76 PHYSIOLOGY CHAP. 



cheinotactic activity in the leucocytes of vertebrate blood. The 

 products of the metabolism of pathogenic bacteria exert a marked 

 chemotactic action upon them (Fig. 19), a fact which is of great 

 importance in the interpretation of the inflammatory phenomena 

 of infective diseases, as we shall see in discussing Blood. 



VIII. Mechanical stimuli (blow, contact, puncture, shake, 

 pressure, etc.) are the simplest means of provoking excitation 

 in living matter. The least shake of the object- carrier on which 

 the movements of an amoeba are being watched under the 

 microscope is sufficient to produce temporary standstill, and if the 

 impact is strong enough a partial retraction of the pseudopodia. 

 If the shock is repeated at frequent intervals the effects induced 

 by each stimulus summate, resulting after a minute or two in a 

 true mechanical tetanus, during which there is a concentric 

 contraction of the whole of the protoplasm, which causes the 

 amoeba to assume a globular form. 



In addition to general mechanical stimulation, the effects of local 

 stimulation have been experimentally studied, by touching or stab- 

 bing the amoeba with a blunt body or with very fine needles. In this 

 case, when a reaction appears, it is at first confined to the point 

 stimulated, whence it is slowly transmitted to the rest of the body. 



The mechanical excitations of the living matter consist 

 for the most part in a modification of the pressure relations 

 under which it exists. In every case in which there is a 

 difference of pressure at two different parts of the body of any 

 organism, phenomena of excitation are manifested, which, since 

 they are produced by a unilateral pressure, are known as barotactic. 

 Several forms of larotaxis may be distinguished according to the 

 kind of pressure, while it also can be positive or negative, according 

 as the organism turns towards the side of greater or less pressure. 



Verworn groups under the name of thigmotaxis the tendencies 

 exhibited by many organisms, both animal and vegetable, to 

 adhere to the surface of more solid bodies, or to penetrate through 

 their pores, even in defiance of gravity. 



Stahl defines as rheotaxis the peculiarity certain organisms 

 exhibit of moving in the direction contrary to a current of water. 

 Since this movement is determined by pressure acting in a particular 

 way, rheotaxis is merely a special form of positive b'arotaxis. 

 Thus far the phenomenon has been studied only in the plasmodia 

 of Myxomycetes and in a few plants ; but it is highly probable that 

 the rise of the spermatozoa in animals and man from the vagina 

 to the uterus, and thence to the oviduct to meet the ovum, is a 

 rheotactic phenomenon, since this movement is accomplished in a 

 direction contrary to that of the current of mucous fluid set up by 

 the cilia of the epithelial cells which line the surface of the uterus, 

 and which vibrate in a direction contrary to the movements of the 

 spermatozoa, 



