The spinal nerves communicate the spinal cord with sensory receptors, muscles, viscera and vessels. In the dog, there are 36 pairs of spinal nerves. Each is formed by a dorsal and a ventral root. Each root is formed by rootlets which fan out along each segment of the spinal cord.
The dorsal root is formed by axons of afferent neurons (sensory) that have their bodies located at the spinal ganglion. The ventral root is formed by axons of efferent neurons (motor) whose bodies are located in the gray matter of the spinal cord. At the intervertebral foramen, each spinal nerve sends out a sensory meningeal branch. It consists of sensory and sympathetic postganglionic fibers that innervate the dura mater, the dorsal longitudinal ligament, the vertebral venous sinuses and other vessels located in the vertebral canal (Page 832 of "Miller's anatomy of the dog" by Evans, H.E. 3 rd Ed.). Once out of the vertebral canal, the spinal nerves from T1 to L5 send out a communicating branch of sympathetic neurons to form the sympathetic trunk. Each spinal nerves forms a dorsal and a ventral branch. The dorsal branch innervates epaxial muscles and the skin that covers them and the ventral branch innervates the hypaxial muscles and the skin of the lateral and ventral portions of the body. The first spinal nerve (C1) leaves the vertebral canal through the lateral vertebral foramen of atlas. The rest of the cervical, thoracic and lumbar nerves leave through the intervertebral foramina. At the sacrum, the dorsal and ventral branches of the first two sacral nerves leave the sacral canal through the dorsal and ventral sacral foramina respectively and the third sacral nerve does so through the intervertebral foramen between the sacrum and the first caudal vertebra.
The brachial plexus is formed by the ventral branches of spinal nerves C6, C7, C8, T1 and T2.
The lumbosacral plexus nerves is formed by the ventral branches of spinal nerves L4, L5, L6, L7, S1, S2 and S3.
This term refers to the area of the skin innervated by afferent nerve fibers from a single spinal nerve. This is brought about by the migration of each dermatome along with the sensory component of a spinal nerve. In the neck and trunk, dermatome has a segmental organization, but in the limbs the cutaneous areas overlap due to the arrangement of the nerves of each plexus. In view of the spinal nature of a dermatome, we cannot consider them to exist in the head but it is possible to identify sensory cutaneous areas innervated by branches of the trigeminal nerve.
 From the Greek derma, dermatos, "skin"; tomos, "section".