776 A MANUAL OF PHYSIOLOGY 



or their collaterals. Some observers, however, suppose that the 

 pyramidal fibres do not come into immediate relation with the 

 anterior horn cells, but that another neuron is intercalated 

 between them and the cells. 



The pyramidal fibres are unquestionably paths for voluntary 

 motor impulses passing down from the cortex to the cord. But 

 they are not the only cortico-spinal efferent paths, and in many 

 animals they are not even the most important paths for voluntary 

 movements. It is the more skilled and delicate movements 

 which the pyramidal tract subserves in man, and it is these 

 movements which are permanently lost when the tract is de- 

 stroyed. The size of the path is proportioned to the degree of 

 development of the brain. Thus, it is larger in the monkey than 

 in the dog, and larger in man than in the monkey. In the lower 

 mammals it is exceedingly small. While in man the pyramidal 

 tracts constitute nearly 12 per cent, of the total cross-section of 

 the cord, they make up little more than i per cent, in the mouse. 

 In some mammals, as the rat, mouse, guinea-pig, and squirrel, 

 the pyramidal tracts lie, not in the antero-lateral, but in the 

 posterior columns. In vertebrates below the mammals the pyra- 

 midal system does not exist as a collection of neurons which 

 send their axons without 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). 



Connections of the Antero-lateral Descending Tract. The main 

 origin of these fibres is the nucleus of Deiters, a collection of large 

 multipolar nerve-cells in the floor of the fourth ventricle near the 

 inner auditory nucleus. These cells give off axons which pass into 

 the posterior longitudinal bundle of the bulb and pons, mostly to the 

 bundle of the same side, but partly into that of the opposite side. 

 Here the fibres bifurcate into an ascending branch, which passes 

 up to the oculo-motor nucleus, and a descending (vestibulo-spinal] 

 branch, which passes downwards to the spinal cord and enters the 

 antero-lateral descending tract. The fibres of this tract ultimately 

 pass into the anterior horn, where most of them end, by arborizing 

 amongst the cells of the horn. Higher up corresponding fibres from 

 the posterior longitudinal bundle arborize in the cranial motor nuclei. 



Thus far, then, 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 capsule into the crust a of the crus cerebri, and thence 

 into the pons (Figs. 326, 327). At this level, the fibres destined 



