DI-ATll. 





Tested even in the lowest grades. But we must 

 observe, that throughout the whole range of 

 animal existence we can more readily ascertain 

 the changes produced in molecular action by 

 diminished respiration, than by the entire sus- 

 pension of this function ; because, in the first 

 place, the arrest of the circulation so soon fol- 

 lows that of respiration, that the subsequent 

 events are assignable rather to the former than 

 to the latter; and in the second place, it is 

 impossible to cause one portion of the body 

 to receive nnaerated, while the others are sup- 

 plied with aerated blood, since the function is 

 in some animals too concentrated to allow of an 

 operation calculated to act upon an isolated 

 part, and in other animals too diffuse to enable 

 us to interfere with it effectually in any given 

 space. In the one case we run the risk of 

 cutting off' the supply of blood from the whole 

 animal ; in the other we should find it im- 

 possible to prevent any one part from receiving 

 from other parts a compensation for what it 

 loses by the obstruction of its own particular 

 allotment of the respiratory function. Nothing 

 however is more common than to witness the 

 degeneration of structure produced by blood 

 insufficiently arterialized, the imperfection of 

 the process depending either upon disorder in 

 the organs of respiration, or upon a vitiated 

 condition of the atmosphere. From facts of 

 this nature it. is legitimate to infer that were it 

 possible for unarterialized blood to circulate, 

 the death of the tissues must sooner or later 

 ensue. Of the destructive tendency of blood 

 depraved by the other causes above enu- 

 merated we can likewise judge approxima- 

 tively ; in other words, while there can be no 

 question of the deterioration of structures under 

 the operation of those causes, we are not ac- 

 quainted with any instances in which we can 

 attribute solely to their agency the entire cessa- 

 tion of molecular actions. It almost always 

 happens that other functions have previously 

 failed, and influenced the result in question. 



Extinction of irritability. Irritability might 

 at first seem rather the result of vitality than 

 one of its conditions ; but whether we look at 

 the textural motions in a complex animal, or at 

 their analogues in the entire systems of the 

 simpler forms, we shall find irritability to be 

 essential to the continuance of those processes 

 in which living action consists. The alimen- 

 tary cavity which contracts upon the nutrient 

 fluid of the zoophyte is no less essential to the 

 existence of the latter, than a similar action of 

 capillary tubes in the tissues of mammalia. In 

 each case the action is requisite, in order to 

 bring the particles within the spheres of the 

 textural affinities. The extinction of irritability 

 is therefore necessarily productive of molecular 

 death. In this instance we are compelled to 

 speak of the privation of a property instead of 

 defining the actual change in the part, because 

 at present it is not ascertained what condition 

 of the part is capable of producing contraction. 

 Irritability is merely an expression of the fact 

 that the substance of which it is predicated, 

 undergoes contractions inexplicable on common 

 physical principles. We detect nothing in the 

 VOL. r. 



substance, the- existence of which enables us 

 to pronounce with certainty that it may be the 

 subject of the actions alluded to. Some have 

 maintained that irritability ought to be ad- 

 mitted as an ultimate fact, of which we know 

 as much as of gravity. But we apprehend that 

 there is this great difference in our knowledge 

 of the two properties, viz. that although igno 

 rant of the cause of the attraction of gravitation, 

 we are certain that the phenomena are co- 

 extensive with the essential properties of mat- 

 ter; but we are utterly unacquainted with that 

 collection of properties to which irritability 

 necessarily belongs. The muscle which has 

 ceased to quiver under the galvanic wire is, 

 for all that we can tell to the contrary, the 

 same in composition as that which is still ca- 

 pable of exhibiting the phenomenon. More- 

 over the action is observed in a great variety 

 of tissues, both in individual animals, and in 

 the whole series ; tissues which appear to have 

 little in common saving a fibrous arrangement 

 of their particles. But as the action in question 

 is stopped by causes which in no way affect 

 the fibre as such, it is plain that this is not the 

 only requisite. Moreover there are unequi- 

 vocal exhibitions of contractility in animals, in 

 which it is difficult to imagine that there can 

 be any shortening of fibres; we allude to the 

 Infusoria, Rotifera, Medusae, &c. Tiedemann 

 makes a separate species of this contractility, 

 under the designation of " contractilite" des 

 animaux gelatineux." * There is reason to 

 suspect that ganglionic tissue is importantly 

 concerned in the action, partly because it is 

 almost universally distributed through irritable 

 substances, and partly because contraction is 

 prevented by causes which operate upon this 

 tissue. As long however as there are animals 

 which manifest contractions, but in which no 

 such tissue can be detected, it is impossible to 

 consider the latter an essential element in the 

 action generally ; though it may be quite es- 

 sential in the animals in which it is found ; 

 just as a heart, though by no means necessary 

 to the function of circulation in the abstract, is 

 indispensable in the animal of whose system it 

 forms a part. 



Irritability may be destroyed by substances, 

 either applied directly to the part or acting 

 upon the general system. Thus, the fibres of 

 the heart may be paralysed by a solution of 

 opium injected into its cavities, or by essential 

 oil of tobacco given by the mouth. Light- 

 ning annihilates the property all over the body. 

 The motions of Infusoria may be arrested by a 

 shock of galvanism,f by solutions of opium 

 and camphor, and by the vapour of sulphur. 

 Arsenical preparations have a similar effect. 

 The contractions of capillary vessels in the 

 higher animals may be arrested by a certain 

 description of injuries of the brain and spinal 

 marrow.]; But it is needless to multiply ex- 

 amples. 



* Traite complet de Physiologie de 1' Homme, tra- 

 duit de TAllemand par A. J. L. Jourdan, D.M.P. 

 2de partie, p. 782. 



t Tiedemann, p. 617. 



J See Wilson Philip on the Vital Functions. 



:; i 



