THE FROG 99 



is known as the utriculus, and the lower smaller one is termed the 

 sacculus. Connected with the utriculus are three hoop-shaped mem- 

 branous tubes, the semicircular canals. The anterior of these lies 

 in a plane practically parallel with the sagittal plane of the head, 

 and its lower front end dilates just as it is joining the utriculus to 

 form an ampulla. The posterior semicircular canal lies in a trans- 

 verse plane almost at right angles to the former and, like it, is dilated 

 into an ampulla at its lower end. The upper extremities of both 

 these vertical canals join together to form a common tube opening 

 into the utriculus. The third or 

 horizontal semicircular canal is in a 

 plane roughly at right angles to the . 

 planes of the other two, and has an 

 ampulla at its anterior end. From 

 the sacculus arises a small sac-like 

 outgrowth, the lagena, . which in 

 higher vertebrates is transformed into 

 a very complex spiral structure, the 



cochlea, and even in the frog indica- FlG 34- The right internal ear 

 . . , , , , . r , , . . i . of the frog, removed from the 



tions of the beginning of this speciah- periotic c * rt ii age and drawn 

 sation are to be seen. The utriculus from the outer surface. 

 also gives rise to a small tube, the From Marshall and Gamble. 



dUCtUS endolymphatiCUS, Which enters ., the anterior vertical semicircular 



, . , n t . canal; 0., its ampulla; A., the horizontal 



the cranial cavity and there unites canal ; ., its ampulla ; p., the posterior 



with its fellow to form a sac, the 

 saccus endolymphaticus, filled by a 



whitish fluid which owes its colour to the presence of very small 

 crystals of Calcium carbonate. A median prolongation from the 

 saccus runs back along the dorsal side of the spinal cord, giving off 

 paired outgrowths which pass out through the intervertebral fora- 

 mina and form the so-called calcareous bodies or periganglionic 

 glands, visible as white masses on each side of the vertebral 

 column. 



The whole membranous labyrinth is filled with a fluid, the endo- 

 lymph, and is closely invested by cartilage and bone ; the small 

 space left between the labyrinth and the surrounding tissues being 

 filled with another fluid, the perilymph. The endolymph contains 

 a number of calcareous granules, the otoliths, which play a consider- 

 able part in the functioning of the ear. 



The labyrinth is lined by an ectodermal epithelium whose cells 

 are differentiated to form sensory patches here and there. The 

 patches contain pear-shaped sensory cells, each bearing a fine hair- 

 like process with long columnar supporting cells running between 

 them. They may be arranged in small ridges or cristae acusticee, or 



