NERVOUS SYSTEM 



867 



higher forms add the middle ear or tympanum, while the highest forms 

 add the outer ear. 



The Inner Ear. This consists of a mere area of thickened ectoderm 

 on each side of the head, between the seventh and ninth cranial nerves. 

 A review of the embryology of the ear will recall the cup-shaped auditory 

 vesicle. In the dogfish, the cavity of this remains connected with the 

 exterior by a slender tube known as the endolymph duct (Fig. 486). In 

 the frog and in higher forms, there is no open auditory cup. There are 

 two layers of ectoderm, the outer one forming an unbroken sheet across 

 the cup. In the dogfish, these endolymph ducts can be seen as external 

 portions on the top of the head. 



The distal end of this endolymph duct becomes enlarged, the enlarge- 

 ment being called the sacculus endolymphaticus. In the frog and other 

 amphibia, the ducts of both sides often unite dorsal to the brain, while 

 the other parts branch and extend into the spinal canal in a root-like 

 manner. In the frog definite diverticula are sent into the so-called cal- 

 careous glands, surrounding the basal portion of the spinal nerves. 



The auditory vesicle constricts into 

 two chambers, an upper vestibule or utric- 

 ulus and a lower sacculus, connected by 

 a narrow sacculo-utricular canal. Three 

 outgrowths now take place, one each from 

 the outer, posterior, and lateral walls of 

 the utriculus ; the one from the lateral 

 svall lies in a horizontal plane, the others 

 in vertical planes. These outgrowths form 

 tubes, and as they are circular in outline, 



they are called the semi-circular canals. 



Some of the sensory epithelium has 



spread into all of these regions, but a defi- 



nite patch of this sensory epithelium can variation of ear . boneg in Mammalia . 



be seen in each of the semi-circular canals, 



A * 



Fig. 487. 

 Four diagrams to illustrate the 



and it is around this patch that the W r all Of the inner side. The tympanic mem- 



f , , r brane is cross hatched and cartilage 



the Canal expands tO form an ampulla, bones are covered with small circles, 



--. A& -11 1 1. j- j i whilst membrance bones are left un- 



Figure 486 will have to be studied and shaded. 



the model of the ear seen or an ear defi- phou 



dition in later Theromorphous Reptile. 

 The dentary has met the squamosal, 



nitely worked out in one of the animals * 



to make this Clear. 



In forms higher than fish and am- an . d the . Q u . adrat e and articular are 



reduced in size. C. Condition in hypo- 



phibia, there is a little pOCket Called the theti cal form, the link between Thero- 



. . morpha and Mammalia. The supra- 



lagena, given Oil from the posterior Side angular has begun to extend along the 



f . , f*> . , ... border of the tympanic membrane. 



Of the SaCCUluS. Sensory epithelium ex- D. Condition in primitive Mammalia 



tends into this pocket, and in the higher tS**1S^^ 



forms the lagena becomes a peculiar struc- itSSfr Jfc 

 ture called the cochlear duct. 



<Fr m Shipley and 



