HANDBOOK OF PHYSIOLOGY ■■^ NEUROPHYSIOLOGY II 



potential evoked in the acoustic area by a sensory 

 volley has been observed as a consecjuencc of callosal 

 inputs (cf. 55). This phenomenon is found in the 

 encephale isole cat but not in anesthetized animals 

 (87). All synchronous volleys reaching a given sensory 

 cortical area in one hemisphere produce a response in 

 the homologous area of the opposite hemisphere, and 

 the impulses responsible for this response have been 

 shown to cross through callosal fibers (60). 



Despite all the evidence indicating an important 

 exchange of messages between the cortex of the two 

 hemispheres through the corpus callosum, no infor- 

 mation as to the function of these connections has 

 been gleaned from transecting it surgicallv. In the 

 cat, dog and monkey, no changes in the spontaneous 

 behavior of the animal, nor deficits in sensory or 

 motor functions, seem to occur as a result of section 

 of the corpus callosum in the absence of intercurrent 

 cortical damage (cf. 58 for references). This factor 

 may well account for gross functional changes seen 

 by early workers after callosal section, as discussed by 

 Bremer et al. (58). Minor changes, as observed by 

 other authors, following this procedure (234), re- 

 semble in many respects the effects of cingulate corti- 

 cal resection as described by Kennard (233). 



In man, agenesis of the corpus callosum is not 

 characterized by any demonstrable deficits in the 

 execution of movements or sensory perception (cf. 58 

 for references), although no substitution would seem 

 possiljle for callosal functions. In primates, other 

 interhemispheric commissures are not concerned in 

 corticocortical relationships with the exception of 

 certain portions of the anterior commissure (34, 296). 



Surgical .section of the corpus callosum in human 

 subjects is rarely followed by any deficits, provided 

 hemispheric lesions are absent (13). Since, however, 

 a temporary motor dyspraxia can occur if motor or 

 sensory deficits were present preoperatively, the 

 hypothesis has been advanced that the corpus callo- 

 sum can mediate a facilitating function in fine move- 

 ments (402). The transient nature of these disturb- 

 ances would not appear to be inconsistent with this 

 hypothesis since compensation may occur on the 

 basis of ipsilateral mechanisms. Indeed, it is clear that 

 motor apraxia cannot be attributed to callosal 

 lesions alone (cf. 58). 



A complete and critical account of the anatomy, 

 physiology and pathology of callosal functions has 

 been provided by Bremer et al. (58). The transfer of 

 memory traces between the two hemispheres through 

 the corpus callosum is discussed in Chapter LXI of 

 this work by Galambos & Morgan. 



M.\JOR EFFERENTS FROM CORTIC-^L ARE.^S 

 CONCERNED IN SENSORIMOTOR INTEGRATION 



Despite the difficulties which may attend attempts 

 to categorize cortical efferent motor pathwavs, with 

 the inevitable aspects of inadequacy entailed in rigid 

 schemes of descending connections, the problem of 

 the.se efferent pathways, depicted in figure 5, may be 

 discussed from two major points of view. There is, 

 first, the consideration of the cortical origin of fibers 

 forming the pyramidal tract, with particular reference 

 to the contriijutions to this tract from areas outside 

 the precentral strip, and including regions of the 

 parietal, temporal and occipital lobes. A corollary 

 to such a study is the assessment of the extent of the 

 distriljution of fibers arising in the cortex and running 

 at least part of their course through the internal cap- 

 sule and basis pedunculi in company with cortico- 

 spinal fillers but terminating in mesencephalic, pon- 

 tine and medullary areas which form part of the 

 reticular formation. These fibers may ultimately 

 exercise a controlling influence on spinal motor 

 centers through reticulospinal pathways. 



The second category of cortical efferent pathways 

 includes those fibers which form an 'extrapyramidal' 

 system, and which nm a subcortical course through 

 the basal ganglia and diencephalic areas to reach the 

 reticular zones of the brain stem. These pathways are 

 presumed to be multisvnaptic in many instances. 



It is obvious that such a subdi\ision is in large 

 measure artificial since the pyramidal tract has many 

 fibers which terminate in brain-stem reticular areas 

 and, thus, exercise their spinal influence through 

 "extrapyramidal" pathways (303). These profound 

 extrapyramidal influences are fully descriljed in 

 Chapter XXX\' by Jung cS: Hassler in this work, 

 and attention will therefore be directed here primarily 

 to the origins and supraspinal distribution of the 

 pyramidal tracts. The long descending tracts in man 

 have been extensively reviewed by Nathan & Smith 

 (345) with a discussion of the earlier literature. Lassek 

 (258) has summarized our knowledge of the pyramidal 

 tract, and aspects of its termination have been re- 

 viewed by Bernhard (46). The pyramidal tract is the 

 subject of Chapter XXXI\" of this work by Patton & 

 Amassian. 



Cortical Origin of Fil>ers of 

 Pyramidal Tract 



In 1874 Betz (48) described the giant cells in the 

 precentral gyrus which bear his name. Since, how- 



