DixieWhistler:
Not being a doctor, I can't be entirely certain, but it's also possible that Lightfoot could have suffered what is referred to as a
contre-coup injury (also known as a
coup-contre coup injury), by the medical profession. That's a particularly severe form of brain injury which occurs as the result of the direct application of force to the head, which is what would have happened to Lightfoot when Red Leary (George Kennedy) kicked him in the head.
Essentially, what happens with this type of injury is that the brain suffers TWO forms of injury: the initial injury itself, or the
coup, in the area where the original blow was struck, and the secondary injury, or
contre-coup, which occurs when the area of the brain
opposite the part of the skull that sustained the impact is damaged, because the victim's brain literally
bounced off of the inside of his skull as a result of the blow.
Judging by the gradual deterioration of Lightfoot's coordination and the slurring of his speech, I would guess that his brain's speech center and motor control center (the part that regulates voluntary motion and coordination,
i.e., walking and such) were affected first, followed by the gradual deterioration of his involuntary actions (heartbeat, breathing,
etc.) as his brainstem or
medulla oblongata (the areas of the brain that control such things) were affected by the injury, which led to his death.
The effect could have been caused by internal hemorrhaging into the brain itself, or by swelling of the injured area as a result of the initial impact, which could bring about death as a result of incresed pressure on the
medulla oblongata or brainstem. Either one is a possibility with that type of brain injury. The effect would be similar to that of a massive cerebral hemorrhage, as Matt mentioned in his post, but would be the result of an externally-applied force, whereas cerebral hemorrhages are usually--but not always--triggered by natural causes, such as high blood pressure, heart disease, or arteriosclerosis.
At any rate, if that was the type of head injury that Lightfoot received, he was pretty much done for. The normal remedy for such an injury is emergency brain surgery to relieve the pressure and repair as much damage as is possible (
i.e., stopping any bleeding), as well as drug treatments to reduce the swelling of the brain itself. I believe that such procedures, while advanced, were available in 1974 (when the film was made).
I myself underwent surgery for what could be called "a hereditary closed-head brain injury" only 2 years later (I'm hydrocephalic--look it up if you're curious, because the explanation is rather complicated--and I've had between 15 and 20 surgeries for it, starting when I was about 2 years old (I'll be 40 on May 30 of this year)). However, I'm not certain whether Thunderbolt (Clint Eastwood) could have gotten his partner to a hospital in time, even had he realized what was happening to him (and besides, Lightfoot's death was part of the script

).
Hope this answers your question.
--John Omohundro