SARGENT : THE OPTIC EEFLEX APPARATUS OF VERTEBRATES. 139 



C. The Brain Ventricles and Central Canal and their Contents. 



/. The Encephalic Cavities. 



The cavities within the central nervous system, their lining and 

 contents, have hardly attracted the attention they deserve, and this is 

 especially true in the lower vertebrates, where they are largest. Most 

 text-books and treatises on the nervous system make no mention of 

 these topics, and others dismiss the subject with a few words, which 

 have been repeated in the text-books, unmodified, for half a century. 

 The small size of the canal and ventricles in the higher vertebrates has 

 probably contributed to this neglect, but in the great majority of adult 

 vertebrates and all vertebrate embryos the volume of these cavities is a 

 considerable i)art of the total volume of the bi'ain. 



Magendie ('42) discovered the foi-amen which puts the intraventricular 

 cavities in connection with the sub-arachnoid space. Key und Eetzius 

 ('75-76) showed that there were also two other foramina in the lateral 

 recess by the side of the flocculus cerebelli. Haller (157-63) found that 

 colored gelatine injected into the veins appeared in the brain ventricles, 

 and Hill ('96) showed by numerous experiments that, under sligbtly 

 increased pressure, the cerebro-spinal fluid was absorbed into the veins, 

 chiefly within the cranium. 



All these observations have been made on the higher vertebrates 

 only, chiefly on man and monkeys. I have been able to demonstrate 

 the foramen of Magendie in sections of the brains of Petromyzon and 

 teleosts. In the former it lies immediately posterior to the border of the 

 plexus chorioidens III., and marks the anterior limit of the dorsal cells 

 of the cord (Hinterzellen). 



1. The Cerebro-Spin^al Fluid. 



The cerebro-spinal fluid is of a clear, watery nature and low specific 

 gravity, with only a trace of proteid. According to Halliburton (:01), it 

 is a true secretion and not a transudation. '' The peculiarity of cerebro- 

 spinal fluid lies in the quality, not the quantity of proteids, that are 

 present " (Halliburton, '88). These proteids constitute about one-tenth 

 of one per cent of the cerebro-spinal fluid, and consist chiefly of globu- 

 lins and albumoses (Hammai'sten, '98, p. 195). There is about nine- 

 tenths of one per cent of inorganic solids, and approximately ninety-nine 

 per cent of water. The composition, however, diff'ers in various physi- 

 ological and pathological conditions. It has been shown by Cavazzani 

 ('92) from experiments on dogs that the cerebro-spinal fluid collected 



