NEURO-MUSCULAR MECHANISM 97 



B. Brain Apes in Reflex Action 



It is obviously of importance to an animal that its anterior 

 end, which in progression first comes into relationship with any 

 change in its surroundings, and in which is situated the mouth, 

 by which it feeds and therefore exists, should be well provided 

 with receptors, so that through them appropriate reactions of 

 the whole body to changes in the conditions may be produced. 



Hence it is in connection with the head that the most com- 

 plex development of extero-ceptive receptors (see p. 104) occurs. 

 Some of them, as those in connection with the spinal cord, respond 

 to stimuli in close proximity, e.g. the sensitive tactile whiskers 

 of the cat, and the receptors in the mouth, which produce 

 different reactions to nocuous and non-nocuous stimuli. But 

 in addition to these, others are developed which are acted upon 

 by stimuli coming from a distance, eg. light, sound, volatile 

 chemical substances, and these may be called distance receptors. 

 Their great importance is that by their action the body is 

 prepared for the more immediate contact with the external 

 condition, and is adjusted either to escape nocuous agencies 

 or to seize nutritive material, e.g. in the capture of prey. So 

 important is the action of these distance receptors in the higher 

 animals, that a mass of nerve tissue, the cerebrum, is developed 

 to bring about the reaction of the head and body generally to 

 these stimuli. In order that these reactions to the distance 

 receptors may be properly co-ordinated with the responses to 

 the non-distance receptors, such as the organs of touch, these 

 latter are closely linked to the cerebrum by definite bands of 

 nerve fibres (fig. 40). 



Still further to secure the appropriate and co-ordinated 

 action of the head and body to these distance stimuli, a 

 special proprio-ceptive mechanism is developed in the head, a 

 mechanism which is called into play by movements of the head, 

 and which thus assists in adjusting the position of the head and 

 body, just as the action of the proprio-ceptive structures in 

 muscles assist in adjusting the position of the limbs (see p. 102). 

 This mechanism is developed from the internal ear, and may be 

 termed the labyrinthine receptor mechanism (see p. 154). 



It is the dominant adjusting or balancing mechanism in the 

 body and it exercises a constant control over the proprio-ceptive 



7 



