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Arterial Supply to the Central Nervous System

Arterial Supply to the Brain

1.1 Basic Pattern of the Main Arteries Supplying the Brain

Five pairs of arteries supply the brain (Figure 1.1). The more rostral four of these arise from the cerebral arterial circle, popularly known as the circle of Willis, on the ventral surface of the brain; the cerebral arterial circle roughly circumscribes the hypothalamus, with the stalk of the hypophysis (pituitary gland) in its centre. The fifth and most caudal arises from the basilar artery. The five pairs of arteries are:

1. the rostral cerebral artery;
2. the middle cerebral artery, this being the largest cerebral artery in most mammals;
3. the caudal cerebral artery;
4. the rostral cerebellar artery; and
5. the caudal cerebellar artery.

Figure 1.1 Diagram of the cerebral arterial circle and its outgoing branches. The connection across the midline at the rostral end of the circle is inconstant in the dog and ruminants.

There are also various smaller arteries, which supply the medulla oblongata and pons.

Although there are minor species variations, these vessels occur in mammals consistently. The cerebellar arteries are variable in number and origin even within the same species: for example, in man and the horse, the rostral one may arise from the basilar artery. The three cerebral arteries are remarkably constant in amphibians and higher forms generally.

1.2 Basic Pattern of Incoming Branches to the Cerebral Arterial Circle

There are four potential incoming arterial channels to the cerebral arterial circle in mammals generally (Figure 1.2):

1. internal carotid artery;
2. basilar artery: This midline artery is a continuation rostrally of the ventral spinal artery. However, the blood that flows within the ventral spinal and basilar arteries has come from the vertebral artery via the segmental spinal arteries.
3. maxillary artery: This artery supplies the arterial circle by its so‐called anastomosing ramus, which joins the maxillary artery to the internal carotid artery.
4. vertebral artery: The vertebral artery connects to the internal carotid artery, and in some species it supplies the arterial circle directly by this route. However, it may also supply the circle indirectly via the ventral spinal artery and therefore the basilar artery (see above).

Figure 1.2 Diagram showing the potential arterial channels to the cerebral arterial circle. There are four such channels, numbered 1 to 4 on the left: 1 = the internal carotid artery; 2 = the basilar artery; 3 = the anastomosing ramus from the maxillary artery to the internal carotid artery; and 4 = the connection of the vertebral artery to the internal carotid artery.

Because of the anatomy of these four arterial channels, the blood which distributes itself over the brain may be internal carotid blood, maxillary blood, or vertebral blood, or a combination of these (Figure 1.3).

Figure 1.3 Diagrams showing species variations in the sources of arterial blood to the brain. In each figure the upper diagram shows the distribution over the brain of internal carotid, vertebral and maxillary blood in the intact live animal (see key); the lower diagram shows the anatomy which accounts for this distribution, based on the four potential arterial channels to the cerebral arterial circle. Arrows show the direction of flow in the basilar artery. The vertebral–occipital anastomosis (VO) can be disregarded in the intact animal. 1 = internal carotid artery; 2 = basilar artery; 3 = anastomosing ramus from maxillary artery to internal carotid artery; and 4 = connection of vertebral artery to internal carotid artery. (a) Dog, man and many other species. 1 (internal carotid artery) and 2 (basilar artery) supply the arterial circle; the basilar artery carries blood to the arterial circle. Neither channel has a rete mirabile. Internal carotid blood reaches all of the cerebral hemisphere except its most caudal part. Vertebral blood supplies the remainder of the cerebral hemisphere, and all the rest of the brain. (b) Sheep and cat. Only 3 (maxillary anastomosing ramus) supplies the arterial circle. It has a rete mirabile. 2 (basilar artery) carries blood away from the arterial circle. Maxillary blood is distributed to all of the brain except the caudal part of the medulla oblongata, which is supplied by vertebral blood. (c) Ox. 3 (anastomosing ramus) and 4 (vertebral artery) both supply the arterial circle. Each has a rete mirabile. 2 (basilar artery) carries blood away from the arterial circle. A mixture of maxillary and vertebral blood reaches all parts of the brain.

1.3 Species Variations

In no domestic mammals are all four of these potential arterial channels to the cerebral arterial circle fully developed. Some of the channels are reduced in calibre or are even totally obliterated. The direction of flow in the remaining channels depends on the pressure gradients within the various vessels. The general relationships of these gradients have been worked out experimentally, thus establishing the direction of flow and the distribution of blood in each species. The following account applies to the intact live animal.

1.3.1 Dog, Man and most Mammals

Most mammals have what appears to be the most usual mammalian pattern of arterial supply to the brain (Figure 1.3(a)). The blood reaching the rostral half of the brain is internal carotid blood, but the caudal half of the brain is supplied by vertebral blood. This is because the pressure gradients are such that the flow of blood in the basilar artery is rostral. Consequently, vertebral blood reaches not only the cerebellar arteries but also the caudal cerebral artery.

The anastomosing ramus of the maxillary artery is much reduced in these species. In the dog there is an anastomotic artery which connects the internal carotid to the external ophthalmic artery, the latter being a branch of the maxillary artery; this anastomosis could possibly provide a supply to the arterial circle. In man, the maxillary artery anastomoses with the internal carotid via the sphenopalatine artery and also via the middle meningeal artery.

1.3.2 Sheep and Cat

In the sheep and cat species the lumen of the proximal two‐thirds of the internal carotid artery becomes obliterated in the weeks or months after birth. (At birth, however, the internal carotid is fully functional.) The whole of the adult brain is supplied by maxillary blood via the anastomosing ramus of the maxillary artery (Figure 1.3(b)). A rete mirabile (see Section 1.6) develops on the anastomosing ramus in these species. The direction of flow in the basilar artery is caudal. Consequently, scarcely any blood from the vertebral artery reaches the brain.

The pressure gradients actually do allow a rostral flow in the caudal end of the basilar artery. As a result, vertebral blood does reach the caudal half of the medulla oblongata (Figure 1.3(b)). It is certain, however, that no vertebral blood reaches the cerebral hemispheres.

1.3.3 Ox

In the ox, the lumen of the proximal two‐thirds of the internal carotid artery is obliterated by 18 months of age, but the distal third remains intact (Figure 1.3(c)). The supply to the whole of the brain is by a mixture of maxillary and vertebral blood (Figure 1.3(c)). This takes place via the anastomosing ramus of the maxillary artery and the vertebral artery, both of which connect directly to the distal third of the internal carotid artery. A rete mirabile develops on both of these channels (see Section 1.6). The direction of flow in the basilar artery is again caudal but, as just stated, vertebral blood already gains access to the brain through the distal remnant of the internal carotid artery.

1.3.4 Summary of Species Variations

An inverse relationship exists between the degree of development of the internal carotid and that of the anastomosing ramus of the maxillary artery: when one is large the other is small. In no species are both of these channels fully developed.

In all species, a vertebral–occipital anastomosis (Figure 1.2) is present and well developed. Despite the fairly substantial calibre of this pathway, it seems to have very little functional importance in the intact animal. It does become of interest, however, if the common carotid artery is cut.

1.4 Summary of the Significance of the Vertebral Artery as a Source of Blood to the Brain

In the normal sheep and cat, little or no blood from the vertebral artery reaches the brain. In the normal ox, vertebral blood reaches all parts of the brain, and it is this point that becomes particularly important in the matter of humane slaughter (see below). In other species vertebral blood reaches only the cerebellum and the caudal part of the cerebrum.

1.5 Humane Slaughter

The...