NECTURUS 73 



The first pair of cranial nerves are the olfactories; they are 

 large nerves which pass directly forward from the anterior end 

 of the hemispheres to the nasal capsules, in some cases branch- 

 ing before they reach the capsules. The second pair are the optic 

 nerves ; these nerves are small and will be seen emerging from the 

 brain just back of the cerebrum and passing to the orbits. The 

 third nerve, the oculomotor, is a very small, threadlike nerve which 

 arises on the ventral surface of the brain beneath the midbrain on 

 each side and passes forward to the orbit, where it supplies the 

 muscles of the eyeball. The fourth cranial nerve, the trochlear, 

 and the sixth, the abducens, are wanting in Necturus, both being 

 incorporated in the fifth nerve. 



The fifth cranial nerve, the trigeminus, is a large trunk which 

 passes forward a short distance and then expands into the Gasse- 

 rian ganglion, which lies against the inner wall of the skull. It 

 then splits into two main branches,— an ophthalmic and a maxillo- 

 mandibular branch,— which pass to the outside of the skull ; trace 

 them as far as possible. 



The seventh, or facial, nerve and the eighth, or auditory, nerve 

 spring from the brain as a single trunk, which, after sending off a 

 large branch to the Gasserian ganglion, divides into two nerves, 

 the facial and the auditory. The former soon splits into two 

 branches ; the latter at once enters the auditory capsule. 



The ninth and tenth nerves, the glossopharyngeal and the vagus, 

 spring from the brain immediately back of the facial and auditory 

 trunk by three roots and at once unite to form a single large nerve. 

 This passes back a short distance and then expands to form a large 

 ganglion, from which numerous nerves pass to the tongue, neck, 

 and gills and to the heart and stomach. 



Observe the anterior spinal nerves. Each one arises from the 

 side of the spinal cord by two roots, a dorsal and a ventral root, in 

 the former of which is a ganglion. These roots pass out of the 

 neural canal by the foramina between the vertebrae. The fibers of 

 the dorsal root have a sensory function, conveying sensory impres- 

 sions to the cord ; those of the ventral root are motor in function. 



Exercise 11. Draw the cranial nerves and the first two or three spinal 

 nerves so far as they have been observed. 



