154 



THE GROWTH OF THE BRAIN. 



quires the length of the fibre having a volume equal to 

 that of the cell-body 50 p in diameter to be 828 /*. 

 The long fibres passing between the cervical and lumbar 

 enlargements are some 300,000 //, in length, so that a 

 fibre of this size and extent would have 375 times the 

 volume of the cell-body just mentioned, the combined 

 volume of cell-body and neuron being '00245 mm. 3 , and 

 the total weight, supposing it to have a specific gravity 

 of ro35, would be but -0025 mgrms. 



FIG. 25. Longitudinal and transverse (A) sections of nerve fibres. 

 The heavy border represents the medullary sheath, which 

 becomes thicker in the larger fibres. Sciatic nerve. Human, 

 X 400 diameters. (Modified from Van Gehuchten.) 



The measurements show that in mature fibres of 

 average size the area of the cross section is divided 

 nearly equally between the axis-cylinder and medullary 

 sheath, so that in the example given the axis-cylinder 

 alone would have a volume 187 times that of the cell- 

 body. 



Similar calculations applied to the peripheral system 

 of man show in even more marked manner the great 

 proportional volume of the neuron, and in the lower 

 mammals also analogous relations are found. At the 

 same time it must be remembered that there are varia- 

 tions in both directions from the relations just described. 



