THE NERVOUS SYSTEAf 33 



known :is tlic luiiidl rtiiutl foriiu'd liy tin- n('ur:il arclics cxtciKiinn 

 tlorsally from the cciilra <»f the vertcltia'. In the mcsal liin' is a 

 longitudinal ^;roov(', the dorsal Jissurc. ()l)S('rv(' tiic cephalic 

 or hrdcludl and tlu' caudal or lumbar cidarncnicnts of the cord. 

 ( 'audad of t he lunil)ar enlargements the cord is reduced to a men; 

 filament, tlie Jiliim Icnninale, which extends into the cavity of 

 the urostyle and may be seen by dissecting away the dorsal 

 portion of this part of the skeleton. 



The Cranial Nerves. — Ten pairs of nerves arise from the brain 

 within the cranial cavity, hence their name. Some are sensory, 

 others motor, and some are both sensory and motor in nature. 

 The roots of some of these may be seen if care is used in making 

 the dissections necessary to expose the sides of the brain. A 

 brief description of these nerves is given as an aid to the identifi- 

 cation of those which may be found without the expenditure of 

 too much time. // time is limited proceed directly to the study of 

 the brain canties. The cranial nerves are in pairs, one leaving 

 the brain on each side. The first is the olfactory and has already 

 been seen, arising from the olfactory lobe. The second, or optic 

 nerve, leaves the ventral surface of the interbrain and extends 

 directly to the eye. The third, the oculomotor, is small and 

 arises caudad of the optic nerve and is distributed to muscles of 

 the eye. The fourth, the trochlearis, is very small and leaves 

 the brain from the dorsal side between the optic lobe and the 

 cerebellum. Both the third and fourth nerves arc motor and are 

 distributed to certain muscles of the eyeball. The fifth, the 

 trigeminal, is one of the largest of the cranial nerves. It leaves 

 the ventral side at the cephalic end of the oblongata. It is 

 distributed to the side of the head and lower jaw. The sixth, 

 the nbducens, arises from the ventral side of the oblongata and 

 supplies certain eye muscles. The seventh, the facial, ari.ses 

 from the oblongata caudad of the fifth and is a mixed nerve to 

 the side of the head and lower jaw. The eighth, the auditory, 

 arises from the dorso-latcral wall of the oblongata and supplies 

 the inner ear. The ninth and tenth, the glossopharyngeal and 

 vagus, respectively, arise together from the sides of the ol)longata 

 a short distance caudad of the eighth. Both contain sensory and 

 motor fibers. 



Both brain and spinal cord are hollow, the cavities of the two 

 being continuous. Carefully slice away the dorsal surface of 

 the brain in order to expose its cavities called ventricles, and il 



