. Evolution and disease . 5° EVOLUTION AND DISEASE. and undergone gradualtransformation into a spinalcord. These facts werebased on the embryologicalhistory of the parts andfrom a study of the malfor-mation exhibited, not rarely,by this central canal andthe permanent alimentarycanal. The mode of developmentof the parts may be brieflydescribed. The alimentarycanal and spinal cord of allvertebrates arise on a com-mon plan. At a certainstage it consists of a U-shaped tube, as in fig. 24,each limb ends in a cul-de-sac, and the ventral limb isconnected by a hollow ductwith the yolk sac. By a series
. Evolution and disease . 5° EVOLUTION AND DISEASE. and undergone gradualtransformation into a spinalcord. These facts werebased on the embryologicalhistory of the parts andfrom a study of the malfor-mation exhibited, not rarely,by this central canal andthe permanent alimentarycanal. The mode of developmentof the parts may be brieflydescribed. The alimentarycanal and spinal cord of allvertebrates arise on a com-mon plan. At a certainstage it consists of a U-shaped tube, as in fig. 24,each limb ends in a cul-de-sac, and the ventral limb isconnected by a hollow ductwith the yolk sac. By a series of secondarychanges the anterior end ofthe ventral limb of this tubeis made to communicatewith the exterior by way ofthe mouth and pharynx, and Fig. 25.—A diagram representing the parts in the adult derived from the U-shaped tube of the embryo. S, stomodoeum; I, infundibulum; Pt, pituitary-body; T, thyroid; L, lung; M, vitello-intestinal duct; A, allantois; C,coccygeal body ; G, post-anal gut; N, neurenteric DISUSE AND ITS EFFECTS. 51 by a similar method a nether opening, the anus, isestablished. The intermediate section becomes thepermanent ahmentary canal. The walls of the dorsalsection of the tube thicken, the cavity becomes restricted,and the bend connecting it with the ventral tube atrophies,thus disconnecting the two limbs ; the dorsal portionfinally developes into the brain and spinal cord. Thevarious connecting parts are represented in the adult bythe following structures, diagrammatically indicated infig. 25. The diverticulum, P, is the infundibulum ; thiswas closely associated with the primitive gullet. Theconnecting limb, N,completely disappears, but the sectionof the gut into which it opens is represented by a smallpedunculated body at the extreme end of the vertebralcolumn, and known as the coccygeal gland. The dorsaland ventral limbs of these tubes in the adult are con-nected in a subtile manner by means of nerves. Thewalls of the dorsal tube contain
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectabnormalitieshuman
