44 THE SKATE 



It is marked by longitudinal and transverse grooves. It 

 is the center of motor coordination. 



e. The Myelencephalon or Medulla Oblongata. Posterior 

 to the cerebellum is the elongated myelencephalon. Its an- 

 terior part consists of the choroid plexus of the fourth ven- 

 tricle, below which is the cavity of the fourth ventricle. 

 There are two earlike projections (auricles) on the anterior 

 end of the medulla. 



The posterior end of the medulla marks the end of the 

 brain which grades indefinitely into the spinal cord. 



Drawing 24. Draw the brain from the dorsal side. 



2. The Cranial Nerves. 



The dissection of the cranial nerves in the skate is easier 

 than in any other vertebrate animal. 



There are ten cranial nerves. These nerves show a defi- 

 nite relation to the original segmentation of the head and 

 to the gill. Throughout the evolution of the vertebrates 

 each nerve continues to be distributed to homologous parts 

 regardless of the animal. So close is this homology that 

 structures of unknown history having the same innervation 

 can be known as homologous regardless of great differ- 

 ence in function and position. 



The cranial nerves are indicated either by number which 

 corresponds to the order of their appearance or by name. 

 Since the roots of some of these as the fifth, seventh, and 

 eighth are intermingled as they leave the brain they should 

 be located some distance from their origin as indicated in 

 the directions. 



a. The first or olfactory tract leaves the olfactory lobe 

 extending to the nostrils and expanding into olfactory 

 bulbs. 



b. The second or optic tract or nerve is a stout, white 

 nerve piercing the orbit and running from the retina to the 



