160 



Basic Structure of Vertebrates 



reptiles, birds, and mammals, the corresponding 10 pairs are present, 

 plus 2 pairs added at the posterior end of the series. Fossil skulls of 

 primitive amphibians give evidence of the presence of 12 pairs of cra- 

 nial nerves. 



A cranial nerve may contain only afferent or only efferent fibers, 

 in which case it is described as, respectively, sensory or motor. A 

 nerve containing fibers of both kinds is called "mixed." Nerves pass- 

 ing to receptors in the body-wall are called "somatic sensory," ex- 

 cept that such fibers as go to structures which are of the "visceral" 

 type but situated in the body-wall (e.g., blood-vessels) may be called 

 "visceral." Nerves connected with striated (voluntary) muscles in the 

 body-wall are somatic motor. Similarly, there are visceral sensory 

 and visceral motor nerves related to the visceral organs. 



Excepting the olfactory and optic nerves and a part of the fifth 

 cranial nerve, the sensory fibers of a cranial nerve are outgrowths of 

 cells located in its ganglion which is external to the brain, appearing as 

 an enlargement of the nerve near its junction with the wall of the brain 

 (Figs. 146, 147). The motor fibers in the root of a cranial nerve arise 

 from cells inside the brain. These motor cells commonly occur in groups 

 or "centers" which are virtually ganglions but are often called "nu- 

 clei" — an unfortunate term: a motor "nucleus" must not be confused 

 with the internal nucleus of a single cell. 



Brief descriptions of the cranial nerves follow. Points of origin of 

 nerves from the brain are shown in Figs. 136, 138, 146, 147. For 

 distribution of nerves, see Figs. 146, 147. 



Fig. 146. Diagram of cranial nerves of lower vertebrate. Eye-muscle nerves are 

 omitted; central nervous system is dotted; fifth nerve is represented as composed 

 of two nerves; lateralis nerves are separated from the ninth and tenth nerves. 

 (I-X) Cranial nerves; (1-5) gill-clefts; (b) buccalis nerves; (c) chorda tympani; 

 (g) geniculate ganglion; (h) hyoid nerve; (£) intestinal (pneumogastric) nerve; 

 (j) jugal ganglion; (I) lateral-line nerve of (X); (m) mouth; (md) mandibular 

 nerve; (ml) mentalis nerve; (mi) maxillary nerve; (op) ophthalmicus profundus 

 nerve; {osV, osVII) superficial ophthalmic nerves of (V) and (VII); (p) palatine 

 nerve; (po) posttrematic nerves; (pr) pretrematic nerves; (pi) petrosal ganglion; 

 (s) semilunar (Gasserian) ganglion; (sp) spiracle. (Courtesy, Kingsley: "Com- 

 parative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



