880 



COMPARATIVE ANATOMY 



vspma. nerve 



deep ophthalmic 



maxillary ' 

 mant^bular 



gill slits 



pretrematic branch 

 posttrematic branch 



Fig. 493. 



Diagram to illustrate the segmentation of the vertebrate head and the relation 

 of the cranial nerves to the segmentation. The numbers above the figure designate 

 the cranial nerves ; the numbers in the figure are situated on the head myoomes ; 

 the sensory part of the nerves is represented by heavy continuous lines ; the motor 

 part by broken lines. The anterior head cavity is the first myotome and therefore 

 the myotome which is numbered 1 is really the second myotome, and so on; But as 

 the myotomes were numbered before the anterior head cavity was discovered, the old 

 numbers are generally retained. The myotomes numbered 1, 2, and 3 produce the eye 

 muscle ; those numbered 4, 5, and 6 degenerate in the majority of vertebrates ; those 

 from 7 on probably contribute to the tongue musculature but never from typical 

 parietal muscles, such as occur in the trunk. It is seen from the figure that the third 

 cranial nerve and, the deep ophthalmic branch of the fifth belong to the first (really 

 second) head segment; the fourth and remainder of the fifth to the second (third) 

 segment; the sixth and seventh to the third (fourth) segment ; the ninth to the 

 fourth (fifth) segment; and the tenth to the fifth to eighth (sixth to ninth) segments. 

 The gill slits are intersegmental in location. The relation of the cranial nerves to 

 the gill slits should also be noted. (From Hyman after Goodrich.) 



From their functions the cranial nerves may be divided into four 

 groups : 



I. Nerves of special sense, namely, the olfactory and optic. These 

 arise in the primitive fore-brain, the olfactory passing to the nose and 

 the optic to the eye. 



II. Nerves of the eye muscles (Fig. 494), namely, the oculomotor, 

 trochlear, and abducens. These are of the somatic motor type, with a 

 few visceral motor and sensory fibers in the oculomotor. They control 

 the muscles of the eye. It is well to mention the hypoglossus at this 

 point, as it belongs to the purely somatic motor group, the spinal acces- 

 sory being of the visceral motor type. 



III. The acustico-lateralis system, namely, the acoustic nerve, and 

 those portions of the seventh, ninth, and tenth cranial nerves which are 

 connected with the sense organs of the lateral line in the gill-breathing 

 fish and amphibians. These nerves have a separate center in the upper 

 anterior end of the medulla oblongata. 



IV. The fifth, seventh, ninth and tenth cranial nerves are more like 

 the spinal nerves than any of the preceding. They all arise in the me- 

 dulla, each having a dorsal root and a ganglion, and each containing 

 somatic sensory (general cutaneous) and visceral sensory and visceral 

 motor fibers. The seventh and tenth may also include some of the fibers 

 running to the lateral line. The ninth nerve is the simplest of these, and 

 arises from the dorsal side of the medulla, dividing just behind its 

 ganglion into two branches, one, the pre-trematic (Fig. 482), which 



