Page:The New International Encyclopædia 1st ed. v. 20.djvu/134

* VERTEBRATA. 100 VERTIGO. Chabt Showing Distribution op Vebtebhates, aud the Chahacteristic Foems of the Diffekent Periods Periods Characteristic Forms Dominant type 1 3 a liecent ' Pleistocene Man (flint implements, skeletons of Spy and Neanderthal caverns). Mam- moth, mastodon, woolly rhinoceros, Elasmotherium, Irish elk, Megatherium, Glyptodon. Diprotodon, one-toed horses, Platygonus, cave-bear, sabre-tooth tiger. Man, estimated 60.000 years o t Pliocene Pithecanthropus, Sivatherium, Samotherium, three-toed and one-toed horses, mastodon, horned rhinoceroses, Chalicotherium, camels, deer, oxen. Dinocyou, Hytenarctos. o a 6 Miocene Apes and baboons, three-toed horses, small-horned or hornless rhi- noceroses, four-tusked mastodons, Dinotherium. Merycochcerus, Pro- camelus, deer, Amphicyon. Mammals. Oligocene Three-toed horses, Titanotherium, primitive rhinoceroses (Cienopus, Hyracodon, Metaniynodou); primitive ruminants (Oreodon, Poebroth- erium, Anoplotherium. Anthracotherium); primitive dogs, and sabre- tooth tigers. estimated 3,000,000 years. Eocene Four-toed horses; primitive types of hoofed and clawed mammals (Uintatherium, Coryphodon. Phenacodus, Hyrachyus, Palaeotherium, Patriofelis, Oxytena) ; monkeys; rodents; serpents. 1 o Cretaceous Great land reptiles (dinosaurs). Trieeratops. Claosaurus; marine reptiles (mosaaaurs and plesiosaurs); flying reptiles (pterosaurs); toothed birds; crocodiles; turtles; sharks; rays; small insectivo- rous mammals. Jurassic Dinosaurs (Brontosaurus. Diplodocus. Stegosaurus. Iguanodon, Megalo- saurim) ; plesiosaurs and ichthyosaurs ; pterosaurs; reptile-birds (Archieopteryx) ; crocodiles; turtles; true sharks and rays; true bony flshes. Reptiles, estimated 7.000.000 years. Triassic Carnivorous dinosaurs (Auchisaurus. Zanciodon); primitive reptiles (Pariasaurus, Cynognathus, Belodon) ; large armored amphibians (Labyrinthodon, Mastodonsaurus) ; luugfish (Ceratodus). Permian Primitive reptiles (Naosaurus. Diadectes. Pariotichus) ; armored amphi- biaus (Eryops, Diplocaulus. Cricotus, Archegosaurus); primitive sharks (Pleuracanthus). Amphibians, estimated o CarboniJerous Small amphibians (Microsauria) ; primitive bony fishes (Palaeoniscue, Platysomus); primitive sharks (Pleuracanthus). 5,000.000 years. Q "5 &4 Deronian Lungflsh (Dinichthys, Coccoeteus, Dipterus) ; primitive sharks; primi- tive bony flshes; ostracoderms (.Pteraspis, Cephalaspis; Pterichthys, Bothriolepis) ; lampreys. Fishes, estimateil 2.000,000 years. Silurian Ordovician Cambrian First vertebrates (Ostracoderms) ; Cyathaspis; Palasaspis. Invertebrates. estimated 10.000.000 years. Eozoic Algonkian No fossils. Archaean Grundziige der Paldontologie (Munich, 1895), translated by Eastman under 'the title Textbook of Paleontolof/t/, vols. ii. and iii. (Xew York and London, 1900-03), gives a more condensed treat- ment. See also hydekker. Oroprdpliical Histoiti of Mammals (Cambridge, 1896), and Gaudry, En- chainements da monde aniiiuil (4 vols., Paris, 1878-96). VER'TIGO (Lat., a turning or whirling round, giddiness, dizziness). Dizziness. A con- dition cliaracterized by an inability to maintain a due equilibrium. Vertigo may be caused by un- usual and powerful visual sensations, such as those produced by water falling rapidly from a great height, or by objects moving swiftly across the field of vision. In some cases the subject feels as though all the visible objects were passing rapidly before him; in other cases he feels as though he were falling in a certain direction. Vertigo may arise from changes taking |)lace in the brain, and, while in no sense a disease, is a common symptom of many diseases and is a result of several poisons. An especially severe form of dizziness is jiroduced by rapidly rotating the body. A rapid and distressing form is pro- duced by sending a galvanic current through the head from ear to ear. Vertigo caused by rota- tion of the body is probably due to false sensa- tion in the semicircular canals of the internal ear. It is known that variations in the pressure of the endolymph within the canals give rise to two kinds of impulses: the auditory impulses and a series of afl'erent impulses which, although conducted to the brain through the auditory nerve, have nothing to do with hearing, but form the basis of our sense of equililirium. Meniere's disease, an affection of the semicircular canals, presents as its principal symptom a constantly recurring vertigo. Disease of tlie middle ear, and also rarel.v of the external ear, is accompanied by the same symptom. Vertigo may be caused by an alteration of the intracranial blood-pressure. It occurs, therefore, in anaemia, in hyper:rmia. in syncope, in epilepsy, in seasickness, in hyper- trophy of the left ventricle of the heart, in iLT-morrhage. in injuries of the brain, after the use of alcoholics, etc. Sometimes merely a sud- den change of position causes it; as bending for- ward, stooping, rising, raising a weight above the head, or any sudden strain on the faculty of eipiililiration. 'ertigo accompanying indiges- tion is due either to reflex cipculatorv distuT-