1002 EVOLUTIONAL MODIFICATIONS 



EVOLUTIONAL MODIUCATIONS IN THE DORSAL SENSORV FIELD 



Another scries of structures represents the centers for myo-articular 

 proprioceptive impulses from the extremities and heacL These comprise 

 the nuclei of the dorsal sensory field, inchiding the nucleus of Goll, of 

 Burdach and of Rolando. In their coellicient expressions these sensory 

 elements maintain a fairly constant equality throughout the primate 

 order. There arc certain lluctuations in their total rt'presentatinn which 

 depend upon extreme developments, such, for example, as tiie prehensile tail. 

 It is claimed that this organ, in many instances, ser\es as a lifth hand. This 

 is true in most of the South American monkeys whose specialization in this 

 respect has been described in Mycetes seniculus. 



More important than the total volumetric representation of the dorsal 

 sensory nuclei are those relations between them which indicate variations in 

 the differentiation oi the extremities. The column and nucleus oi Goll repre- 

 senting the lower extremity, foot and tail, when compared with the column 

 of Burdach and its nucleus, the oblongatal representatives of the upper 

 extremity and hand, show a progressive diminution in M)lume in passing from 

 the lowest to the highest primate. This transitional disproportion from a 

 status in w hich the clava (column and nucleus of Goll) is distinctly larger 

 than the cuneus (column and nucleus of Burdach) to a condition comjjietcly 

 reversing this relation in which the cuneus is at least twice the size of the 

 clava, adxances b\- gradient stages. 



The two sensory elenu'iits areal)()ut (,'C|ual ui \lacacus rhesus and gorilla, 

 but in gibbon, orang and chimpanzee, the balanet' begins to turn iii la\'or of 

 the cuneus. This relation between the two great elements of the dorsal sensory 

 field indicates the evolutional ])rocess which has c-wntually resulted m the 

 production ot a bimanal type, ca])able of the t-rect posturi' and l)i]K'dal loco- 

 motion. Such s]iecialization e\()l\ed out of a common and more generalized 



