398 THE PINEAL ORGAN 



of his silver carbonate process, though their nuclei are re- 

 cognizable with other methods of staining, especially in glial 

 plaques. 



(4) Cells and fibres, which resemble nerve-cells and nerve-fibres 



but are in many cases only with difficulty distinguished from 

 the parenchyma and glia cells and the processes of these cells 

 (Figs. 266, 267, 268). 



(5) Arterioles, capillaries, and venules. 



The Parenchyma Cells 



The form and full extent of the branches of the parenchyma or pineal 

 cells is only fully revealed by the silver methods of Del Rio-Hortega, 

 upon whose description the following account is largely based. The 

 pineal cells of an adult human subject thus shown (Fig. 218, Chap. 22, 

 p. 317) are characterized by an irregularly shaped body containing a 

 clear vesicular nucleus. The amount of cytoplasm varies, in some cells 

 being abundant, in others scanty. The cells thus differ greatly in size. 

 The cell-body is usually branched, and the branches vary in number, 

 size, and complexity. Thus the cell may be unipolar, bipolar, or multipolar. 

 The main branches may give off a group of slender processes which may 

 subdivide and end freely, or more commonly terminate in club-shaped 

 swellings implanted on or in the sheath of a vessel wall. The free ends 

 of the smaller branches are said by Del Rio-Hortega not to communicate 

 with similar branches of neighbouring pineal cells. The processes of the 

 cell may be polarized in one direction or they may be distributed irregularly 

 in any direction. In the former case they are usually directed towards 

 a vessel lying in the periphery of a lobule, and lie in what is termed the 

 marginal zone, where they are radially disposed ; in other cases, more 

 particularly in the centre of a lobule, the cells tend to be multipolar and 

 stellate in form, the branches appearing to form a plexus ; the terminal 

 branches of these cells may end in club-shaped swellings on the sheaths 

 of endolobular vessels, or they may extend to the periphery of the lobule 

 and become attached to the sheath of an interlobular vessel. 



Typical branched cells with club-shaped endings are seldom seen in 

 children below the age of eight years, and the complexity of the branching 

 and the size of the club-shaped swellings appear to increase with the 

 advance of age. 



Although the general form and terminal processes of the pineal cells 

 can only be clearly demonstrated by means of the silver methods of 

 impregnation, very little of the structure of the cells and their content is 

 seen in these preparations ; and in order to form a true estimate of the 

 nature of these cells it is necessary to adopt special methods which will 



