The Somatic System

The Somatic SystemThe somatic nerves are classified in two groups, (a) spinal and (b) cerebral or cranial, according to the portion of the skeleton from which they emerge. The spinal nerves pass out from the sides of the vertebral column between adjacent vertebrae. The cerebral nerves find exit through holes in or between the bones forming the base of the skull.

Spinal Nerves. - In man there are thirty­one pairs of spinal nerves named from the regions of the vertebral column where they emerge:

8 Cervical = neck region.
12 Thoracic = region of chest.
5 Lumbar = region of loins.

5 Sacral + 1 Coccygeal = terminal portion of vertebral column.

Every spinal nerve is attached to the spinal cord by two roots: one to the front of the cord, containing the motor fibres, is termed the anterior root, the other to the back of the cord, consisting chiefly of sensory fibres, is designated the posterior root.

The two roots join together at a short distance from the cord and form a complete spinal nerve. The roots can be readily distinguished from one another by the fact that there is a little swelling in the posterior root due to the presence in it of a ganglion - the spinal ganglion; the anterior root is non-gangliated.

The nerve cells of the spinal ganglia have single processes bifurcating in a T-shaped manner; one limb of the bifurcation (the dendrite of the cell) passes out to the complete spinal nerve, the other limb (the axon) runs into the spinal cord. The fibres in the anterior root are the axons of cells located in the grey matter of the spinal cord.

DEVELOPMENT. - The motor roots are formed as outgrowths from nerve fibres of the posterior root cells in the anterior part of the neural tube. The grow out from the cells of the spinal ganglia. These ganglia arise from the neural crest in the following manner: in each segment the cells of the crest increase rapidly in numbers and then migrate outwards forming thus a series of oval-shaped masses. The central processes of the cells grow inwards to the cord and the peripheral outwards to join the motor root.

Cerebral (cranial) Nerves. - The cerebral nerves do not exhibit the regular segmental arrangement of the spinal nerves, neither are they regularly constituted of two roots-motor and sensory. Some are entirely motor, others purely sensory, and a few are mixed (Fig. 29).

In man twelve pairs of cranial nerves are recognised. The first and second pairs differ from all the other peripheral nerves in that, developmentally, they are really outgrowths of the brain substance. The names and general functions of the cerebral are as follows :

(s.) sensory, (m.) motor.

I. Olfactory (s.), connected with the organ of smell.
II. Optic (s.), connected with the organ of sight.
III. Oculo-motor (m.), to certain muscles of the eyeball.
IV. Trochlear or pathetic (m.), to a muscle of the eyeball.
V. Trigeminal or trifacial (s., m.), sensory to face and tongue, muscles of mastication.
VI. Abducent (m.), to a muscle of the eyeball.
VII. Facial (s., m.), sensory to tongue, motor to muscles of expression.
VIII. Auditory (s.), connected with the organ of hearing.
IX. Glossopharyngeal (s., m.), to tongue and pharynx.
X. Pneumogastric or vagus (s., m.), to the viscera.
XI. Spinal accessory (m.), a portion joins the vagus; the other portion supplies two muscles of the neck.
XII. Hypoglossal (m.), to muscles of the tongue. (s.) sensory, (m.) motor.