i6 Organic Nattcres Riddle 



which a proof of the real existence of such a thing as 

 ' instinct ' must necessarily be fatal. Instinct has been de- 

 fined ^ as a ' special internal impulse, urging animals to the 

 performance of certain actions which are useful to them or 

 to their kind, but the use of Avhich they do not themselves 

 perceive, and their performance of which is a necessary 

 consequence of their being placed in certain circumstances/ 

 Such an impulse is always understood to be the result of 

 sensations; actions which take place in response to unfelt 

 stimuli being referred, not to instinct, but to what is termed 

 reflex action. In such action it is commonly supposed that 

 the mechanism of a living body occasions a prompt responsive 

 muscular movement upon the occurrence of some unfelt 

 stimulation of the nervous system. The nervous system, 

 or total mass of nerve-stuff — which is technically called 

 ' nerve-tissue ' — in the body of an animal, such as a beast, 

 bird, reptile, or fish, is composed of two parts or divisions. 

 One of these divisions consist of a voluminous and continuous 

 mass — the brain and spinal cord (or spinal marrow), which 

 form what is called the central part of the nervous system. 

 The second division consists of a multitude of white threads 

 or cords — the nerves, which form what is called the peripheral 

 part of the nervous system. Of these nerves one set proceed 

 forth from the central part of the nervous system to the 

 different muscles, Avhich they can cause to contract by a 

 peculiar action they exert upon them, thus producing motion. 

 Another set of nerves proceed inwards, from the skin to the 

 central part of the nervous system, and by their peculiar 

 action give rise to various sensations, according as different 

 influences or stimulations are brought to bear upon the skin 

 at, or in the vicinity of, their peripheral extremities. Under 

 ordinary circumstances, different stimulations of the surface 

 of the body convey an influence inwards, which produces 



^ See Todd's Cydopoedia of Anatomy and Physiology ^ vol. iii. p. 3. 



