2. A few biological facts about bird necks

2.1 The neck: the link between the trunk and the head

In vertebrate animals, the nervous system comprises the brain and the spinal cord, from which the nerves innervate the whole body. The spinal cord is protected by the vertebral column. It varies in length and flexibility from one group to another, as in fish, mammals and birds. Within each group, it also differs according to species, their way of life and their environment. Flexibility also varies according to body zone, such as the neck and trunk. The spinal column is an articulated system that must adjust both :

1. protective constraints on the spinal cord (no rupture or elongation);

2. the flexibility/rigidity required for the body to function ;

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