CONNECTIONS OF THE LONG PATHS OF THE CORD 849 



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 destroyed. 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, larger in the 

 anthropoid apes than in the lower monkeys, and larger in man than 



Fig. 344- Paths from Cortex in Corona Radiata (Starr). A, tract from frontal con- 

 volutions to nuclei of pons and so to cerebellum; B, motor pyramidal tract; 

 C, afferent tract for tactile sensations (represented in the diagram as separated 

 from B by an interval for the sake of clearness); D, visual tract; E, auditory 

 tract; F, G, H, superior, middle, and inferior cerebellar peduncles; J, fibres 

 from the auditory nucleus to the posterior corpus quadrigeminum ; K, decussa- 

 tion of the pyramids in the bulb; FV, fourth ventricle. The roman numerals 

 indicate the cranial nerves. 



in even the highest of the apes. In the lower mammals it is exceed- 

 ingly 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, 3 per cent, in the 

 guinea-pig, 5 per cent, in the rabbit, and nearly 8 per cent, in the cat. 

 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 pyramidal system 

 does not exist as a collection of neurons which send their axons with- 



54 



