THE NERVOUS SYSTEM 523 



interrupted at intervals by nodes of Ranvier, but the neurite continues 

 until it terminates on the surface of one or more muscle fibers in the form 

 of a motor end-plate. At this end-plate the neurolemma becomes con- 

 tinuous with the sarcolemma sheath of the muscle fiber. The granular 

 protoplasm of the end-plate contains many nuclei as well as pad-like 

 terminations, the telodendria of the neurite. General somatic efferent 

 fibers supply most of the skeletal muscles of the body. They are absent 

 from cranial nerves with the possible exception of the spinal accessory 

 of amniotes. But this nerve is a spinal nerve in the anamnia. 



2. Special somatic efferent fibers innervate specialized somatic 

 muscles of the head. These muscles are the six eye muscles and some 

 of the muscles of the tongue. The eye muscles are innervated by the 

 oculomotor, trochlearis, and the abducens nerves. The hypoglossus 

 nerve supplies the tongue muscles. The hypoglossus is, however, a 

 spinal nerve in the anamnia. 



3. General somatic afferent nerves supply the skin and skeletal 

 muscles. They are the nerves of general exteroceptive sensibility and of 

 deep proprioceptive sensibility. With a few exceptions already noted 

 the cell-bodies of somatic sensory nerves lie, not in the spinal cord or 

 brain, but in the ganglia of the dorsal roots. Two kinds of ganglion 

 cells may be distinguished, large and small, both usually of the unipolar 

 (disguised bipolar) type. Their fibers are mostly myelinated, and the neu- 

 rilemma sheaths cover both the cell-bodies and the neurites. (Fig. 390) 



As the neurite leaves a ganglion cell, it is twisted into a glomerulus. 

 It soon divides into two branches, one of which passes into the cord, 

 and the other to the skin or to a muscle fiber, where it terminates as a 

 muscle spindle. The ganglion cells are frequently surrounded by a pericel- 

 lular network of nerve fibers which are possibly of sympathetic origin. 



Of the cranial nerves, the trigeminal, glossopharyngeal and vagus 

 contain general somatic afferent fibers from the skin. The dorsal roots 

 of spinal nerves consist largely of such general somatic fibers. The 

 oculomotor, trochlearis, abducens and trigeminal nerves carry proprio- 

 ceptive impulses from the muscles. 



4. Special somatic afferent components are limited to some of the 

 cranial nerves and are not found in spinal nerves at all. The facial, 

 acoustic, vagus and (?) glossopharyngeus nerves contain them. These 

 are nerves of special sense organs, the ear and lateral line organs. The 

 fibers of the cochlear branch of the acoustic nerve carry exteroceptive 

 impulses while those of the vestibular branch are proprioceptive. Latera- 

 lis fibers are contained in the facial and vagus nerves, and, possibly in 

 some fishes, in the glossopharyngeus nerve. If the optic nerve were not a 

 specialized fiber tract of the brain, it might be classed as a special somatic 

 afferent nerve. 



