The question, Why was there no arterial rain on Dr. Kelly's clothing was posed by Rowena Thursby as far back as 2006 here, WHY WAS THERE NO ARTERIAL RAIN ON DR KELLY'S CLOTHING?.
This post is an attempt to answer the question, as posed.
In passing, I'll mention that the forensic biologist's report is still secret and might shed interesting light on the distribution of arterial rain at the scene and, possibly, on Dr. Kelly's clothing.
I'm open to correction, but the only witnesses who commented on arterial rain were Vanessa Hunt (paramedic) and Roy Green (forensic biologist). And each testified that they saw arterial rain on "nettles", whose exact location relative to the body isn't entirely clear to me. (That raises questions that I won't attempt to respond to in this post.)
So Rowena's question seems to accord with the evidence as I recall it. It seems that there was no arterial rain on Dr. Kelly's clothing. Asking "Why not?" is an entirely reasonable question.
For the purposes of this post, I'll ignore the possibility that arterial rain may be simulated. See the post The Death of David Kelly - Is it possible to simulate "arterial rain".
At the risk of stating the obvious it seems to me that there are a number of things one needs to know or assume:
1. Was David Kelly alive when his left wrist was cut? The only relevant evidence I'm aware of is the statement of Nicholas Hunt that there was "vital reaction". Let's for the sake of this post assume that the "vital reaction" was genuine and that David Kelly was alive when his left wrist was cut.
2. Another important consideration is the position of Dr. Kelly's left wrist when each individual incision was made or attempted. It might be that each incision was made in the same position. Or, alternatively, the wrist position may have changed between incisions. For example, hypothetically, the person who made one incision moved the wrist to attempt to get better access or more purchase.
3. Was there a single incision into the ulnar artery? Or multiple? In other words, did a first incision cut into the artery and a later incision completely transect it? I think that both possibilities remain credible.
4.Did Dr. Kelly have enough strength in his right hand to make the incisions which Dr. Nicholas Hunt observed? Remember Mai Pederson's evidence of a right elbow injury and ongoing pain and disability.
As you can see, we are long on uncertainty and short on hard information.
In trying to work out whether the suicide hypothesis was credible or not, I tried to work out the possible positions for Dr. Kelly's wrists.
Try it at home and see what it feels like. Try to imagine, too, if you'd get enough purchase to make deep incisions.
1. The first possibility as I see it, was that he flexed his left elbow and braced his left arm against the left side of his chest. In other words the palm of his hand would be towards his body or face. If the wrist was in that position I'd expect Dr. Kelly to make incisions on the wrist from the left side towards the middle of his body (in other words he cuts from the thumb side or middle of the wrist toward the little finger side). I'll call that Position 1.
2. The other possibility that occurs to me is that he might turn the palm of the left hand away from his body or face, so that it's facing forwards. The palm is facing forwards, the little finger side is towards his left and the thumb side is close to Dr. Kelly's left side. Again the left arm is braced against the left side of his chest, likely with the top of his left fist close to the left side of his chin. In that position I'd expect him to make incisions from the little finger side of the wrist towards the thumb side of the wrist. I'll call that position 2.
Distribution of arterial rain that I'd expect
1. In Position 1, as the first incision is made into the ulnar artery I would expect Dr. Kelly's face and upper chest to be sprayed with arterial rain. I'm not aware of any evidence that such arterial rain was observed on his face or upper chest. So I conclude that he didn't hold his wrist in Position 1 to make any incision which initially breached the ulnar artery.
2. In Position 2, when the first incision is made into the ulnar artery I would expect the bulk of any arterial rain to be directed away from his body, most likely towards the left of where he was standing or sitting or forwards of where he was sitting or standing. If he doesn't turn his wrist round while there is arterial rain then I think it's credible that there is no arterial rain on his clothing if the relevant incision(s) were made when the wrist was in Position 2.
In either Position 1 or Position 2 it seems to me to be credible that once the ulnar artery is completely transected that arterial rain will decrease and, depending on the degree of retraction and contraction of the ulnar artery may be present for aonly a short time.
Assuming that there is no evidence of arterial rain on Dr. Kelly's clothing in Mr Green's written report then I think Position 2 is much more likely than Position 1 for those crucial incision(s) which first enter the ulnar artery. In Position 1 I think that Dr. Kelly's face would receive a significant amount of arterial rain in a very short time.
As hinted at earlier, such a conclusion assumes that Dr. Kelly was alive when the incisions were made and that it was in fact he who made the incisions. Both assumptions are potentially open to question on the available evidence, in my view.