THE CIRCULATION OP CEREBRO-SPINAL SUBSTANCE. 569 



myelin and blood-plasma and may, therefore, be considered as 

 nerves, while others only contain plasma and are, therefore, blood- 

 channels. 



Under these circumstances, are the above-mentioned in- 

 vestigators not justified in considering the net-works referred 

 to as nervous structures? They would be justified in doing so, 

 did all the neuroglia-fibrils contain myelin; but it is the ab- 

 sence of this compound in the fibrils that serve as channels for 

 plasma between blood-vessels and the apical dendrites of the 

 neuron which seems to us to neutralize their view. Were there 

 any evidence that a medullated fiber of any kind connects any 

 portion of the cell with another structure capable of converting 

 chemical energy into nervous energy, the question would re- 

 main an open one; but such is not the case; the absence of 

 myelin in the neuroglia-fibrils connecting the neuron with the 

 source of its blood-supply seems to distinctly point to the need 

 of its absolute isolation, not only to avoid the promiscuous dis- 

 persion of the nervous energy it is able to produce, but also 

 to enable it to store this energy and to direct it in the physio- 

 logical paths. 



The presence of the non-medullated fibers among the 

 cerebro-spinal nervous elements becomes evident when the 

 structural difference between the gray matter and the white 

 matter is interpreted from the standpoint of our views. "Owing 

 to the relative abundance of white refractive medulla," says 

 Professor Foster, "the white matter possesses in fresh spec- 

 imens a characteristic opaque white color; hence the name." 

 . . . "In transverse sections of the cord nearly the whole of 

 the white matter appears, under the microscope, to be com- 

 posed of minute circles, the transverse sections of the longi- 

 tudinally-disposed fibers." . . . "The gray matter, from 

 the relative scantiness of medulla, has no such opaque-whiteness, 

 is much more translucent, and, in fresh specimens, has a gray 

 or rather pi?ikish-giay color, the reddish tint being due to the 

 presence partly of pigment and partly of blood, for the blood- 

 vessels are much more abundant in the gray matter than in 

 the white." That in the cerebral cortex, for instance, these 

 vessels should represent the starting-point of the neuroglia non- 

 medullary fibrils needs hardly to be emphasized. They are now 



