An-i-mal´cule (L. animal´culum, a small animal). Applied to animals which can only be seen with the aid of the microscope. Animalculum (plural, animalcula) is used with the same meaning.
A-or´ta (Gr. ἀορτέομαι, aorteomai, to be lifted up). The largest artery of the body, and main trunk of all the arteries. It arises from the left ventricle of the heart. The name was first applied to the two large branches of the trachea, which appear to be lifted up by the heart.
A´que-ous Humor (L. aqua, water). A few drops of watery colorless fluid occupying the space between the cornea and crystalline lens.
A-rach´noid Mem´brane (Gr. ἀράχνε, arachne, a cobweb, and εἰδος, eidos, like). An extremely thin covering of the brain and spinal cord. It lies between the dura mater and the pia mater.
Ar´bor Vi´tæ (L.). Literally, "the tree of life;" a name given to the peculiar appearance presented by a section of the cerebellum.
Ar´ter-y (Gr. ἀέρ, aer, air, and τέρειν, terein, to contain). A vessel by which blood is conveyed away from the heart. It was supposed by the ancients to contain air; hence the name.
Ar-tic-u-la´tion (L. articulo, to form a joint). The more or less movable union of bones, etc.; a joint.
A-ryt´e-noid Car´ti-la-ges (Gr. ἀρύταινα, arutaina, a pitcher). Two small cartilages of the larynx, resembling the mouth of a pitcher.
As-sim-i-la´tion (L. ad, to, and similis, like). The conversion of food into living tissue.