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HAND-BOOK OF PHYSIOLOGY. 



of the large nerves which, after forming the lumbar and sacral plexuses, 

 are distributed to the lower extremities. The chief cause of the greater 

 size at these parts of the spinal cord is increase in the quantity of grey 

 matter; for there seems reason to believe that the white or fibrous part 

 of the cord becomes gradually and progressively larger from below upward, 

 doubtless from the addition of a certain number of upward passing fibres 

 from each pair of nerves. 



From careful estimates of the number of nerve-fibres in a transverse 

 section of the cord toward its upper end, and the number entering it 

 by the anterior and posterior roots of each pair of nerves, it has been 



FIG. 317. Section of grey matter of anterior cornu of a calf 's spinal cord; n /, nerve-fibres of 

 "white matter in transverse section, showing axis-cylinder in centre of each; a r, anterior roots of 

 spinal nerve passing out though white matter; g c, large stellate nerve-cells with nuclei; they are 

 seen imbedded in neuroglia. (Schofield.) 



shown that in the numan spinal cord not more than half of the total num- 

 ber of nerve-fibres entering the cord through all the spinal nerves are con- 

 tained in a transverse section near its upper end. It is obvious, therefore, 

 that at least half of the nerve-fibres entering it must terminate in the cord 

 itself. 



Grey matter. The grey matter of the cord consists essentially of an 

 extremely delicate network of the primitive fibrillae of axis-cylinders, 

 and which are derived from the ramification of multipolar ganglion c$lls 

 of very large size, containing large round nuclei with nucleoli. This 

 fine plexus is called Gerlach's network, and is mingled with the meshes 

 of neuroglia, which in some parts is chiefly fibrillated, in others mainly 

 granular and punctiform. The neuroglia is prolonged from the surface 

 into the tip of the posterior cornu of grey matter and forms a jelly-like 

 transparent substance, which when hardened is found to be reticular, and 

 is called the substantia gelatinosa of Rolando. 



