- Physically, Klingons are humanoid, with a series of fleshy ridges on the forehead. These ridges run may down the bridge of the nose, and follow the curvature of the skull back to the upper extent of the spine. A narrower series of ridges runs down the spine to the coccyx. Some individuals exhibit ridging on the chest and the instep. Most have a small bony spur above the small toe.
- The pattern of cranial ridging follows House lines, and, usually along the male line. The ridging is a pad of fat, muscle and callous over a central bony process. Heavier ridging is coinsidered more desirable, æsthetically speaking, and is believed to indicate both nobility and warrior prowess. This distinction is enhanced socially by a popular Klingon drinking ritual which involves quaffing half of one's drink, bashing one's forehead against that of the nearest handy person, then (if still able) finishing the drink and clapping the other warrior on the back. Just as in Humans, the fleshy tissue of the nose and ears get larger with age, older Klingons develop more convoluted ridging.
A Klingon skull.
Variations in cranial ridges between Houses.
The frontal bone of the skull is more massive than in humans and has a slight vertical ridge proceeding from the forward end of the sagittal suture to the upper end of the metopic suture, which serves to anchor the ridges (all but invisible in the smooth-headed clans until middle age). At either side of the frontal bone, arching up from where the zygomatic bone meets the supercilliary arch, and proceeding back to the coronal sutures, is another slight ridge which serves to anchor the upper end of the jaw muscles (the dentition pattern of Klingons indicates recent descent from carnivores, rather than Humanity's recent descent from omnivores, thus requiring a heavier bite). In some older Klingon, a minor bone ailment causes small bony protrusions to grow from this ridge (a sort of cranial bursitis), causing some amount of discomfort at first, comparable to teething in a child. These bone spurs rarely, if ever, exceed 5 or 6mm in size.
- Klingons have a very rugged physiology with biological "backups" for most major biofunctions. This characteristic, known as braQ'lul, gives Klingon warriors incredible resiliency in battle. Major organs are duplicated to one degree or another, the heart has eight chambers, and there is an auxiliary nerve nexus at the base of the spine which can tempoararily assume autonomic nerve functions.
- There has been much dispute over the colour of Klingon blood. It has been described as being every shade from pink to purple. In fact, freshly oxygenated arterial blood, which is very bright red in Humans, is a bright lavender pink in Klingons, while unoxygenated venous blood, often almost purplish in Humans, is bright red, almost the colour of oxygenated Human blood.