RAIA. 321 



eyes and auditory sacs. The further structure of these organs 

 will be referred to later. Lying farther out on each side 

 opposite the eyes is a large oval mandibular muscle. Its 

 front end nearly meets the olfactory capsule and its hind 

 border approaches the auditory capsule. Lastly, the spiracle 

 lies slightly in front of the auditory sac. 



Returning to the brain, we notice the large cerebrum at 

 the anterior end which is produced forward as a pair of 

 long olfactory lobes to the olfactory capsules. Behind the 

 cerebrum is the narrow thalamencephalon produced dorsally 

 into a small pineal body and ventrally into a process called 

 the infundibulum. From its ventral surface originate the 

 pair of optic nerves to the eyes. The cerebrum and thala- 

 mencephalon form the fore-brain with the two first cranial 

 nerves — I, olfactory and II, optic. 



The paired optic lobes then succeed. They form the 

 Inid-brain and give off the third cranial nerves or oculomotor 

 (to the eye-muscles) from their ventral surface, and the 

 fourth or trochlear (to a single eye-muscle) from their dorsal 

 surface. Behind them is the hind-brain formed of a large 

 cerebellum which has a large anterior lobe partially covering 

 the optic lobes and a posterior lobe covering the medulla 

 oblongata. The medulla oblongata has a thin dorsal wall 

 and is continued backwards into the spinal cord which 

 passes posteriorly to the tail. From its lateral walls there 

 arise the fifth (trigeminal)^ sixth {abducens\ seventh (facial), 

 eighth {auditory), ninth {glossopharyngeal) and tenth {vagtis) 

 cranial nerves. They can be seen passing out of the cranial 

 capsule by foramina and their subsequent distribution has 

 now to be followed. 



The eye is held in position and moved in the orbit by 

 six eye-muscles which originate in the cartilaginous orbit 

 and are inserted in the sclerotic of the eye. At the 

 anterior end are the obliquus superior and inferior radiating 

 from one point of origin and posteriorly are the four recti 

 muscles. These radiate from one point and are easily 

 identified as the rectus superior, inferior^ internus and 

 externus.^ Without further dissection we can recognise the 



* The names of the last two hav« no meaning in an animal like the skate with 

 lateral eyes, but have been passed down from human anatomy. 



M. 22 



