20 LABORATORY WORK. 



s 



structures are two arches of cartilage , a neural arch around 

 the spinal cord, a hcemal arch around the blood-vessels. 

 The bulk of the section is occupied by the muscles of the 

 tail. Note that the two halves are separate and that each 

 half is subdivided into dorsal and ventral muscles. On 

 either side of the body, just beneath the skin, is a minute 

 tube, the canal of the lateral-line system. Sketch the cut 

 surface of the tail, illustrating all these points. 



Split the skin on the top of the head and pull it 

 off, noticing on its under surface other branching canals 

 of the lateral-line system. 



In the head of the pup it will be difficult to make out 

 much of the brain and nerves, but the following points 

 can be seen and followed with ease. 



Carefully slice off the top of the skull, exposing the brain. 

 Enlarge the opening until the whole brain is visible and 

 then make out the following parts: In front the cere- 

 brum, made up of right and left halves, each prolonged 

 at the lateral anterior angle into an olfactory tract leading 

 toward the nostril. Behind the cerebrum and lying at 

 a lower level is the 'turixt-brain (optic thalamus), which has 

 a thin roof. Next come the optic lobes, a pair of rounded 

 prominences meeting closely in the middle line. The optic 

 lobes are overlapped behind by the unpaired cerebellum, 

 which extends backwards and similarly overlaps the fifth 

 region of the brain, the medulla oblongata, portions of 

 which, the corpora restiformia, are visible at the sides of 

 the cerebellum. Draw the brain in outline, naming the 

 parts. 



The principal nerves you will find will be the olfactory, 

 already mentioned, going to the nose; the optic, arising 

 from the lower surface of the 'twixt-brain and going to 

 the eyes; the trigeminal, arising from the anterior side 



