376 



CHORDATE ANATOMY 



line organs in land animals the lateralis components of cranial nerves 

 disappear. In mammals the facial nerve becomes largely motor. But 

 these are all minor changes. Cranial nerves appear to be among the most 

 conservative of vertebrate organs. 



EVOLUTION OF SPINAL NERVES 



Phylogenetic changes in the spinal nerves are not very great. Man 

 has thirty-one pairs of nerves connected with the spinal cord. These are 



Fig. 333. — The twelve cranial nerves shown as if projected upon a median section 

 of the head. I. Olfactory lobe. II. Optic. III. Oculomotor. IV. Trochlearis. V. 

 Trigeminus. VI.Abducens. VII. Facialis. VIII. Acusticus. IX. Glossopharyngeus. 

 X. Vagus. XI. Accessorius. XII. Hypoglossus. 



metamerically arranged just as are the nerves of Amphioxus. In 

 Amphio.xus nerves are of two sorts, dorsal (sensory and motor) and ven- 

 tral (somatic motor). The two kinds alternate with one another and do 

 not unite. Among vertebrates, however, the spinal nerves of Petro- 

 myzon alone show this primary independence. In all other vertebrates 



