THE TISSUES 75 



becomes limp. In all probability this latter change is due 

 to a solution of the coagulated proteid by the enzyme 

 of the stomach pepsin which seems to exist in all the 

 tissues. This can act only in the presence of an acid, and 

 the appearance of sarcolactic acid, therefore, allows it to come 

 into play. 



II. NERVE. 



It is through the nerves that our surroundings act upon 

 us, and through nerves that our muscles are made to respond 

 appropriately to the surrounding conditions. 



1. Structure and Development. 



In unicellular organisms changes in the surroundings 

 act directly on the cell protoplasm, e.g. an amoeba, when 

 touched, draws itself together. But, 

 even in these simplest organisms, 

 certain kinds of external conditions will 

 produce one kind of change, while others 

 will produce a different one, as has 

 been shown in considering unilateral 

 stimulation (p. 16). Among unicellular 

 organisms somewhat higher than amoeba 

 the infusoria animals are found in 

 which the cell is differentiated into a 

 receiving and reacting part. Poterio- 

 dendron, a little infusorion sitting in 

 a cup-like frame, consists of a long pro- FlG - 33. Poteriodendron 



.,. ,. f r , to illustrate first stage 



cess or cilmm extending up ironi the in the evo ] u tion of a 

 cell body, while a contractile myoid neuro-muscuiar system, 

 attaches the cell body to the floor of 

 the cup. When the cilium is touched the myoid con- 

 tracts, and draws the creature into the protection of its 

 covering. 



But in more highly organised animals, where the reaction 

 has to be more definitely appropriate to the surrounding 

 conditions, and where the complexity of the mechanism 

 involved is greater, there is a development by which special 

 conditions at special parts of the surface each lead to a 



