xxix NERVES 393 



the vestige of a sensory organ, and connected with the ventral 

 surface of the same division is the pituitary body (pty). 



The mode of origin of the nerves is also characteristic. 

 From the spinal cord the nerves arise segmentally, one pair 

 corresponding to each myomere, and pass through aper- 

 tures in the neural arches of the vertebras. Each arises by 

 two roots, a dorsal and a ventral. 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, extending along the dorsal 

 region of the ccelome, is connected with the spinal nerves, 

 and sends branches to the viscera, blood-vessels, &c. 



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 ; the second or optic (Fig. 105, 

 n) the retina of the eye (see below), and the eighth or auditory 

 ( VIII), the organ of hearing. The third or oculomotor, the 

 fourth or trochlear (IV), and the sixth or abducent go to the 

 muscles of the eye ; the fifth or trigeminal ( V) to the snout 

 and jaws ; the seventh or facial ( VII) to the palate, lower 

 jaw, and hyoid arch ; the ninth or glossopharyngeal (IX) to 

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

 to the remaining branchial arches, as well as to the heart, 

 stomach, and lateral line. 



Besides the sensory tubes of the skin (lateral line, &c.), 

 which are probably the seat of a delicate tactile sense, and the 

 rudimentary tongue, which is presumably an organ of taste, 

 there are three pairs of sensory organs, the structure and posi- 

 tion of which is very characteristic of Vertebrates. These 

 are the olfactory organs, the eyes, and the auditory organs. 



The olfactory organs are a pair of cup-like sacs on the 

 under side of the snout, enclosed in the olfactory capsules 



