COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 349 



anterior to the eyelid. The trochlear nerve lies slightly dorsal and medial to 

 the proximal portions of the ethmoidal and infratrochlear nerves. It runs 

 obliquely dorsad and anteriorly, and enters the superior oblique muscle at about 

 the middle of its posterior margin. 



The structure of the eyeball may now be investigated. It is very similar 

 to that of all vertebrates. The outer tough sclerotic coat or sclera is continuous 

 with the transparent cornea covering the exposed surface of the eye. As found 

 above, the cornea is covered externally by the conjunctiva. Cut off the top or 

 dorsal side of the eyeball and look within. The large lens will be observed. 

 Internal to the sclera is the black chorioid coat of the eye and internal to that 

 the greenish-gray retina. Between the lens and the retina is a large chamber, 

 the cavity of the vitreous humor, containing a gelatinous mass, the vitreous humor 

 or vitreous body. Remove the lens. The chorioid coat terminates behind the 

 cornea as a black curtain, the iris, bearing in its center a round hole, the pupil. 

 The space between the cornea and the iris is called the anterior chamber of the 

 eye and is filled in life with a fluid, the aqueous humor. The boundary between 

 the iris and the rest of the chorioid coat constitutes a ring known as the ciliary 

 body. It consists of two parts : a ring of thickened processes, the ciliary processes, 

 next to the iris, and a ring of radially arranged ridges, the orbiculus ciliaris, 

 extending to the main part of the chorioid coat. Both parts of the ciliary body 

 contain the ciliary muscle; this is a smooth muscle having both meridional and 

 circular fibers. It originates on the sclera, is inserted on the walls of the ciliary 

 body, and has the function of changing the shape of the lens. By making a 

 new cut parallel to the first around the equator of the eyeball the relations of 

 cornea, iris, and ciliary body will be more clearly observable. Note the marked 

 thickening due to the ciliary body. Examine the lens. Note its biconvex form 

 as compared with the spherical form of the lens of the fish eye. Around the 

 equator of the lens will be found the torn attachment of a membrane. This 

 membrane holds the vitreous body. Where it is attached to the lens it exhibits 

 parallel ridges, the zonular fibers, which in life fit into the hollows between the 

 ciliary processes. The zonular fibers constitute the suspensory ligament of the 

 lens which passes from the lens to the ciliary processes. By means of this liga- 

 ment traction can be exerted on the lens and its shape altered to some extent. 

 The small space between the suspensory ligament and the iris is the posterior 

 chamber of the eye. Peel the lens and note that it is composed of concentric 

 coats or lamellae like the coats of an onion, each lamella being composed of lens 

 fibers. 



Draw the section of the eye. 



b) The nasal cavities: Detach the head of the animal at the joint between 

 the occipital condyles and the altas, and discard the body. Cut off the pinnae. 

 Clear the dorsal surface of the skull down to the bone. With a saw, saw com- 

 pletely through the head slightly to one side of the median sagittal plane. Use 



