THE PINEAL ORGAN OF MAMMALS 319 



tion of the central and marginal plexuses. It is not at all clear, however, 

 what is the exact relation between the connective tissue elements and the 

 processes of the parenchyma cells. According to the description by 

 Studnicka of the pineal organ in birds, and more especially in Strix 

 flammea (see p. 297, Fig. 204), after the disappearance of the external 

 limiting membrane at an early stage of development the connective tissue 

 elements become inextricably blended with the branched processes of 

 the ependymal cells which in birds line the follicles of the epithelial buds, 

 and thus presumably correspond to the parenchyma cells of the 

 mammalian pineal body, since the latter arise by transformation of the 

 indifferent primary ependymal cells. In the adult both collagenous and 

 glial fibres are found in addition to cellular elements having the dis- 

 tinctive characters of connective tissue cells and glial cells (astrocytes). 

 Also, large areas, or " plaques," of degenerated glial tissue, containing 

 few or no parenchyma cells are found in old subjects and sometimes even 

 in children and infants only 4 months old. These areas are conspicuous 

 owing to their being composed of a network of feebly stained, fine glial 

 fibres, and by the sparseness and small size of the nuclei (Fig. 221, A, B, C, 

 Gli.). The existence of these degenerate areas in the pineal bodies of 

 young subjects, quite apart from any diseased condition of the central 

 nervous system generally, appears to afford very strong evidence of the 

 retrogressive nature of the pineal organ in the human subject. The 

 regressive character of the pineal organ is also plainly indicated by the 

 frequent formation of calcareous deposits, which occur in the parenchyma 

 in the glial plaques, in the connective tissue capsule, and in the trabecular 

 (Fig. 317, B, p. 465, Ca., and Fig. 318, p. 466). The deposits are found 

 in the pineal body and the surrounding vascular pia mater from early 

 infancy to old age, and although they are not always present, their exist- 

 ence in or around the gland is sufficiently frequent to have led some 

 authors to describe the condition as being normal in the adult (Fig. 222). 

 The frequent onset of fibrosis and gliosis in early life and the great 

 variability in the general appearance of the gland which is associated with 

 these states are further evidences of involution. Although the maximum 

 degree of differentiation is usually attained by about the seventh year, in 

 many cases, judging from the microscopic appearances of the cells and 

 of the supporting tissues, the development of the organ appears to have 

 been arrested in early childhood or even in infancy. 



The changes that take place in the normal development of the human 

 pineal body in some respects are similar to those which take place in the 

 development of the central nervous system generally, but in the pineal 

 organ they do not reach the high degree of differentiation which occurs 

 in the central nervous system, and in the pineal there is an outgrowth 



