614 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



in average diameter from 7.6 // to 12.6 //, as the total weight of the frog 

 increased from 1.5 to 63.0 grams. 



The branch which forms the neuron contains an axis-cylinder surrounded by 

 a medullary sheath. There are two views concerning the constitution of the 

 axis-cylinder one 1 that the axis is composed of slender thread-like fibrillse 

 floating in a coagulable plasma, these fibrillse being the conductors of the nerve- 

 impulses. The opposing view is that advocated by Leydig, 2 Nansen, 3 and 

 Schafer, 4 to the effect that the axis-cylinder is formed by a spongy framework 

 in the meshes of which is a semi-fluid plasma. According to this latter view 

 the plasma is the substance through which the impulses pass. Neither view is 

 beyond criticism, nor does either of them admit of detailed correlation with the 

 physiological facts. The conception of the axis-cylinder as composed of fibrillse 

 appears at first sight to offer an anatomical arrangement for a number of isolated 

 pathways within a single fibre, but the fibrillse cannot be unbranched from one 

 end of their course to the other, since many nerve-fibres near their final distribu- 

 tion divide a number of times, the diameter of the individual fibrillse remaining 

 the same ; and the combined cross sections of the axis-cylinders in the subdivis- 

 ions demand, therefore, a far greater number of fibrillae than is contained in the 

 main stem of the fibre. On the other hand, the conception of the axis-cylinder 

 as a series of tubes interosculating at very acute angles does away at the start 

 with any notion of structural isolation of the pathways within the fibres. 

 This latter view is, however, the better supported histologically. 



"When the axis-cylinder increases in diameter, it must, under this view, be 

 by the formation of more of these tubes, for their size, though variable, is not 

 directly in proportion to the diameter of the fibre. While the neuron is growing 

 as a naked axis-cylinder it is usually slightly enlarged at the tip (Cajal), sug- 

 gesting that it is specially modified at that point. The nutritive exchange on 

 which the increase of the entire neuron depends appears to take place along 

 its whole extent, and not to be entirely dependent on material passed from the 

 cell-body into the neuron. 



Medullation. After the production of its several branches the next step 

 in the growth of the cell is the formation of the medullary sheath. Not all 

 neurons have a medullary sheath, nor is any neuron completely medullated. 

 In the sympathetic system there is a very large proportion of unmedullated fibres. 

 In the central system the number is .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 given 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 yet been found. 



In studying the effect of stimulation and of changes in temperature on the 



1 Kuppfer und Boveri : Abhandlungen d. k. bayer. Akad. den Wissenschaften, Miinchen, 1885. 



2 Zette und Gewebe, Bonn, 1885. 



3 The Structure and Combination of the Histological Elements of the Central Nervous System, 

 Bergen, 1887. * Quain's Anatomy, 10th edition, vol. i. pt. 2, 1891. 



