THE FROG 91 



This intervening link is termed the connector neuron. In many cases 

 even more neurons than this are involved, and the reflex involves 

 quite a complex system of neurons and muscles. Examples of these 

 reflexes can readily be shown in a frog whose brain has been 

 destroyed. If such a frog be suspended and a toe pinched, or the 

 toes dipped in weak acid, the foot will be withdrawn. A spot of 

 weak acid placed on the abdomen will be kicked off by the hind foot, 

 and various other similar actions can be brought about, all of them, 

 as we should say, purposeful. It will be seen that in these cases it 

 is not merely the muscle actually touching the skin that is con- 

 cerned, but the whole of the muscles of the leg and foot related to 

 and bringing about the movement of the part. Reflex action plays 

 a considerable part in the ordinary activity of the higher animals, 

 and the life of the lower animals appears to be almost entirely made 

 up of a large number of these co-ordinated reflexes often of a complex 

 nature. 



When the impulse enters the spinal cord by the dorsal root it is 

 not only sent on to the remaining part of the reflex arc, but it is also 

 transmitted to the dendrons of the cells running longitudinally in the 

 spinal cord. These cells, in their turn, hand it on to similar cells and 

 so form a relay path by which the message is conveyed to the brain, 

 where it may produce various results according to the requirements 

 of the animal. In our own case, although many of these reflexes 

 are very powerful, it is possible to override them by an effort of the 

 will. Thus, for example, if we purposely pick up a hot coal we may 

 suppress the reflex to such an extent that we do not drop the coal, 

 but allow it to burn our fingers. One important function of the brain, 

 then, is to receive the stimuli brought in from all parts, and if neces- 

 sary to correlate and modify the local reflexes in such a manner as 

 to make for the well-being of the animal as a whole, and not merely 

 of its individual parts. The better able an animal is to order its 

 reflexes to meet the demands of its environment in the very widest 

 meaning of the word, the more advantages it possesses over its 

 fellows not so well endowed, and the more highly developed we 

 regard it. 



Sense Organs. 



The sense organs comprise the organs of smell, sight, touch, 

 taste and hearing. Three of these, the first two and the last, are 

 compact and surrounded by supporting structures known as capsules, 

 and the other two consist of numerous tiny spots scattered over a 

 more or less wide area. 



The olfactory organ, the organ of smell, is localised in the olfactory 

 capsule, a cartilaginous structure situated at the anterior end of the 



