SENSE ORGAN'S. 569 



the optic nerves, and a tongue-shaped mass (the tuber 



cinereum), to which the pituitary body is attached. 

 The optic lobes, a pair of oval bodies, between and 



below which is the iter. 



The cerebellum, a very narrow transverse band. 

 The medulla oblongata, on the roof of which the pia 



mater forms a very vascular "choroid plexus." 

 The cavities of the brain and the canal of the spinal cord 



are in the adult lined by ciliated epithelium. 

 The cranial nerves are, as usual, on each side the 

 following : 



(1) Olfactory, from the olfactory lobe to the nose ; 



(2) Optic, crossing and interlacing with its fellow ; 



(3) Oculomotor, to four muscles of the eye ; 



(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 palatine 

 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, etc. 



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 accompany 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 organs. The eyes project on the top of the head 

 and on the roof of the mouth. There is a third eyelid. 

 The transparent cornea in front, the firm sclerotic surround- 

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

 usual continuous. The next layer includes the vascular 

 and pigmented choroid and the brilliant iris. Internally is 

 the sensitive retina, while vitreous humour fills the cavity 

 behind the lens. 



