878 



THE CENTRAL NERVOUS SYSTEM 



out interruption down from the cortex to the cord. In birds, e.g., 

 after the removal of a hemisphere, the degeneration does not extend 

 below the mid-brain (Boyce). 



SECTION VI. PATHS FROM AND TO THE CORTEX. 



Thus far we have been able to map out two great paths from 

 the cerebral cortex to the periphery one efferent, the other afferent, 

 (i) The great efferent or motor pyramidal path, which, starting in 

 the cortex in front of the fissure of Rolando, where its axons give off 

 numerous collaterals to the grey matter soon after emerging from 

 the cells, and sweeping down the broad fan of the corona radiata, 



passes through the narrow 

 isthmus of the internal cap- 

 sule into the crusta of the 

 crus cerebri, and thence into 

 the pons (Figs. 354, 355). At 

 this level, the fibres destined 

 to make connection with the 

 motor nuclei of the cranial 

 nerves in the grey matter 

 underlying the aqueduct of 

 Sylvius and the fourth ven- 

 tricle terminate. Most of 

 these fibres decussate to make 

 physiological connection with 

 nuclei on the opposite side, 

 but some join nuclei on the 

 same side. The question 

 whether they arborize di- 

 rectly around the cells of the 

 motor nuclei or make junc- 



Fig. 355. Motor Pyramidal Tracts (Diagram- 

 matic) (Halliburton, after Gowers). The 

 convolutions are supposed to be cut in 

 vertical transverse section, the internal 

 capsule, I, C, and the crus in horizontal 



section. O, TH, optic thalamus; CN, cau- 

 date nucleus; L2 and Lj, middle and ex- 

 ternal portions of lenticular nucleus; /, a, I, 

 fibres from the face, arm, and leg areas of 

 the cortex respectively; E, S, Sylvian fis- 

 sure. The genu or knee of the internal 

 capsule is indicated by the asterisk. 



tion with them through 

 another intercalated neuron 

 is precisely in the same 

 position as the corresponding 

 question for the spinal pyra- 

 midal path (p. 876). On their 

 way through the pons they send off collaterals to the nuclei 

 pontis, as they do higher up to the grey matter of the basal 

 ganglia of the cerebrum and the substantia nigra, and the path 

 may be continued to the motor nuclei by axons arising here. 

 There is no proof, however, that this is the case. The rest of 

 the pyramidal fibres run on into the pyramid of the bulb, 

 where the greater part (usually about 90 per cent.) of the fibres 

 decussate, appearing in the cervical cord as the massive crossed 



