104 MOLAR AGENTS AND PROTOPLASM [Cn. IV 



that non-vital semi-fluid substances tend to assume a spherical 

 form by virtue of the property of surface tensions. That pro- 

 toplasm does not always assume this form is due to special 

 causes. When a Protist or one of its pseudopodia is irritated 

 by contact, it tends to assume a spherical form or a thread tends 

 to aggregate into spherical drops. It seems probable, we can- 

 not say more than that, that this aggregation is due to a dimi- 

 nution in the activity of those causes which oppose the action 

 of surface tension ; and so the latter reasserts itself. It is 

 likewise possible that new attractive centres arise. That a 

 thread should break up into drops indicates, moreover, a loss 

 in cohesion. Loss of cohesion, formation of new centres of 

 attraction, and diminution of the form-maintaining forces, - 

 these seem to be the effects of contact. They must be due to 

 the chemical changes wrought by contact. 



The changes just referred to constitute the essence of con- 

 traction, a phenomenon of widespread occurrence not only among 

 Protista, but among the higher plants and animals ; for ex- 

 ample, in the sensitive plant and in Vertebrate muscle. Into 

 these contraction phenomena which follow contact in the higher 

 organisms we cannot go ; their study belongs to the field of 

 plant and animal physiology. At bottom, however, we must 

 believe many of these phenomena in the higher organisms to 

 be due to the same causes as contraction in Protista. 



A few words concerning rhythmically repeated disturbances. 

 A single disturbance gives rise, as we have seen, to a series of 

 phenomena producing contraction; but in a few seconds the 

 effects of the disturbances are past and the protoplasm returns 

 to its uncontracted form. If, however, the shock is repeated 

 before relaxation has fully occurred a new contraction is super- 

 imposed on the first, and the resulting contraction is more 

 violent than a single one. If now shock follow shock in quick 

 succession, a violently contracted condition, known as tetanus, 

 results. Under the condition of tetanus the amoeba becomes a 

 spherical mass, Actinosphserium retracts all of its pseudopodia, 

 a branching Carchesium stock forms a little ball, and muscle 

 fibres are greatly shortened. In a word, rhythmically repeated 

 shocks are accompanied by an exaggeration of those changes 

 which result from a single shock. 



