Dian Cécht
Irish: swift, powerful judge?
God of medicine; one of the Tuatha Dé Danann. Grandfather of Lugh Lamhfada and maker of the silver arm of Nuada, and who killed his son Miach for being a more skillful doctor. His father is not given, but he is likely the son of Danu and Bíle; the Dagda is sometimes listed as his brother.
According to "The Second Battle of Magh Turedh," when Nuada, king of the De Danann had his arm cut off, he was denied kingship, as all kings of the de Dannan had to be physically flawless. Thus, the kingship was forfit. The Fomorians--the dark gods who inhabited Ireland before the Dé Danann--demanded that Bres, son of Eri (a Dé Danann) and Maeth Sceni (a Fomorian) should be made king. The Dé Danann agreed, but in doing so, allowed a tyrant onto the throne.
So while the Dé Danann suffered, Dian Cécht worked on creating a silver arm for Nuada, hoping this would help him regain the throne--this is the cause for Nuada's full name, Nuada Airgentlamh. However, Dian Cécht's son Miach was more skillful than his father, and was able to magically create a new, biological arm for Nuada. Dian Cécht was so jealous that he struck Miach four times with his sword--each time, Miach would heal himself, until the fourth time, when Dian Cécht struck through the skull to the neck. Out of grief, Dian Cécht buried him, and 365 herbs grew on the grave. Airmed--his daughter--seperated the herbs to their properties, but Dian Cécht scattered them again, so that "no one knows their proper cures unless the Holy Spirit should teach them afterwards" (a Christian interpolation; likely that only the leeches should know them). It is also said that the 365 herbs were put in the Well of Slaine, which could heal the mortally wounded, and was presided over by Dian Cécht and Airmed. It was to this well that Goibniu the smith came to be healed of his spear-wound during the second battle of Magh Turedh.
At this time, Dian Cécht's other son, Cian, had met the Fomorian princess Eithne, and had sired on her a son, who was named Lugh Lamhfada, sometimes called Samildanách, who was raised by Manannán mac Lír and Tailtiu. Lugh then came to Ireland and was able to win the throne back for Nuada.
Dian Cécht is attributed a medical tract, found in the manuscript Phillipps 10297 (now G 11) in the National Library of Ireland. The manuscript is from the fifteenth century, but the tract is much older. Aside from various judgements of payment, it has an interesting paragraph about what it calls "The Twelve Doors of the Soul" or "The Twelve Porals of Life":
There are twelve doors of the soul in the human body: (1) the top of the head, i.e. the crown or the suture, (2) the hollow of the occiput, (3) the hollow of the temple, (4) the apple of the throat, (5) the spoon of the breast, (6) the armpit, (7) the breast-bone, (8) the navel, (9) the side {?}, (10) the bend of the elbow, (11) the hollow of the ham, i.e. from behind, (12) the bulge of the groin, i.e. the bull sinew, (13) the sole of the foot.
Yes, they say twelve, but list thirteen. Why, I'm not sure. What is interesting is that this seems to be almost like an Irish chakra system, attributed to the god of medicine.
SOURCES
1. "The Second Battle of Magh Turedh"--various publications, websites
2. The Judgements of Dian Cécht. trans. D. A. Binchy. Eriu. Vol. XX. Dublin: Royal Irish Academy, 1966.