 attempted, have generally borrowed their foundation 

 from the particular department of science which hap- 

 pened to be most fashionable at the time of their in- 

 vention. Among the oldest of those theories, or rather 

 hypotheses, for none of them amounts to a theory, is 

 that of Borelii, who describes the muscles as consist- 

 ing of rhomboidal vesicles, which are empty and 

 stretched in length when the muscle is relaxed, but 

 filled with a nervous fluid, which enlarges their dia- 

 meter and increases their length when the muscle 

 contracts. The principal objection to this hypothesis 

 lies in the facts, that we are acquainted with no ner- 

 vous fluid which circulate?, and could be injected into 

 muscular vesicles by any mechanical power ; that 

 there is no structure in which such a power could 

 reside, unless we supposed the contractility of the 

 muscle to be transferred to it, which would be no 

 explanation, and that muscles retain this power after 

 all nervous connexion with them is broken off. That 

 action, and very powerful action, mechanical in its 

 effects, is produced by the contraction of muscles we 

 cannot deny, for we have instances of it before us 

 every day, and that the exercise of this action is inti- 

 mately connected with the general energy of all the 

 functions of the animal is also true ; but neither the 

 one nor the other of these, nor any other hypothesis, 

 mechanical or chemical, throws any light whatever 

 upon the real cause in which muscular motion ori- 

 ginates. 



In the mammalia, the muscles are more copiously 

 supplied with blood-vessels than any other parts 

 which are to be considered simply as working struc- 

 tures, and not engaged in secretion, or in specific 

 action upon the blood itself; and a degree of heat is 

 always excited by violent muscular action ; but it does 

 not appear that either of these is connected with 

 muscular irritability. For in other animals, which 

 have the circulation far slower, as for instance in rep- 

 tiles, the irritability of the muscles is much greater, 

 and they can be excited not only after the circulation 

 has been stopped, but after the limb containing them 

 has been separated from the body. 



Observation and experience would thus lead us to 

 suppose that muscular irritability is an antagonist to 

 the stimulus of the blood, to that of the nervous 

 agency, whatever that may be ; and generally speak- 

 ing to all other agents, external or internal of the 

 system, which can have an influence upon muscle ; 

 and that therefore the contractility, in which way 

 soever it originates, is a power perfectly sui generis, 

 and explainable in or by nothing but muscles them- 

 selves. When the muscles serve for the purposes of 

 propelling the contents of vessels, we may general!}' 

 suppose that the contents are the stimuli of these 

 vessels ; and we have pretty convincing evidence of 

 this in the fact that the introduction of artificial 

 stimulants, even in small quantities will increase the 

 excitement, while sedatives have a contrary effect. 

 The stimulus of arterial blood appears to be a power- 

 ful one, and one which is essential to the excitement 

 of muscular action, especially in the mammalia. Dr. 

 Fordyce notices a fact, which shows that the power 

 of muscular contraction can greatly exceed that of 

 the stimulus by which it is excited. Upon intro- 

 ducing a needle into the ventricle of a very recent 

 heart, and touching the inner surface very gently, the 

 ventricle contracted with so much force, as to force 



the needle deep into its surface. This is farther 

 illustrated by the well known fact, that animals, espe- 

 pecially when in a state of excitement, run forcibly 

 upon any weapon which pricks them ever so slightly, 

 and in this way horses, and also men, run upon wea- 

 pons in battle to their certain destruction. 



Every possible component part of muscular sub- 

 stance, and every agent which appears to affect the 

 action of muscles, have been investigated with the 

 utmost care ; but though it has been repeatedly 

 shown that many, if not all of these, are necessary to tho 

 production of muscular action, it has not been proved 

 that any one of them, or even the whole of them 

 taken jointly, are the real sources of that action. It 

 is unnecessary, therefore, to follow them further into 

 the details, although to profound physiologists those 

 details are of the utmost value. 



A power directly the opposite of that of contracting' 

 has sometimes been attributed to muscles ; but it 

 does not appear that there is any reality in the sup- 

 position, and indeed it is opposed by the whole 

 analogies of the case. We never find a contracting 

 muscle which tends to draw a part of an animal to 

 any distance in one direction, which is not accom- 

 panied by a counteracting power which can bring it 

 back again, or move it in the opposite direction. In 

 the limbs of the mammalia, and in all parts of their 

 bodies which are concerned in their external motions, 

 there are antagonist muscles set against each other 

 as it were ; but if the object is merely to bring back 

 a part to a certain position, it is not necessary that 

 the antagonist which effects this when the muscular 

 action is over should be a muscle. An elastic liga- 

 ment attendant, or even the cellular tissue, may effect 

 this purpose, and there is little doubt that the cellular 

 membrane which sheaths the fibres and fasciculi of the 

 muscles tends to restore them to their form in repose, 

 when the contracting action of the muscular fibres, 

 properly so called, is at an end. Many membranes 

 in the living animal possess a power of contracting in 

 every dimension, at least in extent of surface, under 

 the operation of proper stimuli. This is called cor- 

 rugation or crumpling up, and it enters to a consider- 

 able extent into the structure of those animals which 

 which have soft parts. In the mammalia, however, 

 which have the skeleton more perfect, and the mus- 

 cular system more distributed than any of the other 

 animals, muscular irritability is to be regarded as the 

 grand source of motion, and the only one to which it 

 is necessary to have much recourse in treating of 

 such actions of those animals as are necessary for 

 understanding their habits and adaptations in a 

 popular way. 



The action of the nerves is the next department of 

 the physiology of the warm-blooded animal ; and as 

 those nerves in the central part of their system are 

 protected with much more care than any of the other 

 parts, and are much more developed in the mammalia 

 than in animals possessing less sagacity and resource, 

 it is natural to conclude that they form a very im- 

 portant portion of the structure, and one which must 

 not be overlooked. This is, however, a branch of 

 the subject which is exceedingly difficult in itself, 

 and to the consideration of which we are in great 

 danger of bringing some prejudices, or at all events 

 some mistakes, along with us to the contemplation of 

 it. In all the more complicated phenomena of the 

 living animal, particularly those of sensation, and of 

 action as consequent upon sensation, we are in con- 



