THE CEREBRO SPINAL NERVOUS SYSTEM. 475 



the posterior roots of the same (B, 6); a pair of roots on each side cor- 

 responding to each vertebra (Fig. 329). 



White matter. The white matter of the cord is made up of me- 

 dullated nerve-fibres of various sizes, arranged longitudinally around the 

 cord under the pia mater, and passing in to support the individual fibres 

 in the delicate connective tissue or neuroglia made up of a very fine re- 

 ticulum, with both small cells almost filled up by nuclei and stellate 

 branching corpuscles. 



The general rule respecting the size of different parts of the cord 

 appears to be, that the size of each part bears a direct proportion to the 

 size and number of nerve-roots given off from itself, and has but little 

 relation to the size or number of those given off below it. Thus the 



FIG. 331. Section of gray 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 through white matter; g c, large stellate nerve-cells with nuclei; they are 

 seen imbedded in neuroglia. (Schofield.) 



cord is very large in the middle and lower part of its cervical portion, 

 whence arise the large nerve-roots for the formation of the brachial 

 plexuses and the supply of the upper extremities, and again enlarges 

 at the lowest part of its dorsal portion and the upper part of its lum- 

 bar, at the origins 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 gray matter; for there seems reason to believe that the 

 white or fibrous part of the cord becomes gradually and progressively 

 larger from below upwards, doubtless from the addition of a certain num- 

 ber of upward passing fibres from each pair of nerves. 



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



