I 



PHYSIOLOGICAL 379 



straggling cells and a non-living ground substance of very variable 

 nature, according to the particular kind of tissue. This brief survey 

 of cell-forms may fittingly conclude by contrasting the mobile male 

 germ-cell or spermatozoon, with its small head composed almost 

 wholly of nuclear material, and its vibratile tail, with the large, 

 round, sluggish female germ-cell, whose characteristic response to 

 such a stimulus as the entry of the spermatozoon — that of dividing 

 and growing and redividing until the complexity of the parent 

 body is reproduced — is the most marvellous of all the faculties of 

 cells, and one, of course, entirely lacking in such a generalised type 

 of cell as the leucocyte. 



IRRITABILITY. — All living cells have the property of being irri- 

 table or reactive, of changing within themselves in response to 

 changes in their environment. In the case of many irritable 

 tissues, the stimulus of an increase in the sodium content of the 

 medium causes a reaction or response, namely, an increase of per- 

 meability of the cell-wall, while the response to increased calcium 

 content is just the opposite. Cells are sensitive to other changes in 

 the environment besides purely chemical ones; for example, the 

 cells of the organs of smell and taste are sensitive to chemicals, but 

 the cells of the retina to changes in illumination, and various nerve- 

 endings in the skin to changes of temperature or to pressure. In all 

 cases the important response is that a message is sent along a nerve- 

 fibre towards the brain. Single-celled organisms are also sensitive to 

 light, heat, chemicals, contact, or electrical currents, and may 

 react by various changes in their behaviour. Or again, gland-cells 

 secrete or muscle-cells contract in response to a stimulus, which 

 may be a nerve-impulse or an electrical or chemical change. 



The case of muscle-fibres may be examined rather more closely. 

 A change in the relative concentration of the salts of the medium 

 may produce a contraction, but in certain conditions (anaphylaxis) 

 this response is evoked by the appearance of a particular protein 

 and by no other — a remarkably delicate selective reaction. In the 

 case of electric stimulation, an important fact stands out: the muscle 

 is not affected by the continuous flow of a current, but does respond 

 to any change, whether increase or decrease, if it is sufficiently 

 great and sufficiently sudden. Moreover, the reaction is always the 

 same, both in type and in quantity: for striped muscle-fibres, nerve- 

 fibres, and various other kinds of irritable cells, obey the "all-or- 

 none" rule; there is complete contraction or there is none; a nervous 

 impulse is sent at constant strength if it is sent at all, and so in the 

 other cases. Finally, it should be noted that after response there is 

 a refractory period, during which the irritable cell is not sensitive 

 to stimulus, although in the case of nerves this may be exceedingly 

 short : there is some process of reloading, or rebuilding of a structure 



