RATA. 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 sAzracle 
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 offactory lobes to the olfactory capsules. Behind the 
cerebrum is the narrow ¢halamencephalon produced dorsally 
into a small gizneal 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 
mid-brain and give off the third cranial nerves or oculomotor 
(to the eye-muscles) from their ventral surface, and the 
fourth or ¢vochiear (to a single eye-muscle) from their dorsal 
surface. Behind them is the /znd-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 (faciad), 
eighth (auditory), ninth (glossopharyngeal) and tenth (vagus) 
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 ob/iguus superior and inferior radiating 
from one point of origin and posteriorly are the four vecti 
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 have no meaning in an animal like the skate with 
lateral eyes, but have been passed down from human anatomy. : 
M. 22 
