(Dim. of circus, a circle). A circle. Besides its proper signification, it is applied to different parts of the body; as, by Hippocrates, to the balls of the cheeks, the orbs of the eyes, or the cavities which surround the eye. Circulus is also the name of an iron instrument used by the chemists for cutting off a neck of glass vessels. The circulus is heated, then pressed close to the glass where to be divided; and when the glass is hot, the application of a blast of cold air, or a few drops of water, separates it. The circulus is some-limes called abbreviatorium.

It is also reckoned among surgical instruments, figures of which may be seen in Scultetus's Armamentarium Chirurgicum, tab. xxii. fig. 6, 7; tab. xliii. fig. 5.

Circulus quadruplex. See Circus quadruplex.

Circulus arteriosus iridis. It is composed of two arteries going round the basis of the iris. From the arteries of the external lamina are sent several ramifications to the circumference of the iris, where they produce a vascular circle, called circulus arteriosus. From this vascular circle pass off many smaller vessels, which form themselves into arches; and from these arches still finer vessels are sent, which probably secrete the aqueous humour.