332 



THE PINEAL ORGAN 



the lateral horns which bounded the gap on each side gave rise to the 

 squamosal plates and maxillae. In the course of evolution of the brain 

 it seems that the pineal region has — relatively to the cerebral hemispheres 

 — been gradually displaced backwards ; thus the pineal foramen in the 

 earlier types of skull such as Osteolepis lies in the same transverse plane 

 as the orbital cavities for the lateral eyes and between the frontal bones. 

 In later types the foramen lies between the parietal bones, where, owing 

 to the formation of the median anteroposterior crest formed between the 

 two temporal muscles and running backwards to the occipital region (Figs. 

 205, 206, Chap. 22, pp. 300, 301, and Fig. 233), it eventually becomes 

 obliterated. The direction of the pineal foramen or plate seems also to 

 have changed in the course of time. Thus in the restoration of the skull of 



Fig. 233. — View from above of the Skull of an Ictidosaurian Reptile, 

 showing the median parietal crest. all trace of a parietal foramen 

 has disappeared. there are two occipital condyles, and other mam- 

 MALIAN Characteristics. (After Broom.) 



F. : frontal. L. : lacrimal. N. : nasal. 



J. : jugular. Mx. : maxilla. Par. : parietal. 



Dinichthys intermedins (Fig. 234) the plate seems to have been directed forward 

 as well as upwards, in Osteolepis directly upwards ; while the apex of the 

 pineal organ, inside the skull, in adult mammals is directed backwards 

 over the quadrigeminal plate and towards the vermis of the cerebellum. 

 The relative position of the pineal organ to the skull and the fore-brain 

 is greatly affected in the human subject and in mammals generally by the 

 growth of the hemispheres, which as they enlarge in a forward direction 

 are also bent ventral ward, thus forming the primary or cephalic flexure. 

 The pineal organ remains for a time in close relation to the membranous 

 capsule of the brain and skin at the summit of this flexure. In this 

 primarily superficial position the pineal organ of amphibians and reptiles 

 appears to have attained its highest degree of development, as is evidenced 

 by the large size of the pineal foramen in certain of the extinct amphibia, e.g. 

 Protriton (Fig. 170), and Procolophon (Fig. 170, a), Chap. 19, p. 237. More- 



