CORRELATING CENTERS IN THE DIENCEPHALON. 285 



the ciliated cells are said to be absent. A general scheme of the 

 saccus apparatus is given in Fig. 144. 



In mammals there is a sac with epithelial lining and dorsal to 

 it a thick mass containing numerous cells of doubtful character 

 and a rich plexus of nerve fibers (Berkeley, Cajal). From this 

 plexus fibers pass into the epithelium to end freely among its cells 

 (Cajal, Gemelli). The nerve plexus is connected with the brain 

 by a large tract which runs along the raphe of the tuber cinereum. 

 The tract takes origin from a nucleus situated directly over the 

 optic chiasma. The tract and nucleus correspond in position to 

 the efferent tract, and its nucleus in fishes. The epithelial sense 

 cells and the tractus sacco-thalamicus have not been described in 

 mammals. 



In all classes of vertebrates this outgrowth of the brain wall is 

 present and is provided with nervous elements. Although the 

 structure has been very incompletely studied, enough is known 

 from fishes, amphibia and mammals to indicate that the relations 

 of the saccus are fairly constant in the vertebrate series. The 

 only suggestion regarding its function is that it serves as an organ 

 for controlling the character of the cerebro-spinal fluid. Its plenti- 

 ful blood supply and its thin wall adapt it for secreting fluid into 

 the brain ventricle. The existence of a double nerve supply, both 

 centripetal and centrifugal, indicates that it does more than simply 

 secrete. The ciliated cells must be regarded as sense cells and it is 

 conceivable that they may be stimulated by changes of either 

 pressure, density or chemical character in the cerebro-spinal fluid. 

 In response to these stimuli the saccus may secrete some specific 

 constituents of the ventricular fluid. The tract which ends in the 

 saccus epithelium would arouse or control this secretive activity. 



7. Mention may be made here of an epithelial structure resem- 

 bling in some respects the saccus epithelium, which forms the base 

 of the epiphysis in the roof of the diencephalon. This epithelium 

 receives the free endings of nerve fibers and gives rise to fibers 

 which go in various directions. Part of the in-coming and out-going 

 fibers form a commissure over the base of the epiphysis (Holt, 

 Johnston). 



The fate of the four functional divisions in the midbrain and 



