226 HENRY C. TRACY 



connection with the space around the brain (fig. 8). Over the 

 brain there is also a perilabyrinthine structure in the form of a 

 hollow rod or band which arches over the medulla behind the 

 cerebellum and connects the base of the superior sinus of the 

 labyrinth with the corresponding structure of the other side 

 (figs. 12 and 16, PSC). 



Less extensive developments of the perilabyrinthine tissue 

 are found in other places, particularly around the superior 

 sinus and the superior end of the anterior semicircular canal 

 (Alosa) where it lies free in the cerebral cavity (figs. 7 and 12). 

 There is also a plate of this tissue which begins in the roof of 

 the extreme anterior end of the sacculus and passes laterally 

 involving the ventral wall of the utriculus; it is somewhat tri- 

 angular in form and extends under the pterotic segment of the 

 arch which leads into the lateral recess (figs. 9 and 12, TPP). 

 This plate is probably the structure which Breschet called the 

 'bulbe accessoire;' Ridewood describes an expansion of the 

 utriculus in this position. Tysowski, however, describes it as 

 merely a three-sided pyramidal thickening of the utricular wall. 

 A reconstruction of the cavity of the labyrinth definitely proved 

 that this structure contains no diverticulum of the labyrinth. 



Condensed tissue also occurs as a local thickening of the 

 outer wall of the recessus utriculi and under the divisions of the 

 maculi acustica utriculi. It is important to note that the floor 

 of the recessus utriculi between, and on each side of, the anterior 

 and middle division of the macula acustica utriculi is thin and 

 contains little or no dense tissue (fig. 17). 



Through certain parts of these plates and processes of com- 

 pact perilabyrinthine tissue, the forms and relations of which 

 have just been described, and through certain parts of the undif- 

 ferentiated perimeningeal tissue is an extensive system of 

 spaces and canals which result from a rarefaction or thinning 

 out of the tissue elements. In the manner of their develop- 

 ment, these spaces and canals are probably comparable with the 

 perioticular spaces of the mammalian ear as described by Streeter 

 ('17). In some of these spaces the rarefaction of the tissue is 

 merely relative as compared with the compact tissue; in others 



