COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 329 



2. The sense organs of the head. 



a) The nasal cavities: Open one nasal cavity by a longitudinal slit just above 

 the margin of the upper jaw from the external naris to the head. Note 

 the median septum between the two nasal cavities and the swellings, the 

 turbinals or conchae, projecting from the septum into the nasal cavity. There 

 are three turbinals in a row: the first two large and conspicuous, the third and 

 most posterior one consisting only of a small rounded swelling on the roof of the 

 cavity in close contact with the posterior end of the second concha. Only this 

 third concha is provided with olfactory epithelium. Beyond the conchae the 

 nasal passages connect with the pharyngeal cavity. 



b) The eye: Cut through the skin around the eyeball and also remove the 

 roof of the skull between the two eyes. Note the relatively large size of the 

 eyeballs and the interorbital septum between them. Along the dorsal margin of 

 the septum course the two olfactory nerves. Press the eyeball outwardly away 

 from the skull. Two thin, flat muscles will be seen extending to the eyeball 

 from the orbit; the anterior one is the superior oblique, the posterior one the 

 superior rectus. Cut through the superior oblique at its insertion on the eyeball 

 and press it against the orbit. The internal rectus will now be seen extending 

 to the eyeball ventral to the superior oblique. The white nerve crossing the 

 orbit against the internal surface of the superior oblique is the ophthalmic branch 

 of the trigeminus. Dorsal to it the smaller trochlear nerve is seen terminating 

 on the superior oblique. The thin sheet of muscles on the surface of the eyeball 

 is the quadrate, a muscle of the eyelids. On the anterior surface of the eyeball 

 ventral to the superior oblique is a white fatlike mass, the Harderian gland. 

 Press the eyeball posteriorly and find anterior to this gland, against the anterior 

 wall of the orbit, the inferior oblique muscle. On pulling the eyeball forward 

 the external rectus is seen extending to the posterior surface of the eyeball. Free 

 the ventral margin of the eyeball. In the posterior ventral region on raising the 

 eyeball may be seen a small gland, the lacrimal gland. Two muscles will 

 be seen on the ventral surface of the eyeball. The anterior one is the inferior 

 rectus, the posterior one the external rectus. On cutting through the inferior 

 rectus the pyramid, a muscle of the eyelids, will be revealed internal to it. Cut 

 through all of the rectus muscles and the inferior oblique at their insertions on 

 the eyeball and remove the eyeball, severing the optic nerve. The pyramid and 

 quadrate muscles are now more readily seen extending on the surface of the eye- 

 ball to the optic nerve; the quadrate muscle is broad and dorsally situated, 

 the pyramid narrow and ventral. They are concerned in operating the nictitat- 

 ing membrane. In the orbit note the extent of the Harderian gland. 



Cut off the dorsal part of the eyeball and identify the structures of the eye. 

 Note the sclerotic coat, continuing as the transparent cornea over the exposed 

 part of the eye; the conjunctiva, passing over the external surface of the cornea 

 and continuing onto the eyelids; the black chorioid coat internal to the sclerotic 



