18 



into the posterior side of the sacculus. During life the utriculo- 

 sacculus and the semicircular canals are filled with a lymphatic 

 fluid, and the sacculus contains a large calcerous ear-stone (otolith), 

 which is usually dissolved by the formalin used in preserving the 

 dogfish. 



Whitish patches of thickened sensory epithelium may be seen in 

 the ampullae (cristate acusticae) and in the utriculo-saccular cham- 

 ber (maculae acusticae). Branches of the eighth nerve can be fol- 

 lowed to all these areas. 



A projection of the ventral wall of the utriculo-sacculus is the 

 lageria, the rudiment from which the cochlea of higher animals 

 developed. It also contains a macula acustica. 



EXTERNAL FEATURES OF THE EYE. Observe the transparent 

 cornea covering the external surface of the eye; the dark ring of 

 the iris; the central opening in the iris, the pupil; the conjuncti- 

 val sac surrounding the external half of the eyeball. Cut away 

 sufficient of the upper wall of the cartilaginous orbit to expose the 

 eyeball and its muscles. Note the considerable amount of soft con- 

 nective tissue around the eye and explore the orbital sinus (p. 13). 

 Take notice of the following nerves, in order to ensure their preser- 

 vation until the time comes to trace them more completely. A large 

 nerve crossing the medial side of the orbit, the superficial ophthal- 

 mic; a nerve leaving the cranium opposite the optic lobe, passing 

 under the superficial ophthalmic to the anterior muscle of the eye- 

 ball, the trochlear; several long ciliary nerves passing to the eye- 

 ball; several other nerves visible in the deep angle of the orbit. 



Six muscles move the eye. Four of these arise close together 

 at the deep postero-medial angle of the orbit. Diverging, they are 

 inserted upon four sides of the eyeball, and from the position of 

 their insertions are named the superior, posterior, inferior, and 

 anterior recti. Two muscles arise from the antero-medial angle of 

 the orbit, the superior and inferior oblique muscles. 



Between the recti muscles can be seen a mushroom-shaped stalk 

 of cartilage, the ophthalmic peduncle; the eyeball rests against its 

 expanded end. (There is no peduncle in Eugaleus.) 



THE CRANIAL NERVES. The cranial nerves are twelve pairs of 

 nerves arising from the brain, and thus distinguished from the spinal 

 nerves which arise from the sides of the spinal cord. They are dis- 

 tributed chiefly to the head and neck, though branches of the vagus 

 nerve go to the viscera and to the sense organs of the lateral line. 

 Since the nerves are all paired, the distribution of both nerves of a 

 pair being alike, the descriptions will mention but one nerve of a 

 pair. As the cranial nerves are traced dissect away the sides of the 

 cranium down to the foramina penetrated by the nerves, and follow 

 each nerve from its origin on the brain to the parts innervated by it. 

 Features of the dissection which are not found in tracing the nerve 

 of one side should be sought on the other side. 



The olfactory nerve. The anterior surface of the olfactory lobe 

 fills a large foramen in the anterior wall of the cranium and is 

 pressed closely against the posterior surface of the nasal sac. Nu- 

 merous small nerves, collectively forming the olfactory nerve, arise 

 from the anterior face of the lobe, penetrate the membraneous wall 

 of the olfactory organ, and are distributed to its highly folded sur- 

 face. 



The terminal nerve, Nervus terminalis, is a slender nerve run- 

 ning along the medial surface of the stalk of the olfactory lobe. 

 Follow it backward to its origin on the anterior surface of the cere- 



