UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 51 



The ultimate fate of this area membranacea inferior is necessarily involved in 

 the distribution of the tela chorioidea inferior. Likewise it necessitates a discussion 

 of the possible formation of the so-called foramen of Magendie and its mode of 

 origin from the "caudal process" of Blake. It is proposed to discuss briefly some 

 of these questions in the hope that some phases of the problem may be brought forth. 



It must be clearly understood that the questions of the ultimate fate of this 

 area membranacea inferior probably differ considerably in the different species of 

 mammals. In the horse and in the pig the absence of the medial foramen (Magendie) 

 is fairly well established, but in man its existence seems to rest on equally firm 

 grounds. While, primarily, this investigation has not been concerned with the 

 possible existence of the foramen of Magendie, the question has been presented many 

 times in regard to the pig and human embryos examined. 



As far as can be determined, no descriptive study of the development and 

 differentiation of the inferior portion of the rhombic roof has been published. 

 HeuserY 23 ' studies on the form of the cerebral ventricles of the pig have afforded a 

 very good conception of the gradually changing relationships in this region. Hess^ 22 ' 

 has devoted attention to the histological appearances of the inferior roof in the 

 embryo. One of his interesting observations concerns the caudal portion of the 

 rhombic roof in a fetal cat of 10 cm., where he noticed a very sudden interruption 

 in the epithelial lining of the ventricle, with a complete closing by a fibrous net. 

 This description by Hess is the only comment upon the histological appearance of 

 the ventricular roof that has been found. His^ 25 ) pictures, without comment, in a 

 retouched photomicrograph, a differentiated area in the proper situation in his 

 fetus C-l (beginning of the third month). 



The many writers in embryology have commented upon the roof of the fourth 

 ventricle. Minotf 40 ), in 1892, stated regarding it: 



"Several writers have thought that the membrane was broken through at several 

 points, but it probably is really continuous throughout life. The fourth ventricle is to 

 be regarded, then, as an expansion of the central canal permanently bounded by the 

 original medullary walls." 



Kollman^ 32 ', on the other hand, advances the view that during the third month 

 the rhombic roof is broken down to form the foramen of Magendie and the two 

 foramina of Luschka. Streeter^ 54 ), in his chapter on the development of the nervous 

 system in the Keibel-Mall Handbook of Embryology, advances a similar view. The 

 majority of investigators to-day incline to the belief that the roof of the fourth ven- 

 tricle in man is perforated to form the median foramen of Magendie. 



Hess (22) has advanced a conception of the foramen of Magendie that is sup- 

 ported by numerous observations. To test Kolliker's statement that the fourth 

 ventricle remained closed during human embryonic life, Hess sectioned the region 

 in human fetuses, new-born infants, and in adults. The lengths of the fetuses cut 

 were as follows: 7, 12.5, 15, 16, and 17 cm. In the 47 cases the roof showed a 

 medial opening (Magendie), except in one case, in which it was closed by a "thin 

 pial membrane." Hess's conception of the process of formation of this membrane 



