The distribution and extent of the cellular system throughout the temporal bone vary greatly, both in infants and in adults. In discussing this problem, the bone as a whole must be considered and not merely the mastoid process. ANATOMY Anatomically, as is well known, the temporal bone of the fetus consists of three parts, namely, the squamous, the petrous and the tympanic portions. This division still holds at birth or until the squamous and tympanic parts become fused into one. The tympanic ring in the new\x=req-\ born is a flat semicircle which is open above. As there is no meatus at this stage of development, the drum membrane, which fits into a groove in the tympanic ring, lies quite unprotected.In the new-born the mastoid process does not exist. The stylomastoid foramen lies on the lateral surface of what is later to be the prominence of the mastoid. The styloid process is still cartilaginous, partially if not entirely. Shortly after birth the mastoid process begins to appear, and the stylomastoid foramen comes to lie on the inferior surface of the temporal bone as the prominence develops. "The mastoid antrum is practically always present at birth, even in cases in which the external meatus and auricle are underdeveloped." 1 As Cheatle and numerous others since have pointed out, this is not the only cell to be seen at this age. The lateral wall of the antrum is studded with minute cells which enlarge as the mastoid process grows. According to Ruttin,1 the otologist divides the temporal bone into four parts: the squamous, the petrous and the tympanic portions and the mastoid. The anterior part of the mastoid belongs to the squamous portion and the posterior part to the petrous bone. Between these two in childhood is a fissure through which pus easily breaks to the surface. The upper wall of the external meatus is formed by the squamous por¬ tion ; the posterior wall is made up of the mastoid, while the anterior and

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