. Outlines of zoology. Zoology. 440 STRUCTURE OF VERTEBRATA. what is called the choroid plexus of the third ventricle in higher Verte- brates. This choroid plexus is a thin epithelium, with blood vessels in it. But in Elasmobranchs, Dipnoi, and Amphibians the basal parts of the fore-brain have grown upwards to form a nervous roof, and this persists in higher Vertebrates. The optic thalami (thalamencephalon or tween-brain) form the second region of the adult brain. Hence arise the optic out- growths, which form the optic nerves and some of the most essential parts of the eyes. The original cavi
. Outlines of zoology. Zoology. 440 STRUCTURE OF VERTEBRATA. what is called the choroid plexus of the third ventricle in higher Verte- brates. This choroid plexus is a thin epithelium, with blood vessels in it. But in Elasmobranchs, Dipnoi, and Amphibians the basal parts of the fore-brain have grown upwards to form a nervous roof, and this persists in higher Vertebrates. The optic thalami (thalamencephalon or tween-brain) form the second region of the adult brain. Hence arise the optic out- growths, which form the optic nerves and some of the most essential parts of the eyes. The original cavity per- sists as the third ventricle of the brain; the thin roof gives off the dorsal pineal outgrowth or epiphysis, and, uniting with the pia mater, or vascular brain membrane, forms a choroid plexus; the lateral walls become much thick- ened (optic thalami); the thin floor gives off a slight ventral evagination, or infundibulum, which bears the en- y*< igmatical pituitary body or hypophysis. The pituitary 'body.—This is derived in part from the brain and in part from the mouth, and is extremely difficult to under- stand. It is apparently equivalent in part to the sub-neural gland of Tunicates, but this does not carry us much further. Dohrn con- nected it with two abortive gill-slits, but the evidence seems insufficient. Beard has inter- preted it as a residue of the original mouth which Vertebrates are supposed to have pos- sessed before the persistent one with which we are familiar was evolved, and of the in- nervation of that hypothetical structure ; but again confirmation seems wanting. Of its physiological nature we know almost nothing, beyond that a pathological state of this organ is associated in man with certain diseases, acromegaly. The pineal body.—The dorsal upgrowth (or epiphysis) from the roof of the thalamencephalon is represented, though to a varying extent, in all Vertebrates. It is terminally differ- entiated into a little body known as the pineal
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
