. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 792 Biology of the Vertebrates young stranger is a going concern, having marvelously made the critical transition from uterine existence to citizenship in the world. On the face of the tympanic wall opposite the ear drum and next to the inner ear, are two windows, the fenestra ovalis and the fenestra rotunda, similarly curtained by drumhead-like membranes, so that the middle ear bears some resemblance to a hollow drum with very small membranous heads at th
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 792 Biology of the Vertebrates young stranger is a going concern, having marvelously made the critical transition from uterine existence to citizenship in the world. On the face of the tympanic wall opposite the ear drum and next to the inner ear, are two windows, the fenestra ovalis and the fenestra rotunda, similarly curtained by drumhead-like membranes, so that the middle ear bears some resemblance to a hollow drum with very small membranous heads at the two ends (Fig. 692). Endolymphatic Sac Utriculus Sacculus Cochlear Duct Perilymphatic. ,\^~-\^ stapes (Fitting into Fenestra Ovalis) \ Tympanic Membrane Fenestra Rotunda Eustachian Tube Fig. 692. Diagram of the ear region of an adult mammal. (After Goodrich.) The tympanic cavity, or middle ear, in land forms is derived from the inner part of the ancestral spiracular cleft of the elasmobranch fishes that lies between the mandibular and hyoid arches of the primitive splanchno- craniums. In forming the middle ear the pharyngeal pouch never breaks through to the outside. The intervening "skin" remaining at the outer end of the spiracular pharyngeal pouch becomes the external tympanic mem- brane, while the opposite end extends to the pharynx as the Eustachian tube. Thus the external tympanic membrane is made up of an ectodermal layer on the outside and an endodermal layer on the inside. The Eustachian tube is a ventilator of the tympanic cavity. Broader at the pharyngeal than at the tympanic end, it remains closed except during the act of swallowing when it may open sufficiently to permit an adjustment of air within the tympanic cavity. When the pressure of air is so adjusted that it is the same on the inside of the tympanic membrane as on the outside,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabil
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
