Since I dug up the following for a LJ thread I might as well post it here:

Viva! - Vegetarians International Voice for Animals

"The collapse of an animal after cutting the major blood vessels in the neck is caused by cerebral shock due to the sudden fall in pressure of the cerebro-spinal fluid and is not due to lack of oxygen in the blood flow to the brain. The animal may collapse five to 10 seconds after the throat is cut - so long as the major vessels are severed properly or occlusions do not occur (see below). The animal loses consciousness after it has collapsed. There are varying opinions on when the animal stops being able to feel.

Scientific experiments measure the onset of loss of consciousness through observing physical behaviour, spontaneous electroencephalogram (EEG) and evoked responses in the electrocorticogram (ECoG) (22,23,24,25). Cattle stop trying to right themselves between 21 and 47 seconds after the carotids are cut (22); the EEG indicates that there is sensibility for two to 5.2 minutes after cutting in cattle. (Other scientists argue that the EEG is not an accurate measure of the state of consciousness - but theirs is a theory yet to be proven.) Generally, scientists suggest that cattle can take anything from 25 to 90 seconds to lose consciousness after the neck has been cut.

D.K. Blackmore (22) of the Dept. of Veterinary Pathology and Public Health, Massey University, New Zealand who has published many papers on slaughter states:
"A BASIC requirement for humane slaughter is that an animal should be rendered insensible before exsanguination (bleeding) is initiated and this should last until the animal becomes permanently insensible from cerebral anoxia."

Blackmore studied the behaviour of sheep and cattle after being cut; he found that sheep lose consciousness more quickly than cattle. Lambs collapsed after an average of 2.6 seconds and stopped attempting to stand after 10 seconds (not being able to stand does not mean inability to feel); in contrast calves were standing up to 135 seconds after their throat was cut and were attempting to stand up to 385 seconds after that. Even though the calves had both carotid arteries and jugular veins severed three out of four were breathing up to 11.6 minutes later and so were shot. Lambs were gasping for up to 3.8 minutes after carotid arteries and jugular veins were cut. A bull was killed by the Jewish method and was still gasping sporadically for seven minutes after his carotid arteries, jugular veins, oesophagus and trachea were cut.

A huge problem is that millions of animals bleed slowly. Anil et al (21) say: "It is well recognised that unstunned calves which bleed poorly can take a long time to die." It takes more than five minutes for the animals to stop trying to stand normally.

Animals bleed slowly when (21):

1. There is occlusion [blockage] of the cut artery by surrounding tissues (the artery is elastic and can spring back into its connective tissue sheath on being cut).
2. Blood platelets stick together at the end of the cut artery (this leads to the rapid production of a white clot which can plug the artery).
3. The cut is made and the artery goes into spasm.

All three factors tend to restrict blood flow from the cut and can produce a ballooning effect in the severed vessel. Here, the blood goes into the connective tissue sheaths surrounding the artery. The ends can balloon up within 5 seconds of the cut (21). Several studies have shown that this ballooning effect is common (up to 40%) resulting in delay of brain failure and blood pressure being held for longer. Arterial occlusions occurred in half of the calves killed by Anil et al by Halal methods of cutting the carotid arteries and jugular veins. In some of these animals, vertebral artery blood flow was maintained at about one-third of its normal level for approx. 3 minutes (21). Therefore, animals do not lose consciousness immediately. Occasionally cattle have staggered to their feet after being cut and walked some distance before collapsing.

Scientific papers have commonly reported this phenomenon in their results recently and throughout the last century. Back in 1900 Hoffman describes a 12 year old cow lying on the floor for 20 seconds after Shechita. 70 seconds later she lifted her head for 20 seconds. Also, a three year old bull remained standing after Shechita for two and a half minutes, afterwards he fell to the ground but still looked around, slowly bleeding to death.

Ballooning occurs more commonly when blunt knives are used and so is a greater problem with Muslim slaughter. However, it does occur for other reasons when sharp knives are used (though to a lesser degree) and so happens in Jewish slaughter.

It is also a problem in mainstream slaughter as, for example, calves start to regain consciousness between 21 and 59 seconds after electrical stunning (21).

Slaughter expert HE Bywater who witnessed animal slaughter over several years said: "upholders of the Jewish ritual maintain that unconsciousness follows within a few seconds of the ritual cut due to anaemia of the brain, this is not invariably so as experience in the slaughterhouse has shown."

Bywater's observations lead him to conclude that: "a state of diminished consciousness or semi-consciousness follows soon after the large blood vessels are severed, but that thereafter consciousness can, and does, return in a number of animals - adult cattle, sheep and calves....

"The blood vessels which supply the brain in these animals differ from those which are present in the human and this fact has never been sufficiently appreciated by the eminent medical men who maintain that the Jewish ritual invariably causes immediate loss of sensation in food animals." (4)

Thornton's Textbook of Meat Inspection says: "A factor of considerable importance, and to which not sufficient attention has been paid in considering the problem of Jewish slaughter, is that after the carotid arteries of cattle are severed transversely, they tend, by virtue of their elasticity, to retract rapidly within their own external connective tissue coat, and as a result the sealing of the cut ends of the vessels may occur. As the blood pressure...is then maintained by the heart, the blood pressure in the vertebral arteries may likewise be maintained at a substantial level and unconsciousness therefore delayed".

This explains why "some cattle have regained their feet and walked a considerable distance before they succumbed some minutes later."

The Textbook of Meat Inspection continues: "these occurrences have been attributed to the fact that all the neck vessels may not have been severed completely, but observations lead one to the conclusion that the cut is invariably made dextrously... There is therefore considerable doubt as to whether unconsciousness always follows rapidly in cattle after the severance of the neck vessels, for by the very nature of the neck cut made in Jewish slaughter it is impossible to ensure that self-sealing of the cut ends of the carotid arteries will not occur." (4)

In religious slaughter, the animal's vertebrae, spinal cord and vertebral arteries and veins are not severed. Scientists argue over the significance of this. Some say that one of the reasons some animals take several minutes to lose consciousness is because the vertebral arteries continue to supply blood to the brain after the animal has been cut. Others say that the blood flow from the vertebral arteries is insufficient to maintain the brain's functioning (40). Of course, if occlusions occur, the vertebral arteries can supply enough blood to the brain to keep the animal conscious.

Some scientists argue that measuring evoked responses in the cortex of the brain is the best method to evaluate consciousness. Daly et al (25) showed that evoked responses disappeared instantaneously after the captive bolt pistol was used correctly in cattle. However, after Shechita the cortex was responding to stimuli from 20 seconds to over two minutes."