COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 323 



Follow it here and note the ganglia which it bears at intervals and the branches 

 from these ganglia. Note particularly the branches between the sympathetic 

 ganglia and the adjacent spinal ganglia. These branches constitute the ramus 

 communicans and consist of the visceral motor and visceral sensory fibers passing ' 

 between the sympathetic and central nervous systems. The ganglia and 

 branches of the sympathetic are particularly noticeable in the urogenital region. 



Draw spinal nerves, plexi, and sympathetic system as far as seen. 



2. The sense organs of the head. — 



a) The nasal cavities: The external nares lead into wide chambers, the nasal 

 cavities. Cut off the external nares and the roof of the skull posterior to them, 

 thus revealing the nasal cavities. .They are separated by a median septum, 

 partly bony. From the ventral region of the septum a conspicuous fold projects 

 into the nasal cavity. On the posterior wall of the nasal cavity is a slight pro- 

 jection, a concha or turbinal. Posterior to this the nasal cavity connects by a 

 passage with the roof of the mouth cavity, the nasal cavities thus serving as 

 respiratory passages. 



b) The eye: Although the eye is small it can be dissected with a little care. 

 Its parts are very similar to those of the fish eye. Make an incision through the 

 skin around one eye and with the bone scissors remove the skull dorsal to and 

 between the eyes. The two eyes are seen to be close together, separated by a 

 median membranous interorbital septum. Near this septum on each side runs 

 an artery. On the anterior dorsal surface of the eyeball is a gland, the Harderian 

 gland. Over the posterior and ventral surface of the eyeball extends the much 

 larger lacrimal or tear gland. Remove these glands, thus exposing the surface 

 of the eyeball and the eye muscles. Extending from the interorbital septum to 

 the dorsal surface of the eyeball is the superior oblique muscle. Posterior to 

 this and inserted in the eyeball near it is the superior rectus. Between and ven- 

 tral to these two is the internal rectus. Passing above the internal rectus are 

 two nerves, the trochlear to the superior oblique muscle and the ophthalmic 

 branch of the trigeminus. Loosen the eyeball ventrally and raising it as far as 

 possible examine the ventral surface. The anterior part of this surface is covered 

 by a flat muscle, the pyramidalis, which originates on the eyeball and passes to 

 the eyelids and nictitating membrane. Remove this and clean the ventral 

 surface of the eyeball. The inferior oblique and inferior rectus muscles are then 

 seen converging to their insertions on the ventral surface of the eyeball. The 

 external rectus is posterior to them. 



Remove the eyeball and open it by cutting off its dorsal side. Identify the 

 coats of the eyeball, the lens, the cavities of the eye, and the two humors as in 

 the elasmobranch eye, as the structure is practically identical with the latter. 

 Note, however, the difference in the shape of the lens of the turtle eye. 



c) The ear: The ear consists of two parts, a middle ear and an internal ear. 

 The former is located posterior to the angle of the jaws internal to a circular area. 



