DEPARTMENT OF EMBRYOLOGY. 87 



slowly effected changes in the arterial tension have but little influence upon 

 it. Thus, though the pressure of the cerebrospinal fluid is dependent upon 

 the intracranial venous and arterial pressures to the extent of being influenced 

 passively and in the same direction by both, yet it is relatively independent 

 of each of them, in that normally it maintains an individual, fairly constant 

 level far below that of the intracranial arteries and somewhat above that of 

 the intracranial veins. 



By introducing hypertonic and hypotonic solutions of various concentra- 

 tions into the blood-stream, these investigators have been able to study the 

 general systemic and intracranial vascular alterations that occur under these 

 conditions, and there is thus afforded a unique opportunity for a study of the 

 mechanisms which normally control the pressure of the cerebrospinal fluid. 

 Analysis of their data demonstrates that alterations in the pressure of the 

 cerebrospinal fluid can be effected and maintained independently of change 

 in the intracranial and peripheral vascular pressures. These changes in pres- 

 sure must be explained by the alteration of the osmotic pressure of the cir- 

 culating blood. Where the volume of the blood is increased by the injection 

 of a considerable amount of isotonic solution, there is a brief rise of the cere- 

 brospinal fluid pressure, long enough for the organism to compensate for the 

 increased volume of fluid. When, however, the volume of circulating blood 

 is increased and at the same time its salt-content relatively diminished by the 

 intravenous injection of distilled water, there is a prolonged increase in the 

 pressure of the cerebrospinal fluid, accompanied by an increase of venous 

 pressure of lesser degree and duration. We are here concerned with two pro- 

 cesses of adjustment: (1) the giving off of water to the tissues with attraction 

 of salts from them, as evidenced by the increased brain volume and pressure 

 of the cerebrospinal fluid; (2) compensations in the vascular bed, indicated 

 by the return of the vascular pressure to normal levels, while the pressure of 

 the cerebrospinal fluid remains high. The intravenous injection of strongly 

 hypertonic solutions causes a prolonged and profound fall in the pressure 

 of the cerebrospinal fluid, preceded usually by a sharp rise. Here the chief 

 increase of fluid volume in the circulating blood is due to the attraction of 

 water from body-tissues, to which is to be attributed the decrease in brain 

 volume and the reduction of pressure of the cerebrospinal fluid. 



Working with Dr. Weed, Dr. J. C. Nafiagas undertook to determine the 

 effect of intravenous injections of hypertonic and hypotonic solutions on the 

 brain bulk and the pressure of the cerebrospinal fluid in the dilated cerebral 

 ventricles of hydrocephalic kittens. Though the pressure in the kittens was 

 considerably higher than in normal animals, it could be decreased or increased 

 at will and in some cases even reduced below zero. Subsequent histological 

 examination of the material yielded the important observation that the 

 increased absorption of the ventricular fluid occurring after injections of 

 hypertonic solutions takes place through the ependyma into the underlying 

 capillary network. There is no evidence of absorption by the choroid plexuses. 



During the year a review has been prepared and published by Dr. Weed, 

 outhning what is now known concerning the cerebrospinal fluid in its embryo- 

 logical, anatomical, and physiological aspects. Such a complete and authori- 

 tative account should prove of very great value to those who have occasion to 

 concern themselves with this important body-fluid. 



