24 THE HUMAN BODY. 



in Physiology as a stimulus) and the result, which is the es- 

 sential characteristic of irritability when the term is used in 

 a physiological connection. The granular cells of the blood 

 can take foreign matters into themselves in exactly the same 

 manner as an Amoeba does; and in this and in other ways, as 

 by contracting into rigid spheres under the influence of elec- 

 trical shocks, they show that they also are endowed with irri- 

 tability. 



Conductivity. Further, when an Amoeba or one of these 

 blood-cells comes into contact with a foreign body and pro- 

 ceeds to draw it into its own substance, the activity excited 

 is not merely displayed by the parts actually touched. Dis- 

 tant parts of the cell also co-operate, so that the influence of 

 the stimulus is not local only, but in consequence of it a change 

 is brought about in other parts, arousing them. This prop- 

 erty of transmitting disturbances is known as conductivity. 



Finally, the movements excited are not, as a rule, random. 

 They are not irregular convulsions, but are adapted to attain 

 a certain end, being so combined as to bring the external par- 

 ticle into the interior of the cell. This capacity of all the 

 parts to work together in definite strength and sequence to 

 fulfil some purpose, is known as co-ordinations 



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 (stim- 

 ulus) 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 physio- 

 logical 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. Speak- 

 ing broadly, these properties are more manifest in animals 

 than in plants, though they are by no means absolutely con- 

 fined to the former. In the sensitive plant touching one leaflet 

 will excite regular movements of the whole leaf, and many of 



