20 GUIDES FOR VERTEBRATE DISSECTION 



THE HEAD 



Cut off the head just behind the pectoral fins and throw the 

 body away. Remove the skin from the dorsal side of the head 

 as far back as the gill-slits, noting in the operation the distal ends 

 of the endolymph ducts (p. 3) passing from the skull to the skin, 

 and also the numerous nerve-twigs (branches of the ophthalmicus 

 superficialis nerve) going to the skin at the tip of the snout. 



The Ear. Remove, one by one, very thin slices of cartilage 

 from one side of the skull between the endolymph duct and the 

 spiracle, keeping close watch for the appearance of tubes in the 

 hard material. First to be found will be the longitudinal 

 semicircular canal, consisting of a tube in the cartilage filled with 

 a fluid (perilymph), and in this a smaller tube free from the walls 

 on all sides, a part of the membranous labyrinth. Continue to 

 develop slowly in the same matter, making out a horizontal 

 canal, lateral to that first found, and a transverse canal, posterior 

 to the other two. Dissect these out very carefully and work out 

 the entire membranous labyrinth consisting of the three semicir- 

 cular canals (known also as external, anterior, and posterior) 

 and a two-chambered portion with which the canals are con- 

 nected at either end. How many enlargements (ampullae) does 

 each canal bear? Observe that the three canals are nearly at 

 right angles to each other. (The two chambers, an upper utric- 

 ulus and a lower sacculus, are not so easily made out as the rest 

 of the labyrinth, but a little patience will accomplish the task.) 

 Cut into the central portion and extract some of the matter 

 contained; place it in a drop of water on a slide and examine 

 with the microscope, drawing some of the crystalline bodies (oto- 

 liths) which occur. Is the labyrinth in front of or behind the 

 spiracle? 



Also make a diagrammatic sketch of the membranous labyrinth. 



Brain. Now, in the same way, cut off the roof of the cranium 

 in the middle line a little in front of the ear, removing the car- 

 tilage in small bits, thus exposing the brain. See that the cavity 

 of the cranium is lined with a rather tough membrane, the endo- 

 rachis (really the perichondrium). Take care that the nerves 

 lying just lateral to the cranium are not injured. The brain as 

 thus exposed is seen to be covered by a thin membrane, the 



