590 THE ORGANS 



pletely fills in the space between the cochlear duct and the bony canal. Ab- 

 sorption of this tissue takes place, resulting in formation of the scala tympani 

 and scala vestibuli. 



During the differentiation of the above parts a constriction appears in the 

 body of the primitive otic vesicle. This results in the incomplete septum 

 which divides the utricle from the saccule. 



The middle ear is formed from the upper segment of the pharyngeal 

 groove, the lower segment giving rise to the Eustachian tube. 



The external ear is developed from the ectoderm of the first branchial cleft 

 and adjacent branchial arches. The tympanic mem]:)rane is formed from the 

 mesoderm of the first branchial arch, its outer covering being of ectodermic, its 

 inner of entodermic origin. 



TECHNIC 



(i) For the study of the general structure of the pinna and walls of the exter- 

 nal auditory meatus, material may be fixed in formalin-Miiller's fluid (technic 6, 

 p. 7) and sections stained with haematoxylin-eosin (technic i, p. 20). In sections 

 of the wall ot the cartilaginous meatus the ceruminous glands may be studied, 

 material from children and from new^-born infants furnishing the best demon- 

 strations of these glands. 



(2) For the study of the inner ear the guinea-pig is most satisfactory on 

 account of the ease with which the parts may be removed. Remove the cochlea 

 of a guinea-pig with as much as possible of the vestibule and semicircular canals 

 and fix in Flemming's fluid (technic 8, p. 8). A small opening should be made 

 in the first turn of the cochlea in order to allow the fixative to enter the canal. 

 After forty-eight hours the cochlea is removed from the fixative and hardened in 

 graded alcohols (page g). The bone is next decalcified, either by one of the 

 methods mentioned on page 10 or in saturated alcoholic solution of picric acid. 

 If one of the aqueous decalcifying fluids is used, care must be taken to carry the 

 material through graded alcohols. Embed in celloidin or paraffin, cut sections 

 through the long axis of the modiolus, through the utricle and saccule, and 

 through the semicircular canals. Stain with haematoxylin-eosin and mount in 

 balsam. 



(3) The neurone relations of the cristte, maculcC, and cochlear duct can be 

 demonstrated only by means of the Golgi method. The ear of a new-born mouse 

 or guinea-pig furnishes good material. The cochlea together with some of the 

 base of the skull should be removed and treated by the Golgi rapid method (page 

 36). Sections should be thick and must of course be cut through undecalcified 

 bone. Good results are difiicult to obtain. 



The Organ of Smell 



The olfactory organ consists of the olfactory portion of the nasal 

 mucosa. In this connection it is, however, convenient to describe 

 briefly the olfactory bulb and the olfactory tract. 



The Olfactory Mucosa. — This has been described (page 305). The 



