480 THE DOGFISH CHAP. 



gill-clefts ; b, the visceral branch ; arid c, the lateral line 

 branch, above and to the inner side of the branchial branches. 



8. Separate some of the ampullary sensory tubes from one 

 another, and note the ampullce and the nerves supplying 

 them. 



9. Carefully slice away the cartilage of the auditory 

 capsule of the side you have not already dissected so as to 

 expose the membranous labyrinth. Examine under water, 

 and make out the vestibule (utriculus and sacculus) with its 

 contained otolithic mass, the three semicircular canals with 

 their ampulla;, and the branches of the auditory nerve 

 (compare Fig. 59). 



III. Now examine the preserved brain from above, from 

 below, and from the side, making out, in addition to the 

 parts already noticed (F, I, i) 



1. The optic chiasma, infundibulum (with an oval lobe 

 and a vascular sac on either side) , pituitary body, crura ccrebri, 

 and, as far as possible, the origins of the nerves. Sketch 

 from above and below. 



2. On one side of the brain, cut into the olfactory lobe, 

 prosencephalon, optic lobe, and cerebellum from above, so 

 as to expose the olfactory ventricle, lateral ventricle, optic 

 ventricle, and cerebellar ventricle. Then bisect the entire 

 brain into right and left halves with a sharp scalpel, and 

 examine the uninjured half in longitudinal section, noting, 

 in addition to the parts mentioned above, the third ventricle, 

 foramen of Monro, Her, and fourth ventricle. Sketch. 



G. Transverse Sections. Cut thick transverse sections of 

 an entire dogfish with a knife through a, the anterior, 

 and b, the posterior part of the head (pharyngeal region) ; 

 c, about the middle of the body ; and d, the tail. Make out 

 the relations of the various parts and organs, and sketch 

 the lateral half of each section. 



A more satisfactory method than this is to obtain a very 

 young dogfish, not more than J inch in diameter, and after 

 cutting it transversely into pieces about J inch in thickness 

 in the regions named above, stain, embed, and mount a few 

 sections from each piece (see p. 136). These can first be 

 examined with a lens or with the low power of the micro- 

 scope, and then, by putting on the high power, important 

 points in the histology can be made out. In addition to 

 the minute structure of the tissues and organs described in 

 Part I. of this book, the structure of the nolochord (p. 441), 

 integumentary sense-organs (pp. 432 mid |<>|), dermal 

 teeth (pp. 433 and 4.4 \), &c., should be studied. 



