The development of the human body; a manual of human embryology . while externally it is separated from theupper part of the ectodermal groove of the first branchialcleft by the thin membrane which forms the floor of thegroove. It has been seen in an earlier chapter that the axialmesoderm of each branchial arch gives rise to skeletalstructures and muscles. The axial cartilage of the ventralportion of the first arch is what is known as Meckelscartilage, but in that portion of the arch which forms theroof and anterior wall of the tympanic cavity, the car-tilage becomes constricted to form two ma
The development of the human body; a manual of human embryology . while externally it is separated from theupper part of the ectodermal groove of the first branchialcleft by the thin membrane which forms the floor of thegroove. It has been seen in an earlier chapter that the axialmesoderm of each branchial arch gives rise to skeletalstructures and muscles. The axial cartilage of the ventralportion of the first arch is what is known as Meckelscartilage, but in that portion of the arch which forms theroof and anterior wall of the tympanic cavity, the car-tilage becomes constricted to form two masses which later 47° THE DEVELOPMENT OF THE HUMAN BODY. ossify to form the malleus and incus (Fig. 248, m and i),while the muscular tissue of this dorsal portion of the archgives rise to the tensor tympani. Similarly, in the case ofthe second arch there is to be found, dorsal to the extrem-ity of the cartilage which forms the styloid process of theadult, a narrow plate of cartilage which forms an invest-ment for the facial nerve (Fig. 248, VII), and dorsal to. Fig. 248.—Semi-diagrammatic View of the Auditory Ossicles of an Embryo of Six , Incus; /, jugular vein; m, malleus; mc, Meckels cartilage; oc, capsule of otocyst; R, cartilage of the second branchial arch; st, stapes; VII, facial nerve.—(Siebenmann.) this a ring of cartilage (st) which surrounds a small arteryand represents the stapes, in connection with which amuscle, termed the stapedius, develops. Since, as has already been stated, the two arches meetdorsally above the primitive tympanic cavity, the struc-tures just mentioned lie embedded in the mesenchymeforming the roof of the cavity, as does also the chorda THE MIDDLE EAR. 471 , . ? v __ _/ \M ?M tympani, a branch of the seventh nerve, as it passes intothe substance of the first arch on the way to its destina-tion. The mesenchyme in which these various structuresare embedded is rather voluminous (Fig. 250), and afterthe end of the seventh monthit becomes
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
