. Evolution and disease . sauropsida and mammalia. This is much more probable than to attribute the change to the evolution of lungs froma swim-bladder, for functionalgills and lungs co-exist in suchforms as the mud-fish {Lepido-siren) and ceratodus. The history of the pinealeye is an instructive with the vertebratemid-brain is a structure knownas the pineal body, whichhas long puzzled investigators have re-garded it as vestigial ; that is,it was of some functional valuein the ancestors of exist- Fig. 20.—The head of a Lizard ing vertebrata. The truth of (Var
. Evolution and disease . sauropsida and mammalia. This is much more probable than to attribute the change to the evolution of lungs froma swim-bladder, for functionalgills and lungs co-exist in suchforms as the mud-fish {Lepido-siren) and ceratodus. The history of the pinealeye is an instructive with the vertebratemid-brain is a structure knownas the pineal body, whichhas long puzzled investigators have re-garded it as vestigial ; that is,it was of some functional valuein the ancestors of exist- Fig. 20.—The head of a Lizard ing vertebrata. The truth of (Varanus), showingthe-bright this opinion has been demon-scale indicating the pineal eye, P. r strated by the admirable re-searches of De Graaf and Baldwin Spencer. On the dorsal aspect of the skull in lizards asmall opening exists, known as the parietal some lizards — as, , Varanus—the situation ofthis foramen is indicated by a bright scale (fig. 20).On making a longitudinal section of the head, so as ?-. DISUSE AND ITS EFFECTS. 43 to include this parietal foramen, we find it occupiedby an organ representing an eye in miniature, con-nected with the pineal body by nerve-fibres. Therelation of the parts is indicated in fig. 21, represent-ing a lateral view of the brain and adjoining partsof the skull in the New Zealand lizard, Hatteria(Sphenodoii). When the eye is examined microscopically, it pre-sents the structural details found in functional eyes,such as cornea, lens, retina, pigment, &c. (fig. 22).
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Keywords: ., bookcentury1800, bookdecade1, booksubjectcongenitalabnormalities
