578 



GENERAL PHYSIOLOGY 



the more we find the tendency of the central nervous system to 

 extend its authority toward a unified control of all cells and cell- 

 complexes of the animal body. In order to make graphic the 

 principle upon which the mechanics of the central nervous system 

 is based, it will be advantageous to consider the simplest form in 

 which the function of the latter is expressed, namely, the reflex 

 action. 



The essence of the reflex action consists in the fact that an element 

 that appreciates stimuli and an element that reacts to stimuli are 

 so put into relation with one another by a central bond, that every 

 stimulus acting upon the appreciating element is conducted first 

 to the centre, and thence, as an impulse to a reaction, to the 

 reacting element. Such a mechanism, in which every stimulus 

 acting upon the sensory end calls out with machine-like certainty 

 a reaction at the other end, is a reflex arc. The most primitive 



Fia. 279. Primitive reflex arc in a single cell. /, Poteriodendron , a flagellated cell fixed in a 

 cup-shaped sheath upon a myoid-fibre. //, Neuro-muscular cells from an actinian. (//, 

 after Hertwig.) 



form of a reflex arc exists in unicellular organisms, the cell-body 

 of which possesses both the sensory and the motor elements, and 

 even functions also as the central bond for the two. A single 

 Poteriodendron represents a reflex arc of the simplest kind 

 (Fig. 279, /). The cell-body, fixed upon a myoid-fibre at the 

 bottom of a delicate, cup-shaped sheath, bears a flagellum which is 

 extremely sensitive. The slightest stimulus which acts upon the 

 latter is conducted centripetally to the cell-body, and from there 

 centrifugally to the myoid-fibre, and the action of the stimulus 

 upon the flagellum is followed at once by the contraction of the 

 fibre. Wholly analogous to this is the behaviour of Vorticella, 

 except that in the latter the sensory elements are present chiefly 

 in the form of the cilia of the peristome. The same relations, 

 further, exist in the so-called neuro-muscular cells of the Ccelen- 



