SENSE OR RECEPTOR ORGANS 855 



The endolymphatic duct appears to lack specialized sensory areas. In the 

 elasmobranch fishes, the endolymphatic ducts open by means of small pores 

 at the top of the head. The endolymphatic-sac area of the duct may be absent 

 in some fishes, but in many teleosts, reptiles, and amphibia the endolymphatic 

 sac is greatly enlarged. In the frog group, the endolymphatic sac is most ex- 

 tensive, protruding itself into the brain and spinal cord areas. 



Sensory patches of epithelium are present in the lagena. In reptiles, birds 

 and mammals, the lagena is extended considerably. In birds and mammals 

 the lagena is called the cochlear duct, and it contains an extensive area of 

 sensory epithelium known as the organ of Corti. 



In tetrapod vertebrates a middle ear containing a specialized ossicle or 

 ossicles, is added to the hearing mechanism (fig. 3611, J, K and L), and in 

 reptiles, birds and mammals an external meatus or specialized structure for 

 receiving sound waves is found. The external auditory meatus in mammals is 

 supplemented by the addition of an external ear or pinna, a funnel-shaped 

 structure for collecting sound waves (fig. 3611). 



b. Development of the Internal Ear 



The internal ear arises from the otic placode which sinks inward to form 

 the otic vesicle. The otic vesicle gradually transforms into the shape and 

 structure of the internal ear peculiar to the species. The transformation of 

 the otic vesicle in the human embryo is shown in figure 361C-H. 



c. Development of the Middle Ear 



The development of the middle ear results from an evagination of the 

 pharyngeal wall which primarily involves the region of the first branchial 

 pouch. This evagination unites distally with spaces forming around the ossicles. 

 The opening into the pharynx is retained, and the narrow passageway between 

 the pharynx and the middle ear cavity containing the ossicle or ossicles of 

 the ear is called the eustachian duct or tube. 



d. Development of the External Auditory Meatus and Pinna 



The external auditory meatus forms from an epidermal invagination in the 

 area of the first visceral groove, that is the region between the mandibular and 

 hyoid visceral arches. The pinna of the external ear in mammals arises from 

 swellings on the mandibular and hyoid arches. These swellings enlarge and 

 fuse to form the complicated form of the pinna (figs. 328 and 329). 



F. Nerve fiber-effector organ relationships 



(Consult figure 358F-G.) 



