22 THE HUMAN BODY. 



ence of the stimulus is not local only, but in consequence 

 of it a change is brought about in other parts, arousing 

 them. This property of transmitting disturbances is known 

 as conductivity. 



Finally, the movements excited are not, as a rule, ran- 

 dom. They are not irregular convulsions, but are adapted 

 to attain a certain end, being so combined as to bring the 

 external particle into the interior of the cell. This capa- 

 city of all the parts to work together in definite strength 

 and sequence, to fulfil some purpose, is known as co-ordi- 

 nation. 



These Properties Characteristic but not Diagnostic. 

 These four faculties, irritability, conductivity, contractility, 

 and co-ordination, are possessed in a high degree by our 

 Bodies as a whole. If the inside of the nose be tickled 

 with a feather, a sneeze will result. Here the feather- 

 touch (stimulus) has called forth movements which are 

 mechanically altogether disproportionate to the energy of 

 the contact, so that the living body is clearly irritable. 

 The movements, which are themselves a manifestation of 

 contractility, are not exhibited at the point touched, but at 

 more or less distant parts, among which those of abdomen, 

 chest, and face are visible from the exterior; our Bodies 

 therefore possess physiological conductivity. And finally 

 these movements are not random, but combined so as to 

 produce a violent current of air through the nose tending 

 to remove the irritating object; and in this we have a 

 manifestation of co-ordination. Speaking broadly, these 

 properties are more manifest in animals than in plants, 

 though they are by no means absolutely confined to the 

 former. In the sensitive plant touching one leaflet will 

 excite regular movements of the whole leaf, and many of 

 the lower aquatic plants exhibit movements as active as 

 those of animals. On the other hand, no one of these four 

 faculties is absolutely distinctive of living things in the 

 way that growth ~by intussusception and reproduction are. 

 Irritability is but a name for unstable molecular equilibrium, 

 and is as marked in nitroglycerine as in any living cells; in 

 the telephone the influence of the voice is conducted as a 



