SARGENT : THE OPTIC KEFLEX APPAKATUS OF VERTEBRATES. 211 



between the elongated ependymal cells, and undergoing fusion, so that 

 before entering the ventricle they are consolidated into three definite 

 trunks. The more posterior of these is much the largest. In one 

 specimen these trunks, beginning with the most posterior, measured, 5, 4, 

 and 2 micra respectively in diameter. These trunks before emerging 

 into the ventricle run for some distance caudad just under the mem- 

 brane limiting the ventricle, and emerge successively just under the 

 posterior commissure. Within the mesocoele they are joined by some 

 finer branches from farther forward, with which they coalesce to form 

 Reissner's fibre. This has a diameter of 8 micra. The membrana 

 limitans interna, formed by the ependyma which covers the ' Schalt- 

 stiick,' is deflected outward over the fibre, forming a loose, wrinkled, 

 membranous sheath, extending nearly to the cerebellum. 



Reissner's fibre is of such size in this species that it has been found 

 practicable to follow its course through the ventricles and canal in series 

 of transverse sections, though it often requires careful search to distin- 

 guish it among the coagulum and blood corpuscles about it. In trans- 

 verse sections the portions of the fibre appear as cylinders, viewed from 

 the end. These are often toppled over, or otherwise displaced, or they 

 are sometimes missing, having dropped out in the process of making the 

 preparation. 



In one series of sagittal sections studied, the fibre was found irregu- 

 larly coiled within the fourth ventricle, and the adjacent anterior portion 

 of the central canal contained no fibre. The coiled portion of the fibre 

 was shrunken and thicker than the normal fibre where the course was 

 straight. The coiled portion was from 12 to 14 micra in diameter, the 

 normal fibre 9 to 10 micra. In this case the roof of the ventricle was 

 entire, so the fibre could not have been drawn out of its natural position 

 by any external force. "When in the fresh condition the spinal cord was 

 severed 2 cm. posterior to the brain, the fibre probably by its own 

 elasticity recoiled into the ventricle. This shrinking and coiling was 

 doubtless accentuated by the action of Flemming's fluid, in which the 

 preparation was fixed. 



The large size of Eeissner's fibre in this species makes it a favorable 

 subject for the study of its finer structure. In thin transverse sections 

 it is seen to have a very definite internal structure. There is a thin 

 medullary sheath about the fibre, having a thickness one-tenth to one- 

 twelfth of the diameter of the fibre (Plate 7, Fig. 53). This takes stains 

 in precisely the same manner as the myelin of ordinary medullated fibres. 

 The central portion of the fibre has in section a finely punctate appear- 



