SOME EYOID UINTS. 137 



the ear of Picus the tongue conveys aid, and 

 from the olfactory organ of Colaptes the 

 tongue receives aid — that in each case the 

 hyoid cornua are auxihary to an organ other- 

 wise powerless to perform the task required 

 of it by a narrow and exacting habit of life. 

 In other words, if Picus could have heard the 

 worm stir in the wood, we should not have 

 found his tongue-bones seeking his ear, and 

 if Colaptes had been blessed with a fine sense 

 of taste, his nostrils would not have been in- 

 vaded by his hyoid cornua in quest of aid. 



The sheath or nerve-wisp in which the 

 tongue-bones (hyoid cornua) of Colaptes are 

 inclosed, runs to near the anterior extremity 

 of the upper mandible, after passing into the 

 right nostril. Each movement of the bird's 

 tongue must be felt in its nasal cavity, there- 

 fore, and to what extent the gustatory sense 

 may thereby be aided is not altogether infer- 

 ence. The extremity of the sheath is nerve- 

 tissue and muscular fiber, and is obscurely 

 connected with the tissue lining the nasal 

 cavity, and this, taken along with the facts 

 already given, makes it clear to my mind that 

 the intrusion of the tongue-bones and tongue- 

 nerves into the olfactory space is not acci- 

 dental any more than their projection around 

 the right eye to the ear of Picus is accidental. 



To a degree, atrophy and hypertrophy of 

 the limbs and organs of animals are the out- 

 come of what may be called hereditary desire 

 arising out of a need, negative or positive. 

 A negative need induces atrophy, as where 

 the need of losing a useless limb slowly with- 

 ers it by non-use. A positive need induces 

 hypertrophy, as where the over-development 

 of a limb is caused by a continuing demand 

 upon its strength and action. 



In all birds the motor nerves of the tongue 

 pass directly to the brain, and in the wood- 



