STRUCTURAL ARRANGEMENTS OF CEREBRUM 479 



(4) THE OPTIC RADIATION. These diverging fibres in the back part 

 of the corona radiata are mixed up with fibres which are partly cortici- 

 fugal. The corticipital fibres arise in the pulvinar and the external 

 geniculate body and end in the occipital cortex. 



(5) THE AUDITORY RADIATION. These fibres consist of the axons 

 of cells situated in the internal geniculate body. They pass through 

 the posterior limb of the internal capsule under the lenticular nucleus 

 to end in the temporal lobe. 



B. THE EFFERENT PRO- 

 JECTION FIBRES. 



(1) THE PYRAMIDAL TRACT. 



This is composed of fibres which 



arise from the large Betz cells in 



the ascending frontal convolution, 



the ' motor area.' They pass 



through the corona radiata into 



the internal capsule, where they 



occupy the genu and the anterior 



two-thirds of the posterior limb. 



Hence they pass into the crusta, 



where they occupy the middle 



two-fifths of this structure, and 



are continued as the pyramids of 



the pons and medulla to the 



upper part of the spinal cord, 



where most of them decussate 



to the other side to form the 



crossed pyramidal tracts. Some 



of the fibres do not cross at 



the pyramidal decussation, but 



are continued down in the same 



position in the anterior columns of the spinal cord of the same side, 



forming the direct or anterior pyramidal tracts. These fibres cross 



for the most part lower down in the cord, so that the direct pyramidal 



tract is not seen below the cervical region. The pyramidal tracts are 



not found in lower vertebrates, and make their first appearance in the 



mammalia. Their development corresponds with the gradual increase 



in the direct interference of the cerebral cortex in the reactions of the 



organism as a whole and are an index to the gradual shifting of 



these reactions from the inevitable to the educated reflex. The fibres 



of the pyramidal tract end at various levels of the spinal cord and can 



be traced to the lower end of the sacral region. According to Schafer 



they end in the posterior cornua, so that their action is to set going a 



reaction which could otherwise be elicited by stimulation of the afferent 



FIG. 214. Diagrammatic representation 

 of the internal capsule, as seen in hori- 

 zontal section. (CUNNINGHAM.) 



