INJECTIONS AND REPLACEMENTS IN THE CEREBRO-SPINAL SYSTEM. 29 



THE INJECTION OF INDIA INK. 



The objections to the use of any fluid of insoluble particles in suspension have 

 already been discussed in considering the methods of injection which were possible 

 for use in this study ; but for comparison with results obtained by more promising 

 methods and to ascertain to what extent injections with india ink are reliable they 

 will be further considered here. No granular substance other than india ink (car- 

 bon granules) was employed in this investigation. In every way this suspension 

 possesses advantages over other possible masses in its ease of preparation, in the 

 small size of the granules, and the insolubility in the reagents used for microscopic 

 technique. 



Suspensions of india ink (diluted from 4 to 10 times) were introduced first into 

 the medullary-canal system of living pig embryos by the replacement method. In 

 no case, however, even though the circulation of the embryo may have continued 

 for 90 minutes, was there any evidence of an extension of the replaced mass outward 

 into the periaxial spaces. The carbon granules remained wholly within the ven- 

 tricles, a striking difference from the results obtained by the ferrocyanide replace- 

 ments. It would appear, then, without the further evidence afforded by micro- 

 scopic section of the specimens, that there is an existing mechanism which prevents 

 the passage of the carbon granules from the fourth ventricle into the periaxial spaces. 

 This finding was found to be constant in all the living embryos subjected to the 

 cerebro-spmal replacement. 



Quite similar to these results by the replacement method are those from the 

 injection of a suspension of india ink under mild syringe-pressure. In no instance, 

 provided the pressure was maintained at a low enough degree, was there any passage 

 of the granular material into the periaxial tissue. In embryos of over 30 mm., 

 however, even with the lowest pressure, it becomes increasingly difficult to prevent 

 a sudden spread into the periaxial spaces. The type of spread indicates a sudden 

 release of some restraining agent and suggests a rupture of a membrane. This 

 spread is usually local and takes place from the roof of the fourth ventricle. 



With moderate and strong syringe-pressures, however, it is possible to secure a 

 periaxial spread, but this is quite different from the distribution of the injections by 

 the use of ferrocyanide solutions. Figure 10 illustrates a specimen of a pig embryo 

 of 21 mm. into whose central spinal canal india ink was injected under strong 

 syringe-pressure. The resultant spread of the injection is easily discerned; the 

 cerebral ventricles are quite filled with the carbon, while from the superior portion 

 of the roof of the fourth ventricle a dense but localized periaxial spread is made out. 

 This extraventricular extension of the ink is well defined; it stretches caudalwards 

 for a slight distance, curving about the bulbous caudal portion of the ventricle 

 and extending lateralwards but a short distance. The median portion of the 

 cerebellar lip is covered by the granules. Evidences of the excessive pressure at 

 which the injection was made are shown by the lines of invasion of the spinal cord 

 and mid-brain. A comparison of the spread of this injection mass with the exten- 

 sion of a ferrocyanide replacement in an embryo of the same size (21 mm.) is afforded 

 by figures 10 and 6. With such a divergence in the results obtained by the two 



