282 NERVOUS SYSTEM OF VERTEBRATES. 



mass of central gray and in more highly specialized forms (teleosts, 

 birds, mammals) special nuclei are present in addition to those 

 mentioned. The functional relations of these are not yet suf- 

 ficiently understood. It must be noted that some of these nuclei 

 which constitute correlating centers of a higher order than those 

 described above, by some authors have been assigned without 

 reason to the hypothalamus. Thus the nucleus rotundus and 

 even the corpus interpedunculare of bony fishes have been included 

 in the hypothalamus. Equally unfounded is the assignment of 

 the corpus Luysii, the field of Forel and adjacent centers in mam- 

 mals to the hypothalamus. The term hypothalamus should 

 be strictly limited to the infundibular and mammillary regions 

 which have secondarily bulged ventrally and which serve as 

 centers related to olfactory conduction paths. 



6. THE SACCUS APPARATUS. 



In all lower vertebrates the saccus vasculosus is a wide sac, 

 sometimes much branched, with epithelial lining and with an ex- 

 tremely rich blood supply in its walls. This outgrowth of the 

 brain wall comes into very close relation with the vestige of the 

 hypophysis, the branching tubes of the saccus often being inter- 

 digitated with the epithelial sacs of the hypophysis. This fact 

 has led many authors to ascribe to the hypophysis nervous struc- 

 tures which really belong to the saccus. 



The saccus is a part of the brain wall and is composed of elements 

 characteristic of the brain wall. The vascular plexus is of course 

 of mesodermal origin. Within this the saccus consists of a layer 

 of nerve fibers and an epithelium bounding the cavity which is 

 a part of the brain ventricle. The epithelial lining is made up of 

 supporting cells and nerve cells. The supporting cells form an 

 internal limiting membrane as in the rest of the brain and extend 

 through the fiber layer in which they form a supporting meshwork. 

 These cells are therefore comparable with the ependyma cells of 

 the brain. The nerve cells have been described in several fishes 

 both in embryonic and adult stages (Lundborg, Johnston). They 

 are rather large spindle-shaped cells whose inner ends bear a tuft 

 of cilia projecting into the ventricle. Such cells are already 



