SENSE ORGANS. 539 



(4) Pathetic, to the superior oblique eye muscle ; 



(5) Trigeminal, with ophthalmic, maxillary, and mandibular 



branches ; 



(6) Abducens, to the external rectus eye muscle ; 



(7) Facial, arising along with the auditory, with a ganglion uniting 



with the Gasserian ganglion of the trigeminal, with a pala- 

 tine branch to the roof of the mouth, and a hyoid branch to 

 the lower jaw ; 



(8) Auditory, to the ear ; 



(9) Glossopharyngeal, to the tongue and some of its muscles ; with 



a ganglion which unites with that of the tenth ; 

 (10) Vagus, with branches to lungs, heart, stomach, (S;c. 

 The student should refer back to the description of the skate, and to 

 the chapter on the structure of Vertebrates. 



The spinal cord gives origin to ten pairs of spinal nerves, 

 and is swollen at the origin of those which go to the limbs. 

 Around the union of the anterior and posterior roots lie sacs 

 with crystals of carbonate of lime. 



The sympathetic system consists of about ten pairs of 

 ganglia — {a) united by branches to the spinal nerves ; {b) 

 united to one another by longitudinal trunks which accom- 

 pany the dorsal aorta and the systemic arches, and end 

 anteriorly in the Gasserian ganglion ; {c) giving off branches 

 to the heart, the aorta, and the viscera in the pelvic region. 



Sense Orga?is. 



The eyes project on the top of the head and on the roof 

 of the mouth. There are two lids, the upper thick and very 

 slightly movable, the lower transparent and movable. The 

 transparent cornea in front, the firm sclerotic surrounding 

 the eyeball, and the sheath of the optic nerve, are as usual 

 continuous. The next layer includes the vascular and pig- 

 mented choroid and the brilliant iris. Internally is the 

 sensitive retina, while vitreous humour fills the cavity behind 

 the lens. 



The internal ears have the usual parts and lie within the 

 auditory capsules, which are in great part bounded by the 

 pro-otics. Connecting the fenestra ovalis of the ear with 

 the tympanic membrane, which is flush with the skin, there 

 is a delicate bony rod — the columella. This lies in the 

 Eustachian tube, which opens into the mouth at the corner 

 of the gape. 



The nostrils open into small nasal cavities, with folded 



