102 Journal of Comparative Neurology. 



Owing to the greater development of the secondary rhom- 

 boidal lip in man, the parts of the recess which it forms are as 

 a rule entirely retained, while in the lower mammals these parts 

 consisting from the start only of epithelium, become lost. 



In man the floor of the recess is formed chiefly from the 

 secondary lip where it extends from the medulla at the region 

 of greatest width. It here forms a plate of nervous matter 

 peculiarly convoluted, which usually not only extends to the 

 extremity of the recess, but folds on itself both at its caudal 

 and lateral portions and so enters into the formation of the roof. 

 In the instance where it is well developed, it may form a con- 

 siderable portion of the roof as well as the floor. Fig, 37. In 

 these cases a more or less complete pouch is formed at the ex- 

 tremity of the recess, which Retzius proposes to call the Mar- 

 supium or Pera. Figs. 35, 36 and 37. 



In other cases the lip at its extremity does not turn over 

 into the roof but ends as a free rounded margin. Fig. 33. In 

 contrast to the condition found in the lower mammals the cau- 

 dal part of the recess in man is, as a rule, well developed and 

 the recess is here closed. Exceptionally the condition pre- 

 sented in Figs. 32, 33 and 34 is found, a condition which closely 

 approximates that found in the lower mammals. The caudal 

 and lateral extremity is here lost and the recess opens widely 

 into the subarachnoid cavity. In this brain the metapore was al- 

 most completely filled by the choroid plexus (Fig. 31) which 

 may explain the unusual size of the openings in the lateral 

 recesses. 



In a large number of brains of adults and children exam- 

 ined, I found the openings in the lateral recesses (foramina of 

 Luschka) present and almost invariably they conformed to the 

 description as given by Retzius, with the single exception 

 figured here. 



Sections of this region of the brain of children at term un- 

 der the microscope show the margins of the foramen as in Figs. 

 35 and 36. The wall of the recess ends in a smooth rounded 

 margin and the abrupt termination of the epithelium is readily 

 made out. 



