Slide 5. C6 ventral horn. Nissl stain. See Fig. 33 for orientation of section.
Note the prominent lateral extension of the ventral horn. Letters A-D point to the individual motor nuclei of lamina IX, each of which provides innervation to a single muscle or a group of muscles having a similar action. Distal muscles (i.e., those of the hand) tend to be innervated by the more lateral nuclei, such as A. Proximal muscles (like the deltoid) are innervated by the most medial nuclei, such as C and D.
This type of organization, in which the body is represented spatially within the nervous system in a reliable and orderly fashion is called a somatotopic one. Somatotopy is conceptually similar to the organization in other sensory systems (retinotopy for the visual system and tonotopy for the auditory system).