THE CIRCULATION OF CEREBRO-SPINAL SUBSTANCE. 571 



Berkley's experiments have shown that they become the seat 

 of swellings under the influence of poisons, there must be no 

 escape for fluids through their walls; indeed, gemmulation 

 would become impossible were the centrifugal pressure neces- 

 sary counteracted by the escape of the plasma into lymph- 

 spaces. That the blood returns toward the cell-body and 

 through some of the central fibrils is therefore probable. Un- 

 der these circumstances we can say, as a working proposition, 

 that the plasma which enters the collaterals is returned to the apical 

 dendrite, and to the cell-body with the blood of the latter. The Uood 

 of the cell-body then passes out through the axon. 



How does the blood leave the axon of the neuron in the 

 substance of the brain and cord? This question plainly re- 

 solves itself into the following: How does the blood reach the 

 veins from the axon? "The perivascular lymphatics . . . 

 are especially found in connection with the vessels of the brain" 

 says Gray 54 ; "these vessels are inclosed in a sheath which acts 

 as a lymphatic channel, through which the lymph is carried to 

 the subarachnoid and subdural spaces, from which it is returned 

 to the general circulation." This familiar fact would be un- 

 explainable after the views we have advanced concerning the 

 circulation of arterial blood were the return of blood to the 

 veins not the purpose of the lymphatic sheaths, for the same 

 authority states that lymphatic vessels "have not at present 

 [1901] been demonstrated in the dura mater or the substance 

 of the brain/' Again, when we consider that perineural, as 

 well as perivascular, spaces exist, we are brought to realize that 

 by linking the axon of a neuron to a venule, with a lymphatic 

 space as intermediary, we have the elements of a mechanism 

 which not only utilizes structures that are known to be present 

 in the cerebro-spinal axis, but which also satisfy the needs of 

 the function. Finally, if an axon is itself buried (up to the 

 neck of its bulbous terminals) in a perineural sheath, which in 

 turn communicates with a vein through stomata, as is the case 

 in nerves, the blood of the axon is provided with a clear path 

 to the general blood-stream. 



That the blood of the neuron is eliminated as it is in other 



"Gray: Loc. cit. 



