Human anatomy, including structure and development and practical considerations . of the accessory air-spaces. The earliest establishment of these spaces pre-cedes the appearance of the carti-lage that later encloses them, theirrelations to the skeleton being,therefore, secondary (Kallius).The ethmoidal air-cells and thesphenoidal sinus are primarily con-strictions from the nasal fossae, whilethe maxillary and frontal sinusesare more or less direct extensionsfrom the same cavities. The maxillary sinus ap-pears about the middle of the thirdfoetal month as a minute epithe-lium-lined sac within t
Human anatomy, including structure and development and practical considerations . of the accessory air-spaces. The earliest establishment of these spaces pre-cedes the appearance of the carti-lage that later encloses them, theirrelations to the skeleton being,therefore, secondary (Kallius).The ethmoidal air-cells and thesphenoidal sinus are primarily con-strictions from the nasal fossae, whilethe maxillary and frontal sinusesare more or less direct extensionsfrom the same cavities. The maxillary sinus ap-pears about the middle of the thirdfoetal month as a minute epithe-lium-lined sac within the mesoblast at the side of the nasal fossa, from which ithas been evaginated ; by the sixth month it measures some 5 mm., and at birthhas acquired the size of a pea. Until the eruption of the milk teeth provides the In Hertwigs Handbuch d. Entwikelungslehre, Lief. 4 and 5, 1902. Fig. 1190. Fore-brain Nasal apertureLateral nasal process processPrimitive choana Palatal process Part of head of fcetus 15 mm. in length, showing primitive choanaeand palate. -8. [Peter.). 1432 HUMAN ANATOMY necessary room for expansion, its j^routh is retarded. After the sixth year, whenthe eruption of the i)erniani-nt teeth betjins, tlie antrum loses its j^eneral sphericaloutline and i^radually acciuires the definite pyramidal form. The frontal sinus formed as an extension of the nasal fossa durinj^ the thirdfcjetal month, is for a time so small that it is usually regarded as absent at )u<;h indistinctly seen during; the third year, not until about the seventh is the sinus a definite space; it remains small, how-ever, until puberty, after which its adultproportions are gained. The sphenoidal sinus,])rimarily arisesby the constriction and jiaitial isolatif)n of apart of the primitive nasal fossa. Althoughits development begins during^ the thirdfoetal month, the space remains so rudimen-tary that not until the seventh year hasabsorption of the cancellous bone progressedsuffici
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