GENERATION AND DEVELOPMENT. 559 



by internal causes. Neuralgic pains, the sensation of rigor, 

 formication or the creeping of ants, and the states of the sexual 

 organs occurring during sleep, afford striking examples of sub- 

 jective sensations. 



The mind, also, has a remarkable power of exciting sensa- 

 tions in the nerves of common sensibility ; just as the thought 

 of the nauseous excites sometimes the sensation of nausea, so 

 the idea of pain gives rise to the actual sensation of pain in a 

 part predisposed to it. The thought of anything horrid excites 

 the sensation of shuddering; the feelings of eager expectation, 

 of pathetic emotion, of enthusiasm, excite in some persons a 

 sensation of " concentration" at the top of the head, and of cold 

 trickling through the body ; fright causes sensations to be felt 

 in many parts of the body ; and even the thought of tickling 

 excites that sensation in individuals very susceptible of it, 

 when they are threatened with it by the movements of another 

 person. These sensations from internal causes are most fre- 

 quent in persons of excitable nervous systems, such as the 

 hypochondriacal and the hysterical, of whom it is usual to say 

 that their pains are imaginary. If by this is meant that their 

 pains exist in their imagination merely, it is certainly quite in- 

 correct. Pain is never imaginary in this sense ; but is as truly 

 pain when arising from internal as from external causes ; the 

 idea of pain only can be unattended with sensation, but of the 

 mere idea no one will complain. Still, it is quite certain that 

 the imagination can render pain that already exists more in- 

 tense and can excite it when there is a disposition to it. 



CHAPTER XX. 



GENERATION AND DEVELOPMENT. 



THE several organs and functions of the human body which 

 have been considered in the previous chapters, have relation 

 to the individual being. We have now to consider those 

 organs and functions which are destined for the propagation 

 of the species. These comprise the several provisions made 

 for the formation, impregnation, and development of the 

 ovum, from which the embryo or foetus is produced and gradu- 

 ally perfected into a fully-formed human being. 



The organs concerned in effecting these objects are named 



