352 NERVOUS SYSTEM OF VERTEBRATES. 



bordering on them. A comparison of Figures 175 and 176 with 

 Figures 177 and 178 shows that around the senso-motor areas 

 a number of border zones have been myelinated while the central 

 zones of the association fields remain without myelin. This 

 seems to indicate that when the senso-motor areas become func- 

 tional the association areas immediately adjoining them first come 

 into relation with them, and the central parts of the association 

 fields become functional last. A difference in function, then, is 

 probably to be attributed to the border zones and the central 

 zones of the association fields. 



By association fields must not be understood areas in which 

 the functions of association are carried out without the aid of the 

 senso-motor areas. This would be physically impossible and 

 the term correlation centers would more truly express the function 

 of the association fields. In other words, the function of the 

 association centers is to correlate the actions of the senso- 

 motor centers. The cells in the senso-motor areas are by no 

 means all connected with projection fibers; only a few of them 

 give rise to such fibers. The remainder give rise to association 

 fibers of greater or less length. The shorter ones serve to connect 

 the layers in the same gyms or to carry impulses to adjacent gyri. 

 The longer ones carry impulses to more distant parts of the cortex. 

 It appears that these fibers do not go from one senso-motor area 

 to another, but that the several senso-motor areas are brought 

 into relation through the association centers. The fibers reach 

 first to the adjoining border zones of the association fields and 

 later to the central zones and even to more distant association 

 centers. The longer association fibers enter into the formation 

 of certain long association tracts of which there may be mentioned : 

 (i) the fasciculus longitudinalis superior or arcuatus, apparently 

 connecting the occipital and frontal lobes; the fasciculus long- 

 itudinalis inferior, connecting the occipital and temporal lobes; 

 the cingulum, connecting the hippocampus and perhaps other 

 olfactory nuclei with the lower gyri of the frontal lobe; the fas- 

 ciculus uncinatus, connecting the temporal lobe with the same 

 region of the frontal lobe; and the fasciculus occipito-frontalis 

 (tapetum) which Dejerine believes consists of fibers arising in 



