SENSE ORGANS AND NERVOUS COORD/NAHON 593 



their cell bodies. The cell bodies of afferent neurons are nearly always 

 located in ganglia on both spinal and cranial nerves. The afferent neu- 

 rons enter the spinal cord, and generally terminate in synapses with the 

 dendrites or cell bodies of internuncial neurons. These cell bodies are 

 located in the dorsal portion of the gray matter of the cord. The ventral 

 root contains the efferent neurons, and their cell bodies nearly always 

 lie in the ventral portion of the gray matter of the cord. 



The spinal nerves of most vertebrates are essentially alike, although 

 in the most primitive vertebrates the roots do not unite peripherally, and 

 the segregation of afferent and efferent neurons within the roots is not 

 as clear-cut. In most vertebrates, the roots unite to form a spinal nerve 

 that divides into a dorsal branch, or dorsal ramus, which supplies the 

 skin and muscles in the dorsal part of the body, a ventral ramus, which 

 innervates the lateroventral parts of the body, and frequently one or 

 more communicating rami to the visceral organs. Afferent and efferent 

 neurons occur in each ramus. Man has 31 pairs of spinal nerves. Those 

 supplying the receptors and effectors of the limbs are larger than the 

 others, and their ventral rami are interlaced to form a complex network, 

 or plexus, from which nerves extend to the limbs. 



Cranial Nerves. The nerves from the nose, the eyes and the ear 

 contain only afferent neurons, and have evolved along with the organs 

 of special sense. The other cranial nerves are mixed, and they are con- 

 sidered to be serially homologous with the separate roots of the spinal 

 nerves of primitive vertebrates. Some of them are essentially the cephalic 

 counterparts of dorsal roots; others, the counterparts of ventral roots. 

 The location of the cell bodies of the neurons of cranial nerves, and 

 of their endings within the brain, follows the pattern described for spinal 

 neurons. 



Reptiles, birds and mammals have twelve pairs of cranial nerves, 

 if we omit the minute and poorly understood nervus terminalis. Though 

 distributed to the nasal mucosa, this nerve is not olfactory. The other 

 cranial nerves and their distribution are shown in Table 6, and their 

 stumps can be seen in a figure of the brain (Fig. 29.11). 



Fishes and amphibians lack discrete spinal accessory and hypoglossal 

 nerves. The homologues of neurons that are segregated in the spinal 

 accessory of higher vertebrates are included in the vagus of fishes and 

 amphibians, and the homologues of neurons in the hypoglossal are in- 

 cluded in several minute nerves emerging from the occipital region of 

 the skull. The trigeminal, facial, glossopharyngeal and vagus nerves of 

 fishes are primarily associated with the muscles of the visceral arches, 

 and, as shown in Table 6, they supply the derivatives of this musculature 

 in the higher vertebrates. Muscles change in shape and function during 

 the course of evolution, but their innervation remains remarkably con- 

 stant. 



Autonomic Nervous System. Most of the efferent fibers in the 

 spinal and cranial nerves supply somatic muscles of the body and visceral 

 muscles associated with the gill region. But in addition to these, certain 

 of the cranial and spinal nerves contain other efferent fibers going to 

 muscles in the walls of the gut, heart, blood vessels and other internal 



