168 Basic Structure of Vertebrates 



XI. Accessory (or spinal accessory), a purely motor nerve 

 partly associated with the vagus — i.e., "accessory" to the vagus (Figs. 

 138, 530) — and in part resembling a spinal nerve. Some of its centers 

 are in the medulla and others in the spinal cord. Some of its fibers may 

 join vagus fibers in supplying thoracic and abdominal viscera. Its chief 

 distribution is to certain muscles of the neck (cleidomastoid, sterno- 

 mastoid) and to the trapezius, an important superficial dorsal muscle 

 extending between the vertebral column and the shoulder girdle 

 (Fig. 91). 



XII. Hypoglossal, purely motor, innervating small muscles of 

 the hyoid and larynx and the important muscles which constitute the 

 tongue (Fig. 530). 



The accessory and hypoglossal nerves appear, as such, only in 

 reptiles, birds, and mammals, although possibly present in ancient am- 

 phibians. Their origin is problematic. Some fishes have several pairs of 

 nerves which are typically spinal in character but arise from the ex- 

 treme hind region of the medulla and, therefore, inside the cranium. It 

 has been mentioned (see p. 112) that, in development of the cranium, 

 several embryonic "occipital vertebrae" may become incorporated into 

 its occipital region. There is much evidence that the vertebrate head 

 and neck have been, in effect, so pushed into one another that some 

 vertebrae have been "absorbed" into the skull and some spinal nerves 

 find themselves inside the cranium (spino-occipital nerves). The ac- 

 cessory nerve may be regarded as a hybrid compounded partly of fibers 

 extracted from the vagus and partly spinal in origin. The hypoglossal 

 is evidently constituted entirely of spinal motor roots. 



Groups of Cranial Nerves 



The 10 or 12 cranial nerves fall naturally into definite groups. 



I and II are nerves of special sense organs. The unique manner 

 of origin of the olfactory fibers and the fact that the retinal receptors of 

 the optic nerve are themselves derived from the brain set these two 

 nerves apart from all the others. 



Ill, IV, and VI constitute an oculomotor group. It is a striking 

 fact that the small muscles of the eyeball monopolize 3 of the 12 nerves. 



V, VII, and VIII are closely related in their embryonic origin and 

 there is much overlapping in the distribution of V and VII. The sense- 

 organs within the ear, like the neuromasts of the lateral-line system, are 

 stimulated by vibrations of the external medium. This similarity in 

 function, and peculiarities in the manner of development and in the 

 structure of the ear, indicate strongly that the sensory mechanism of 



