sinus

n cavity in a bone, esp any of the air-filled spaces in the skull that are connected to the nostrils 窦（骨骼中的空穴, 尤指颅骨中的）.

single-minded, singlet, singsong, singular, singularity, singularly, sinister, sink, sink in, sinker, sinkhole, sinking, sinking fund, sinlessness, sinner, Sino- (also sino-), sinologist, sinology, sinuosity, sinuous, sinusitis, sip, siphon, sir, sire, siren, sirloin, sirocco, sirup, sisal, sissy (also cissy), sister, sister-in-law, sisterhood, sisterly, sit, sit down, sit down on, sit for, sit in,