180 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



in the growth of the cell is the formation of the medullary sheath about the 

 axone. Not all axones have a medullary sheath, nor is any axone com- 

 pletely medullated. In the sympathetic system there is a very large pro- 

 portion of unmedullated axones. In the central system the number is 

 also very large, although their mass is small. Of the significance of the 

 medullary sheath we know nothing-. The suggestion that it acts to insulate 

 the nerve impulse within a giveD axis-cylinder has little or no evidence 

 in its favor. The suggestion that it is nutritive is plausible, but important 

 differences in the physiological reactions of the two classes of nerve-fibres 

 have not vet been found, if we except the observation that the non -medullated 

 nerves rapidly lose their irritability at the point of stimulation with the 

 faradic current, thus exhibiting a "stimulation fatigue" not found in nerves 

 unquestionably medullated. 



Growth of Medullary Sheath in Peripheral Nerves. — Whatever may 

 be the significance of the medullary sheath, it is usually formed before the 

 neurone has attained its full size. In the peripheral system it depends on 

 the presence of mesodermal cells which envelop the axis-cylinder, forming a 

 tube about it. Each ensheathing cell is physiologically controlled by a nu- 



Fig. 73.— Longitudinal (B\ and transverse (A) sections of nerve-fibres. The heavy border represents 

 the medullary sheath, which is thicker in the larger fibres. Human sciatic nerve. X200 diameters 

 (modified from van Gehuchten). 



cleus which becomes situated about midway between its extremities. Accord- 

 ing to Ranvier and his school, the cell-substance is largely transformed into 

 myelin, and the line of junction between two of these sheathing cells forms a 

 node of the nerve-fibre. In the sheath of a growingaxone at least two changes 

 can be readily followed : As the axis-cylinder increases in diameter the medul- 

 lary sheath becomes thicker. The change is such that in the mammalian 

 peripheral system the areas of the axis-cylinder and of the medullary sheath, 

 as shown in cross-sections of osmic acid preparations, remain nearly equal 

 (Fig. 73). On the other hand, the length of the intemodal segments tends to 

 increase with an increase in the diameter of the nerve-fibre, and for nerves of 

 the same diameter it is less in man than in the lower animals. In a given 

 fibre the segments are shorter at the extreme peripheral end ( Key and Retz- 

 ius). In the young fibres, also, they are shorter and increase in length 

 with age. 



A physiological significance attaches to these segments, because, as Ran- 

 vier long since pointed out. it is at the nodes that various staining reagents 

 easily reach the axis-cylinder. This suggests that normal nutritive exchanges 

 may follow the same path, and thus short intemodal segments giving rise to 



