Acetabulum
A cup like depression on each side of hip girdle into which head of femur (thigh bone) fits, forming hip joint, in tetrapod vertebrates.

Acetylcholine
(Ach). Substance (neural transmitter) secreted at the ends of many nerve fibres (cholinergic fibres) when nerve impulses (action potentials) arrive there. Where such a nerve fibre ends at a synapse e.g. in a sympathetic or parasympathetic ganglia. A Ch is the agent which stimulates the contiguous nerve cell and hence in effect passes the impulse on’; and similarly where it connects with an effector, e.g. at the nerve-muscle junction. After secretion A Ch is very rapidly destroyed by the enzyme cholinesterase, being broken down into acetate and Choline. It is the acetyl ester of Choline.

Acid
Any chemical substance that acts as a proton donor. acids dissolve in water with the formation of hydrogen ions which may be replaced by metals to form salts.

Acid dyes
Dyes consisting of an acidic organic grouping of atoms (anion) which is the particular staining part, combined, with an inorganic cation, e.g. eosin. stain particularly cytoplasm and collagen.

Acidophil
Having tissues that are easily stainable with acid dyes.

Acidosis
A condition in which the concentration of bicarbonate in blood is below normal.