u6 



THE KINGDOM OF MAN 



the thyroid yields a substance which influences the 

 growth of the skin, hair, bones, &c. ; the pituitary gland, 

 an extract which is a specific urinary stimulant. Quite 

 lately the mammalian ovary has been shown by Starling 

 to yield a secretion which influences the state of nutri- 

 tion of the uterus and mammae. A great deal more 

 might be said here on topics such as these topics of 

 almost infinite importance ; but the fact is that the 

 mere enumeration of the most important lines of pro- 

 gress in any one science would occupy many pages. 



Nerve-physiology has made immensely important ad- 

 vances. There is 

 now good evidence 

 that all excitation 

 of one group of 

 nerve-centres is ac- 

 companied by the 

 concurrent inhibition 

 of a whole series of 

 groups of other cen- 

 tres, whose activity 

 might interfere with 

 that of the group 

 excited to action. 

 In a simple reflex 

 flexure of the knee 

 the motor-neurones to the flexor muscles are excited, 

 but concurrently the motor-neurones to the extensor 

 muscles are thrown into a state of inhibition, and so 

 equally with all the varied excitations of the nervous 

 system controlling the movements and activities of the 

 entire body. 



The discovery of the continuity of the protoplasm 

 through the walls of the vegetable cells by means of con- 



FIG. 33. 



The continuity of the protoplasm of neigh- 

 bouring vegetable cells, by means of threads 

 which perforate the cell- walls. Drawing 

 (after Gardiner) of cells from the pulvinus of 

 RoUnia. 



