CHAPTER 22 



THE PINEAL ORGAN OF MAMMALS 



Only the basal segment of the organ, the epiphysis or conarium, is 

 recognizable in man and mammals generally ; the terminal parts, namely, 

 the parietal sense-organ, the end-vesicle or pineal sac, and the stalk, are 

 seldom present in the adult animal, and in many cases it is doubtful whether 

 any vestige of these parts, even in a rudimentary condition, is found in 

 the embryo. It is possible, however, that the occasional occurrence of 

 bifid pineal diverticula and accessory organs and the development in the 

 foetus of an anterior lobe may be attributed to an inherited trait, which 

 has not yet been completely exhausted or suppressed, and which serves 

 as an indication of its primary dual origin. The sagittal or parietal 

 fontanelle of the human skull and the parietal foramina which are formed 

 at its lateral angles, each of which transmits a small vein and artery, may 

 also be regarded as possible vestiges of the parietal foramen which in 

 certain reptiles and fishes lodges the parietal sense-organ. Although 

 in the adult human skull the roof is separated from the pineal organ by 

 the whole depth of the falx cerebri and the splenium of the corpus callosum, 

 in the human foetus at the 7th week, when the rudiment of the pineal is 

 first recognizable, the roof of the diencephalon is quite near the con- 

 densation of mesenchyme which represents the future membranous 

 capsule of the brain, and there is a special development of endothelial 

 lined vascular spaces in this situation. Further, the occasional appearance 

 of pigment in the skin of the head of the pineal region in certain swimming 

 birds (Klinckowstroem), mentioned on p. 75, and the occasional occur- 

 rence of a parietal foramen in the skull of the goose (Mrazek), more 

 especially in those examples which possess a tufted crest, indicate that 

 remnants of a parietal fleck and a parietal foramen may persist in the higher 

 classes of vertebrates long after all remnants of the parietal eye have dis- 

 appeared ; and that, if this is true, the validity of the supposed morpho- 

 logical significance of the parietal foramen in the human subject is not 

 so difficult to accept as was formerly thought. The hypothesis that the 

 parietal fontanelle and the parietal foramina of the human skull are 

 homologous with the parietal foramen of reptiles, amphibia, and fishes 

 has, moreover, received a considerable amount of support from recent 

 geological evidence of the foramen in fossil skulls of the Therapsids or 



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