THE FUNCTIONS OF THE EPENDYMA 389 



Secretory. 



Pigment formation. 



Special sense. 



Receptive cells concerned in reflex mechanisms. 

 The generative capacity has already been referred to in connection 

 with the differentiation from it of spongioblasts, neuroblasts, and the 

 definitive ependyma, as has also the supportive character of the early 

 ependymal elements. The limiting function of the definitive ependymal 

 layer is, moreover, obvious, and the supposed action of the cilia in pro- 

 ducing movement of the cerebrospinal fluid is well known. The special 

 problems concerning dialysis, filtration, and the formation of a barrier to 

 the passage of certain fluids or substances, whether normal or extraneous, 

 into the cerebrospinal fluid are familiar to neurologists. 



The secretory function of the modified ependymal epithelium which 

 covers the choroidal plexuses is well established, and is definitely proved 

 by inferences made in cases of obstruction to the outflow of cerebrospinal 

 fluid from the ventricles and by direct observation of secretion of the 

 fluid on the surface of the choroid plexus — in the human subject by Mott 

 and Cushing, and in animals by Dandy and Blackfan. It is generally 

 thought that the ependyma in other situations has a limited power of 

 secretion of cerebrospinal fluid, but normally only to a very small extent. 

 The question of the power of the ependymal cells to absorb cerebrospinal 

 fluid has been studied by Nahagas and others. According to Nanagas 

 a very small amount of cerebrospinal fluid may be absorbed through the 

 ependyma lining the ventricles of the brain in normal animals (kittens), 

 and evidence obtained from post-mortem examinations shows that 

 obliteration of the lumen of the central canal of the spinal cord, which 

 sometimes occurs in old age, is not followed by distension of the cana 

 below the obstruction. 



We have already considered the formation of pigment in the outer 

 layer of the retina, in nerve tissues, and in the epidermis, and it will thus 

 be only necessary to recall its frequent presence in the parenchyma cells 

 and fibro-glial tissue of the pineal body : a condition which may be 

 partly due to degenerative changes setting in at an early age, before 

 degeneration has commenced in the nerve tissues as a whole, either as a 

 result of disease or old age ; or it may in part be due to an hereditary 

 trait, which has been preserved from the remote period when it may be 

 inferred pigment was normally present in large quantity in the parietal 

 sense-organ of our reptilian ancestors. 



The possible function of the ependyma as a sensory layer containing 

 receptive nerve-cells which are concerned in reflex action is strikingly 

 suggested by Agduhr's demonstration of nerve cells in the ependyma 



