98 Instinct and Intelligence 



being sensory in function. An irritant applied 

 to parts of the skin containing tactile organs 

 travels along sensory or afferent nerve fibres to 

 corresponding central nerve cells, thus releas- 

 ing a part of their potential energy which 

 extends to a motor cell, and thus reaches 

 muscles or other structures to which the motor 

 fibres are distributed, becoming manifest in a 

 definite kind of work. A system of this kind 

 is known as being a nervous arc; when stimu- 

 lated it gives rise to what is termed reflex 

 action, which, though strictly automatic, is 

 generally adapted to promote the well-being of 

 the organism. 



The spinal cord after entering the skull 

 expands to form the hind-brain; this is pro- 

 longed upwards by means of strands of nerve 

 fibres into the "most constant portion of the 

 brain of vertebrates, the mid-brain." * (Figs. 1 1 

 and 13.) This latter part of the brain is con- 

 tinued onwards into the fore-brain. The cere- 

 bellum is situated above the bundles of nerve 

 fibres which connect the hind- and mid-brain. 



1 The Nervous System of Vertebrates, by Prof. J. B. Johnston, 

 pp. 223, 261. 



