16 



DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



by Evans^ 14 ') phagocyte the small colloidal particles. In addition, the true solution 

 must be readily precipitated as an insoluble salt, capable of remaining unchanged 

 in histological technique. After trying many salts in long-continued injections into 

 the adult cerebro-spinal spaces, it was found that solutions of potassium ferro- 

 cyanide and iron-ammonium citrate in equal parts were admirably adapted to the 

 purposes of the experiment. By the addition of a mineral acid (preferably hydro- 

 chloric) ferric ferrocyanide could be precipitated. This prussian-blue is insoluble 

 in the routine technique and is readily identified in sections. After mounting in 

 damar or balsam the blue granules can be observed unchanged for several months, 

 but after a year there is some deterioration in the specimen, due to a conversion 

 of the blue into indefinite greens. 



TEXT-FIGURE 1. Schematic sketch of mechanism used for replac- 

 ing ventricular and spinal fluid of an embryo with a foreign 

 solution or suspension. The system is here shown in balance, 

 the difference in fluid-level in reservoirs and needles repre- 

 senting the hydrostatic pressure necessary to overcome the 

 capillary resistance of tubes and needles. The stands hold- 

 ing the injecting needles may be moved about without altering 

 the balance of the system. As one reservoir is raised, the 

 other is lowered in a corresponding degree. 



In regard to these two major factors in the employment of injections (pressure 

 and true solution) it was found necessary to devise a method of experimentation 

 which would satisfy the requirements of the problem. Solutions of the ferrocyanide 

 and of the citrate were non-toxic within the central nervous system and afforded 

 an excellent histological means for following the fluid-pathways. It was hoped at 

 first that a simple "replacement" type of injection might be employed, as in the 

 adult animals. In this procedure a given amount of fluid was withdrawn from the 

 subarachnoid spaces and immediately replaced by an equal quantity of the injection 

 fluid. The method was successfully tried on fetal cats of considerable size, but 



