Hypodermic Needles
The hypodermic needle was invented independently by Charles Gabriel Pravaz in France and by Alexander Wood in England in 1853. Since then, needles have become the most widely used medical device, with an estimated 16 billion injections administered worldwide. Currently, needles are available in a wide range of lengths and gauges (i.e., diameters) either to enable delivery of drugs, vaccines, and other substances into the body or for extraction of fluids and tissue (Figures 1 and ​22). The appropriate needle gauge and length are determined by a number of factors, including the target tissue, injection formulation, and patient population. For example, venipuncture requires the use of needles typically as large as 22–21 gauge inserted to depths of 25–38 mm to withdraw milliliters of blood. In contrast, vaccines usually require injection of less than 1 ml of fluid and, therefore, 25- to 22-gauge needles with a length of 16–38 mm are adequate. Insulin delivery, which involves even smaller volumes and is typically carried out by patients in diverse everyday settings, benefits from still smaller needles, usually of 31–29 gauge inserted to a depth of 6–13 mm.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2769648/

The diameter of the needle is indicated by the needle gauge. Various needle lengths are available for any given gauge. There are a number of systems for gauging needles, including the Stubs Needle Gauge and the French Catheter Scale. Needles in common medical use range from 7 gauge (the largest) to 33 (the smallest) on the Stubs scale. 21-gauge needles are most commonly used for drawing blood for testing purposes, and 16- or 17-gauge needles are most commonly used for blood donation, as the resulting lower pressure is less harmful to red blood cells (it also allows more blood to be collected in a shorter time). Although reusable needles remain useful for some scientific applications, disposable needles are far more common in medicine. Disposable needles are embedded in a plastic or aluminium hub that attaches to the syringe barrel by means of a press-fit or twist-on fitting. These are sometimes referred to as "Luer Lock" connections, referring to the trademark Luer-Lok.

https://en.wikipedia.org/wiki/Needle_gauge_comparison_chart
Hypodermic needles are available in a wide variety of outer diameters described by gauge numbers. Smaller gauge numbers indicate larger outer diameters. Inner diameter depends on both gauge and wall thickness. The following chart shows nominal inner diameter and wall thickness for regular-wall needles. Thin-wall needles (not shown) have identical outer diameters but larger inner diameters for a given gauge.
Needle wire gauge is derived from the Birmingham Wire Gauge. #1 = 18½ B.W.G.; #2 = 19 B.W.G., and so on to #14 = 31 B.W.G.

https://en.wikipedia.org/wiki/Stubs_Iron_Wire_Gauge
The gauge starts at the lowest gauge number of 5Ø or 00000, corresponding to the largest size of 0.500" (12.7mm) to the highest gauge number of 36, corresponding to the smallest size of 0.004" (0.102mm). Size steps between gauges range from 0.001" between high gauge numbers to 0.046" between the two lowest gauge numbers and do not correspond to a particular mathematical pattern, although for the most part the steps get smaller with increasing gauge number. Concerning wire and fine tubing, the gauge number is used to specify the outside diameter of the product, whereas for larger mechanical tubing the gauge number specifies the wall thickness independent of the overall size of the tube.
In medicine, the Stubs system specifies the outside diameter of hypodermic needles, catheters, cannulae and suture wires. It was originally developed in early 19th-century England for use in wire manufacture, and it began appearing in a medical setting in the early 20th century.

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Note added at 6 hrs (2016-03-01 08:48:47 GMT)
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