SUMMARY OF STRUCTURES 461 



mediate primates. In the first place, the median unpaired nucleus of Bischoff 

 has entirely disappeared and no structure resembling this aggregation of cells 

 in the dorsal cokimn is found in any of the three species here compared. 

 Inasmuch as this speciaHzation of the dorsal sensory nucleus appears only in 

 relation with those animaLs possessed of a prehensile tail or a tail used for 

 propeUing purposes, it has its importance in this fact. The nucleus of GoII, 

 according to its measurements, is larger in both the baboon and macacus 

 than it is in the gibbon. This disparity is considerable and stnkmg, for the 

 nucleus in gibbon is about half as large as in the other two forms. The impor- 

 tance of this inequality is notable because the gibbon belongs to those apes 

 which possess no tails; hence all of the sensory specialization pertaining to that 

 organ, in the capacity either of a prehensile, a steering or a balancing struc- 

 ture, has disappeared in the fiylobate species. The volume of atlerent 

 impulses arising from the tail in the other monkeys has also decreased in the 

 absence of prehensile function and by just so much the nucleus of Goll has 

 become reduced in size. In addition, it must be borne in mind that the lower 

 extremities in the gibbon are poorly developed ; that is to say, the legs are not 

 differentiated for locomotion on the ground. The main burden of locomotion 

 falls upon the upper extremities inasmuch as the animal's extreme develop- 

 ment of brachiation is the one outstanding physical feature of its motor 

 organization, from which fact the family has received the name of "tree- 

 walkers. " But their walking is conducted by means of the arms and hands 

 rather than the legs and feet. Thus the gibbon's inferiority in the develop- 

 ment of the column of Goll may be attributed to the dual elfect of absence of 

 a tail and certain functional depreciations in the lower extremities. The 

 baboon and macacus, on the other hand, have a fair representation of nuclear 

 substance in the nucleus of Goll, indicative of hind extremities equipped with 

 adequate proprioceptive organs for the receipt of discriminative impulses. 

 The afferent influx from the tail makes a not inconsiderable addition. 



