1 54 ZOOLOGY. 



and v. There is also a solar plexus in the mesentery, 

 connected by splanchnic nerves with the main chain. 



16. The Sense-organs of the frog resemble those of the 

 rabbit in all essentials, but differ in details. The olfactory 

 organs are a simple pair of sacs, opening to the exterior by 

 external nares, and to the mouth by internal nares. We 

 can thus see that the nasal passage of the rabbit includes 

 two things the pair of olfactory sacs plus a portion of the 

 mouth -cavity cut off by the formation of a palate. 



The eye has almost exactly the structure of the rabbit's, 

 but the lens is nearly spherical ; it has the same six muscles, 

 but its lubricating glands are less developed. 



In the internal ear the utriculus and sacculus com- 

 municate by a wide aperture, and the endolymph-duct is 

 unsplit. The endolymph contains a mass of calcareous 

 matter, known as the otolith. In the place of the four 

 auditory ossicles of mammalia there is only one, consisting 

 partly of cartilage, partly of cartilage-bone. This is 

 certainly homologous with stapes plus orbiculare : whether 

 it also represents the other two is very doubtful, but we 

 shall return to this in the next chapter, on the skull. 

 This one ossicle is called the columella auris, and of course 

 extends from the tympanic membrane to the cartilage that 

 encloses the internal ear. As already mentioned (1) the 

 tympanic membrane is flush with the general skin, and 

 there is no outer ear. 



17. Spleen, Thymus, Thyroid and Adrenals, The 

 spleen is a spherical red body attached to the mesentery 

 between stomach and rectum. 



The thymus is a paired yellow body, and lies just beneath 

 the skin, behind the angle of the jaw. This is a very 

 different position from that which it occupies in the rabbit, 

 but the identity of the organs is provfed by their develop- 

 ment, during which they shift in both cases from their 

 original position. 



The thyroid is also paired, and lies just ventral to the 

 external jugular veins, on either side of the larynx. 



The adrenals are not as clearly defined as in the rabbit, 



