578 THE ORGANS 



ciliary body, iris, and lens. Before altempting to cut this section almost all of 

 the lens should be picked out of the block, leaving only a thin anterior and 

 lateral rim attached to the capsule and suspensory ligament. The block should 

 then be so clamped to the microtome that the lens is the last part of the block 

 to be cut. The above precautions are necessary on account of the density of the 

 lens, making it difficult to cut. 



(b) Section through the postero-lateral portion of the eyeball to show struc- 

 ture of sclera, chorioid, and retina. This section should be as thin as possible 

 and perpendicular to the surface. 



(c) Section through the entrance of the optic nerve. Haematoxylin-picro- 

 acid-fuchsin also makes a good stain for this section. It is instructive in cutting 

 the eye to cut a small segment from the optic nerve and to block it with the optic- 

 nerve entrance material in such a manner that it is cut transversely. In this way 

 both longitudinal and transverse sections of the optic nerve appear in the same 

 section. 



(d) For the study of the neurone relations of the retina, material must be 

 treated by one of the Golgi methods (page 36). 



(4) The connective-tissue cells and cell spaces of the cornea may be demon- 

 strated by means of technics 8 and 9, page loi. 



(s) The different parts of the lacrymal apparatus may be studied by fixing 

 material in formalin-IMuller's fluid and staining sections in haematoxylin-eosin. 



(6) The Eyelid. An upper eyelid, human if possible, should be carefully 

 pinned out on cork, skin side down, and fi.xed in formalin-Miiller's fluid. Vertical 

 sections should be stained with haematoxylin-eosin or with haematoxylin-picro- 

 acid-fuchsin. 



The Organ of Hearing 



The organ of hearing comprises the external ear, the middle ear, 

 and the internal ear. 



The External Ear 



The external ear consists of the pinna or auricle, the external 

 auditory canal, and the outer surface of the tympanic membrane. 



The pinna consists of a framework of elastic cartilage embedded 

 in connective tissue and covered by skin. The latter is thin and con- 

 tains hairs, sebaceous glands, and sweat glands. 



The external auditory canal consists of an outer cartilaginous por- 

 tion and an inner bony portion. Both are lined with skin continuous 

 with that of the surface of the pinna. In the cartilaginous portion of 

 the canal the skin is thick and the papillas are small. Hair, sebaceous 

 glands, and large coiled glands {ceruminous glands) are present. The 

 last named resemble the glands of Mall (page 574) and are probably 

 modified sweat glands. Their cells contain numerous fat droplets and 



