104 THE MECHANISM 0^ LIFE 



velops, do the interactions of one part of the cortex with other 

 parts by means of the commissural and association tracts come 

 to possess significance in the development of the manifestations 

 of intelligence and higher mental faculties. 



From all parts of the cortex bundles of fibres radiate inwards 

 towards the cerebral peduncles; these form the "projection 

 tracts " — those that connect the cortex with the lower brain and 

 spinal cord. Some of the projection tracts are represented in 

 Fig. 28 in a very schematic way. One, which is called the 

 " great sensory tract," connects the parts of the cortex lying 

 behind the Rolandic fissure (see Fig. 38) with the mid-brain 

 ganglia; another tract of fibres, starting from the cortical region 

 well in front of the Rolandic fissure, travels downwards and ends 

 in the grey matter of the pons varolii, where it forms a series of 

 relays with the fibres coming from the two sides of the cerebellum 

 via the middle peduncles of the latter. This is the " tract from 

 the cortex to the pons and cerebellum " of Fig. 28. 



Thus the cortex is connected with the lower brain on the one 

 hand by a direct tract, and with the cerebellum on the other 

 hand by two indirect tracts, one via the mid-brain and the other 

 via the pons and the middle peduncles. 



The most important of all the projection tracts in man and 

 the higher mammals is the fyramidal tract, that represented in 

 Fig. 28, and called " the motor tract from the cortex to the 

 cord." The fibres composing it start in the peculiar pyramidal 

 cells of the cortical region lying in front of, and immediately 

 round and in the depths of, the Rolandic fissure — that is, in the 

 " motor area " of the cortex. These fibres are gathered up into 

 two bundles which travel down in the crura of the brain towards 

 the medulla, where they decussate, or cross, to the other side of 

 the body. The crossing is not, however, complete, as the figure 

 shows, and some of the pyramidal fibres remain on the same side 

 of the body as that in which they originate. But most of them 

 cross over. Running down in the white matter of the cord are 

 therefore two descending tracts of fibres, " the direct and crossed 

 pyramidal tracts." All these fibres pass into the grey matter of 

 the cord, and end there as synapses round the cells that give off 

 the fibres of the motor roots of the spinal nerves. 



The pyramidal cortical tracts, with their nuclei in the cortex, 

 are the typically " higher " parts of the mammalian brain. They 

 are relatively small in such animals as the mole or the rabbit. 



