THE PINEAL BODY 131 



different in different periods of life. The foramen is not the 

 result of direct pressure of the eye, but occurs for the purpose of 

 permitting the passage of light rays. As a rule, the parietal eye 

 lies in the foramen or under it, so that its optic axis corresponds 

 to that of the foramen. In Sphenodon a single exception to 

 this rule is noted by Spencer. 306 Here the organ is tipped for- 

 ward so that the light rays cannot reach the entire retina. The 

 size of the foramen differs and bears no direct relation to the 

 size of the parietal eye. The third eye is connected to the 

 foramen by means of connective tissue and is surrounded by 

 lymph spaces while blood vessels make up a net about the 

 edges of the foramen. No mention of muscular tissue or dis- 

 crete muscles has been made in connection with the parietal 

 eye. 



Leydig 238 in 1891 found in Lacerta muralis, near the tip of 

 the epiphysis, four round, free, calcium bodies. Similarly in 

 Varanus nebulosus many small pieces of calcium carbonate have 

 been observed. These, however, have nothing to do with the 

 more common deposits of brain sand in the pineal organ of 

 mammals, as Leydig 238 originally thought. 



The interior of the parietal eye contains a coagulum, the 

 vitreous or the corpus vitreum. This consists of a syncytial 

 layer of cells entirely free of pigment. A sclera has been de- 

 scribed as developing in connection with the membrana limitans 

 externa which passes over into the connective-tissue sheath of 

 the eye. There is a space between these two layers which was 

 originally supposed by Leydig 238 to be a large lymph space. In 

 most cases the connective tissue forms a sheath for the eye 

 which may be considered as a sclera. In other instances it is 

 absent. The connective-tissue capsule of the parietal eye is 

 considered analogous to the sheath of the eye in Petromyzon. 

 The connective tissue above the eye becomes differentiated as a 

 cornea and contains no pigment. It is almost fiberless connective 

 tissue. A parietal spot is absent in those saurians in which 

 no parietal eye or no parietal foramen develops. It is recognized 

 as a less pigmented area in the skin and presents many different 

 appearances, as well as differences in size, in the several species 

 of saurians (fig. 68). 



