388 THE PINEAL ORGAN 



From the morphological standpoint it must be remembered also 

 that the lining membrane of outgrowths from the cerebral vesicles is 

 homologous with the ependyma, and thus the epithelial lining of the 

 following parts is ependymal in origin : the olfactory lobes ; the optic 

 vesicles, including the pigment and sensory layers of the retina ; the 

 infundibulum of the hypophysis ; the choroidal epithelium ; the para- 

 physis ; the epiphysis and the subcommissural organ ; also the lamina 

 terminalis and the roof and floor-plates of the brain and spinal cord. 

 The shape, the structure, and the function of the ependymal cells in these 

 different situations varies. Thus the cells may be flattened, cubical, or 

 columnar in form. They may be club-shaped, flask-shaped, or bottle- 

 shaped. They may be specialized in form, as in the hexagonal pigment 

 cells, or rod and cone cells of the retina ; they may be glandular in type, 

 as in the choroidal epithelium or in the paraphysis. They are, in some 

 situations, devoid of cilia, in others they possess well-defined cilia, having 

 minute rod-like particles or blepharoplasts at their bases, and their free 

 ends converging to a point from the wide basal or ventricular end of the 

 cell. The cells may be close together or separated by an interval. The 

 basal ends of the cells may be joined by intercellular bridges so as to 

 form an internal limiting membrane or separated so that the spaces 

 between the cells open into the ventricular cavities or central canal of the 

 spinal cord. 



Large intra- and inter-ependymal nerve cells have been demonstrated 

 by Agduhr (1922) in the ependyma lining the central canal of the spinal 

 cord in the human subject and in various mammalian animals. These 

 cells frequently end in a club-shaped process which projects into the 

 central canal and resembles the club-shaped projections of the sensory 

 cells described by Dendy and Studnicka in the parietal organ of cyclo- 

 stomes and fishes. These cells have been beautifully demonstrated by 

 the Nissl method of staining, and Agduhr has shown connections (synapses) 

 between peripheral processes of intra-ependymal cells and the processes 

 of cells lying in the nerve substance external to the ependyma. 



The Functions of the Ependyma 



These may be enumerated in the following order : 

 Generative. 

 Supporting. 



Lining membrane for protection and limitation of the nerve-tissues. 

 Causation of currents in the cerebrospinal fluid by means of its 



cilia. 

 A membrane concerned in dialysis and filtration, or serving as a 



limiting barrier. 



