The development of the human body; a manual of human embryology . the first arch, from the second arch, from neither one northe other, but from the cartilaginous investment of the otocyst,or, finally, it has been held to have a compound origin, its archbeing a product of the second arch while its basal plate was apart of the otocyst investment. Recent observations seem toplace its independence of the otocyst investment beyonddoubt, in which case its origin from the second arch seemsfairly certain. The Development of the Tympanic Membrane and of theOuter Ear.—Just as the tympanic cavity is form
The development of the human body; a manual of human embryology . the first arch, from the second arch, from neither one northe other, but from the cartilaginous investment of the otocyst,or, finally, it has been held to have a compound origin, its archbeing a product of the second arch while its basal plate was apart of the otocyst investment. Recent observations seem toplace its independence of the otocyst investment beyonddoubt, in which case its origin from the second arch seemsfairly certain. The Development of the Tympanic Membrane and of theOuter Ear.—Just as the tympanic cavity is formed fromthe endodermal groove of the first branchial cleft, so theouter ear owes its origin to the ectodermal groove of cleft and to the neighboring arches. The dorsaland most ventral portions of the groove flatten out and THE EXTERNAL EAR. 473 disappear, but the median portion deepens to form atabout the end of the second month, a funnel-shapedcavity which corresponds to the outer portion of the ex-ternal auditory meatus. From the inner end of this a. 3—me :?-? Fig. 250.—Horizontal, Section Passing through the Dorsal Wallof the External Auditory Meatus in an Embryo of cm. c, Cochlea; de, endolymphatic duct; i, incus; Is, lateral sinus; m, malleus;me, meatus auditorius externus; me, cavity of the meatus; s, sac-culus; sc, horizontal semicircular canal; sc, posterior semicircularcanal; st, stapes; t, tympanic cavity; u, utriculus; 7, facial nerve.—(Siebenmann.) solid ingrowth of ectoderm takes place, and this, enlargingat its inner end to form a disk-like mass, comes into rela-tion with the gelatinous mesoderm which surrounds themalleus and chorda tympani. At about the seventh4o 474 THE DEVELOPMENT OF THE HUMAN BODY. month a split occurs in the disk-like mass (Fig. 250),separating it into an outer and an inner layer, the latter ofwhich becomes the outer epithelium of the tympanicmembrane. Later, the split extends outward in the sub-stance of the ectodermal in
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
