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the cerebrum of the 2 adult sheep after bilateral severance of major blood vessels. In sheep when the vessels were severed on one side of the neck only, the passage of dye was noted for at least 53 seconds. The authors concluded that there are major differences between sheep and calves in the blood supply to the brain due to the vertebral arteries in cattle. The vertebral arteries of cattle are not severed by the neck cut due to their passage close to the spinal cord. Unlike sheep, the vertebral arteries in cattle are capable of maintaining the cerebral blood flow. This effect is supposed to be even stronger in unanesthetised animals, because anaesthesia is known to reduce cerebral blood flow (Blackman et al., 1986). Levinger (1961) concludes from similar experiments that the cerebral blood flow through the vertebral arteries would not be sufficient to supply the brain. Nevertheless, even if the blood flow from the vertebral arteries may not be sufficient to supply the whole brain, it is likely that it contributes to prolong brain function and consciousness. Anil et al. (1995a) found that, in electrically stunned calves suspended upside down by a hindleg, carotid occlusion delayed the time to isoelectric ECoG (brain failure). The mean arterial blood pressure was held for longer when carotid occlusion occurred and vertebral artery blood flow could be maintained at about 30% of its initial level for up to 3 minutes. In some animals vertebral artery blood flow increased substantially following sticking. Shaw et al. (1990) ligated the vertebral arteries in 4 out of 8 calves and measured the onset of brain failure by EcoG. They concluded that factors other than blood flow from the vertebral arteries contribute to the prolonged time to loss of electrocortical activity after slaughter observed in calves. Bager et al. (1988) looked at cerebral blood flow by measuring the venous blood leaving the head ends of the jugular veins in calves, and suggested that factors impeding the retrograde blood flow from the brain and thus rising cerebral blood pressure might be important. Daly et al. (1988) suggested two explanations: first there are differences between animals in the proportion of the total cerebral blood flow which is contributed by the vertebral arteries. Secondly the amount of blood reaching the brain via the vertebral arteries after slaughter is very close to the minimum blood flow necessary to sustain electrical activity in the brain cortex, so that slight differences in individuals would result in large variations (Daly et al., 1988). It is important to note that the above mentioned experiments have been conducted with only limited numbers of animals and already important individual as well as species differences have been found (Levinger, 1961). In small ruminants Levinger (1961) found that animals collapsed but were able to regain posture when the carotid arteries were clamped, whereas loss of posture was definite when the collateral pathways via vertebral and occipital arteries were also blocked. Even in sheep, where the vertebral arteries pathway to the brain is usually stated to be of minor importance, this route could be found in some animals and activated in others (Nangeroni and Kennet, 1963). Cerebral hemodynamic compensatory mechanisms will also help to maintain brain function during reduced systemic blood pressure. Cerebral perfusion pressure (CPP), the driving force for blood through the cerebral circulation is defined as the difference between mean arterial pressure and venous backpressure or intracranial pressure. As CPP falls, cerebral blood flow is initially maintained by vasodilation of resistance arterioles, a reflex known as autoregulation. With further reductions in CPP, the autoregulatory capacity is exhausted and cerebral blood flow falls as a function of pressure, but increases in oxygen extraction fraction will maintain cerebral oxygen metabolism and tissue function up to a point (Derdeyn, 2001). To summarize factors influencing the dynamics of cerebral blood flow after neck cutting seem to be very complex and individual differences as well as age, weight and breed have an