The misshapen heads described in the Mishnah are examples of craniosynostosis. In the course of the development of a child’s head, the skull is made up of a number of bones with spaces between them, allowing them to grow. In normal development, as each of these bones grows, they ultimately fuse together forming a skull of normal size, proportion and shape. Craniosynostosis is a condition in which one or more of the fibrous sutures in an infant skull fuses prematurely by ossification, thereby changing the growth pattern of the skull. Because the skull cannot expand perpendicular to the fused suture, it compensates by growing more in the direction of the open areas between the bones. Sometimes the resulting growth pattern provides the necessary space for the growing brain, but results in an abnormal head shape and abnormal facial features.
Although the seventh perek (=chapter) of Masechet Bekhorot that begins on today’s daf (=page) continues the theme of mumin – blemishes – in it we switch from focusing on animals to people. In Sefer Vayikra (21:16-21) the Torah teaches that kohanim – priests who ordinarily serve in the Temple – even as they retain their unique status as kohanim, cannot serve if they are blemished. What specific blemishes mark a kohen?
A comparison between the list that discusses kohanim and that that discusses sanctified animals (see 22:21-24) shows that there are both parallels and discrepancies between them. One major difference is that the blemishes in animals are all injuries to an animal or to one of its limbs, while with regard to kohanim there is another category of blemishes, when a kohen appears odd or is strange looking. Thus we find in the Mishnayot on today’s daf that among the kohanim who are unfit to serve in the Temple are people with misshapen heads or who are entirely bald or who have a single eyebrow.