27O THE PINEAL ORGAN 



The size and shape of the parietal foramen differ considerably in 

 different types. Apart from variations in the actual diameter of the 

 foramen which accompany variations in the size of the animal, there is a 

 wide difference in the proportional size of the foramen relative to the 

 size of the skull. Thus in Procopholon (Fig. 171, A, Chap. 19, p. 237), 

 in which the skull is only 5 cm. in length, the diameter of the circular 

 parietal foramen is 3-3 mm. ; whereas in Ichthyosaurus, which has a skull 

 50 cm. long, the parietal foramen is 16 mm. long and 8 mm. wide. The 

 foramen is thus actually much larger in Ichthyosaurus than in Procopholon, 

 although it is relatively smaller. In both animals the orbits are very large 

 relative to the size of the head, but they are exceptionally large in Pro- 

 copholon. The shape of the foramen also varies, the superficial opening 

 usually being either oval or circular in outline. In Mixosaurus the anterior 

 margin of the opening is slightly concave forward, but it is not definitely 

 heart-shaped. In section the parietal canal may be tubular with straight 

 sides, or the edges may be bevelled, the central part being narrow (Lacerta 

 agilis). In other cases it is funnel-shaped, the narrow end of the funnel 

 being either superficial, as in Pseudopus Pallasii (Fig. 177, p. 248), or the 

 smaller opening may be below and the larger above, as in Varanus 

 giganteus (Fig. 176, p. 247). The opening is usually closed superficially 

 by a fibrous membrane continuous at the margin of the foramen with 

 the periosteum and the membrane lining the canal. The membrane is 

 destitute of pigment, as are the structures which lie superficial to it ; 

 moreover, the superficial part of the parietal eye is firmly adherent to 

 the membrane, so that if traction is employed in removal of the organ 

 from the foramen the eye is liable to be damaged. Exceptionally, the 

 lower aperture may be closed by membrane continuous with the dura 

 mater, so that the parietal organ is, strictly speaking, outside the cranial 

 cavity, as in certain amphibia (Fig. 161, Chap. 19, p. 228). This con- 

 dition is found in chameleons. The foramen in the adult animal is in 

 bone ; in the embryo it is in cartilage, as in the Selachia. Its contents 

 vary ; as a rule the parietal organ is situated superficially in contact with the 

 external closing membrane and its diameter is about one-third the diameter 

 of the foramen. It is surrounded by a loose, fibrous connective tissue, 

 often containing pigment and sometimes fat. In Sphenodon there is a 

 special inner and outer fibrous capsule containing blood-vessels and 

 traversed by the parietal nerve. In addition to these structures, the 

 parietal recess may contain a part of the end vesicle of the pineal organ 

 and one or more accessory organs (Fig. 177). In some instances it has 

 been noted that in the embryo the parietal organ does not lie in the 

 foramen, but below this, and it is only at a later period that it is found 

 actually in the foramen, e.g. Lacerta vivipara (Owsjannikow, 1888), and 



