REGULATORS 587 



the heart, and these manifest their characteristic regular 

 rhythmic pulsation. 



Scraps of some organs when removed and transplanted in 

 other parts of the body may grow, and the cells may multiply 

 and develop into those characteristic of the organ from which 

 they were taken. 



Hereditary inertia seems to be all-powerful in early embry- 

 onic life, and its influence extends on into the adult condition. 

 Hooker found that, even after all that part of the central nervous 

 system from which the nerves to the heart arise has been 

 destroyed in the tadpole, the heart develops and beats in the 

 usual way. 



Its influence is dominant both in structural develop- 

 ment and in the development of functional activity, not only 

 in such simple actions as cardiac contraction, but in the most 

 complex responses of the central nervous system upon which 

 the conduct of the individual depends. 



II. The Nervous System. 



As development advances, the nervous system comes to 

 play a part in the regulation of metabolism, and thus in the 

 development of the static adaptations. Its effect is seen in 

 the failure of recreneration after removal of structures in some 

 invertebrates and in many amphibia if the nerves to the part 

 are destroyed. It is also seen in infantile paralysis, a disease 

 which follows destruction of the cells of the anterior horn of 

 grey matter. The growth of the limb connected with that part 

 of the spinal cord becomes arrested. The condition of herpes 

 zoster, or shingles, a painful eruption of vesicles on the skin 

 over a nerve, has already been considered (p. 91) as an example 

 of the trophic influence of the nervous system. 



The part played by the nervous system in regulating and 

 co-ordinating the functional adaptations, i.e. the activities of one 

 structure with those of others has been repeatedly indicated in 

 the previous pages. The effect of a heightened arterial 

 pressure in inhibiting the heart through the inferior cardiac 

 branch of the vagus may be taken as an example; and that this 



