378 MANUAL OF ZOOLOGY SECT. 



The fourth ventricle is continuous behind with the 

 -central canal of the spinal cord. In front it is continuous 

 with a narrow passage, the iter (iter.) which opens 

 anteriorly into a wider space, the diacale or third ventricle 

 (dia.) occupying the interior of the diencephalon. From 

 this opens in front a median prosoccele, which gives off a 

 pair of paracceles or lateral ventricles (para.) extending into 

 the two lateral portions of the prosencephalon. 



A series of nerves arise in pairs from the brain and spinal 

 cord. From the spinal cord the nerves arise segmentally, 

 one pair corresponding to each myomere. Each arises by 

 two roots, a dorsal and a ventral, and passes through 

 independent apertures in the neural arches of the vertebrae. 

 The dorsal root is dilated into a ganglion, and contains only 

 sensory fibres ; the ventral root is non-ganglionated, and is 

 motor. A longitudinal ganglionated sympathetic nerve, ex- 

 tending along the dorsal region of the ccelome, is connected 

 with the spinal nerves, and sends branches to the viscera, 

 blood-vessels, etc. 



From the brain arise ten pairs of nerves, some of which 

 are sensory, others motor, others mixed. Three are the 

 nerves of the principal sense-organs, the first, or olfactory, 

 supplying the organ of smell (Fig. 212, olf. s.) ; the second, 

 -or optiC) the retina of the eye, and the eighth, or auditory, 

 the organ of hearing. The third, or oculomotor, the fourth, 

 or trochlear (path.), and the sixth, or abducent, go to the 

 muscles of the eye ; the fifth, or trigeminal (oph. V., mx. V., 

 mnd. I 7 .), to the snout and jaws ; the seventh, or facial (oph. 

 VII., pi. VII., hy. mnd. VIL), to the snout, palate, lower jaw, 

 ^and hyoid arch ; the ninth, or glossopharyngeal (gl. ph.), to 

 the hyoid and first branchial arches; and the tenth, or vagus 

 4vag.), to the remaining branchial arches, as well as to the 

 heart, stomach, and lateral line. 



