160 



KNOWLEDGE 



[June 1, 1889. 



growth of bone until several months after birth. The term 

 " foutanelle " was no doubt applied to these localities on 

 account of a fxnoied resembUiuce of the pulsations there 

 seen to a fountain (Jons). The particular foutanelle to 

 which attention is now drawn is called by anatomists the 

 parietal foramen, because it is a hole thi-ough the walls of 

 the skull. 



Now that foramen has a very remarkable story to tell of 

 the ancestry of the race of animals in whose skulls it is 



Section (mag.nified) through 1'ineal Eyk oi- " Varanus 

 GiGANTEUS." o V, optic vesicle; P s, pineal stalk (optic 

 nerve) ; L, lens ; P, pigment ; R, rods of retina ; r', specialised 

 rods at entrance of nerve fibres ; c T, connective tissue ; 

 B V, blood vessels ; p B, parietal bone. 



SECTIOS (MAGMHED) THEO01 h Pineal Ete ot ' Hattekia 

 PUKCTATA." References as in section of Pineal Eye of Varanus. 



found. It marks, in fact, the position occupied in former 

 generations by an eye. It is in the middle line of the skull, 

 and hence reminds us of the Cyclops. 



In some of the lower vertebrates the foramen remains un- 

 closed throughout life, and in some a peculiar structure lies 

 beneath it, of which the function has remained until very 



recently a complete mystery. It was spoken of by Descartes, 

 if not by more ancient writers, as " the seat of the soul." 

 On account of its shape — somewhat conical, and resembling 

 the foim of a pine-cone — it is called the "pineal body." In 

 the. human brain it is only seen after separation of the 

 hemispheres, and it lies just in front of the corpora quadri- 

 (jemina. It is larger in children than in adults, and in 

 females than in males. In some amphibians and reptilians 

 the structure is less simple. Lying between the wall of the 

 skull and the connective tissue beneath, it is very liable to 

 be overlooked. 



That the pineal body is connected with an atrophied 

 median eye will hardly be doubted after an examination of 

 the structure and its connections in two genera of reptiles 

 — Varanus and Ilatteria — both very old types. The gigantic 

 lizard, Varanus, may be looked at in tlie Zoological Gardens, 

 Regent's Park. In the median line of the skull is one dermal 

 scale of lighter colour than the surrounding ones. That scale 

 maiks the position of the obscured eye. Beneath there lies a 

 perfect structure, a magnified section of which we show in an 

 illustration. There is a crystalline lens, an optic vesicle, a 

 lit'autiful retina (with its "rods iind cones" turned in an 

 oi)posite direction to that in which they occur in the paired 

 vertebrate eyes — viz., in the same direction as they occur in 

 the eyes of invertebrates), and an optic nerve connecting the 

 structure with the brain. And may not the modified dermal 

 scale represent an atrophied cornea? In Ilatteria the eye 

 is just as perfect, nay more so, since the pigment spots are 

 absent from the crystalline lens. 



In the little legless lizard known as the " slow- worm " or 

 " blind-worm," the atrophied eye ma}' be found, but no longer 

 united by a nerve-strand with the brain. In the embryo 

 frog it exists in complete connection with the brain ; but in 

 the adult the skull has severed the connection, and the eye 

 lies oulsule. In man the eye has entirely disappeared, and 

 what is probably the base of the nerve-strand, the " pineal 

 body," alone remains 



In the long-extinct Labyrinthodonts, as well as in some 

 of the Saurians, the parietal foramen presents i-idges and 

 grooves which probably indicate muscular attachments con- 

 cerned in the movement of the eyeball. 



Thus the old ancestral invertebrate eye must have sur- 

 vived long after certain oflshoots of the invertebrate stock 

 had taken up a distinctlj' vertebrate structure, and must 

 have been used side by side with, or rather between, the 

 paired vertebrate eyes. The ancient beasts, thus doubly 

 provisioned, must have found the paired eyes more useful 

 than the unpaired median eye. It woidd be impossible to 

 say what different effects upon the brain and consciousness 

 are produced by the turning back of the retinal rods and 

 cones, since we have not the opportunity of comparison, 

 being possessed of only one of the types of eye. But that 

 ancestral forms suffered the one to become functionless by 

 want of use proves the other to have been the better 

 adapted to their requirements. No vertebrate now uses its 

 ancestral invei-tebrate eye. Where the latter is possessed, 

 it is hidden away beneath opaque tissues, a useless heirloom, 

 but of intensest interest to those higher animals who feel a 

 certain antiquarian affection for what is so pregnant with 

 biological lore. Genealogies which carry us back only some 

 twenty or thirty human generations have entrancing inte- 

 rest for many minds : how does the lesson of the pineal 

 body and the parietal foramen in man compare with these, 

 throwing back his ancestry through long geological ages ! 



The Queen has been pleased to confer the dignity of a 

 Baronetcy on Professor G. G. Stokes. 



