INJECTIONS AND REPLACEMENTS IN THE CEREBRO-SPINAL SYSTEM. 27 



The most important feature of these findings in the embryo pig injected with 

 true solutions under moderate pressures from a syringe concerns the fact that the 

 extension of the injection coincides, except as to the size of the embryo, hi every 

 instance with that obtained by the replacement method. Thus similar and analo- 

 gous spaces are filled by injections under syringe-pressures in small embryos and by 

 the solution under normal tension in larger embryos. It must be assumed, then, 

 that the pressure of injection is sufficient to dilate potential cerebro-spinal spaces 

 which normally would not be concerned in the pathway taken by the cerebro-spinal 

 fluid. No evidence of new or abnormal pathways for the fluid is afforded by the 

 observations made with the increased pressure; these phenomena indicate great 

 potential strength in the tissues which limit the immature cerebro-spinal spaces. 



Injections with a simple syringe may be made with such a degree of pressure 

 that gross rupture of the tissues becomes apparent. In such an injection into the 

 central canal of the spinal cord the infundibular region ordinarily ruptures in the 

 smaller embryos (under 15 mm.), while in larger embryos rupture usually occurs 

 into the subcutaneous tissues of the back of the neck over the fourth ventricle. 



In discussing the effects of the introduction of solutions of ferrocyanide under 

 pressures higher than normal into the central canal of the spinal cord, it may be 

 appropriate to record observations made in the attempt to inject the cerebro-spinal 

 spaces from the perispinal space. In embryos under 15 mm. in length it is quite 

 difficult to make a perispinal injection. As the embryos exceed this measurement 

 the injection becomes increasingly easy, but not until a length of 20 mm. is attained 

 can it be made under the mild pressure advisable. These observations tend to 

 substantiate the findings already recorded in both the intramedullary replacements 

 and the injections under mild pressure. 



RESULTS OF INJECTIONS OF NITRATE OF SILVER. 



In a number of experiments a dilute solution (0.5 per cent) of nitrate of silver 

 was injected into the central canal of the spinal cord and the salt then reduced in 

 the sunlight. This solution, although a true one, is wholly unsuited for the replace- 

 ment type of injection, on account of its great toxicity and its power to coagulate 

 protein. It was employed here only for the simple type of injection. 



The results obtained by this intraspinous injection of solutions of nitrate of 

 silver were of but little value in the determination of a pathway for the cerebro- 

 spinal fluid, but they vividly present certain aspects of the problem. Thus, in 

 figure 11, a drawing of a specimen (pig) of 16 mm., the area through which fluid 

 passes in the superior portion of the roof of the fourth ventricle is clearly outlined 

 by a denser deposition of the silver. This specimen was prepared by introducing 

 the solution of nitrate of silver into the central canal of the spinal cord under the 

 so-called moderate syringe-pressure. The drawing shows a slight, cone-shaped 

 extraventricular spread of the injection fluid. This spread takes place solely from 

 the superior area of fluid passage, a result in accord with the finding that the solu- 

 tion of potassium ferrocyanide and iron-ammonium citrate passed first through the 

 superior area. Of course it is realized that the precipitant action of the silver may 



