344 NATURAL HISTOltY. 



the brain and spinal cord of the Frog be taken as the type of the higher group, it may be said that 

 the first is small. The front of the brain, or the region where the olfactory nerves arise, is a rounded 

 swelling, and behind it are feebly-developed and rounded-off cerebral hemispheres, which contain, 

 cavities or ventricles. Behind is a space, in which is the pineal gland and the region of the optic 

 thalami, and the optic lobes are distinct and in front of a band-like cerebellum. The medulla 

 oblongata has a ventricle in it. 



The spinal cord is long in some of the Amphibia, but it is not so in the Fi-og. The cranial 

 nerves are small as a rule, as there is little expression in the Amphibia and the 

 nerves of special sense are not highly developed. The eyes always exist, although 

 they may be hidden beneath the skin, and they are very small in some kinds which 

 inhabit subterranean waters or burrow in the earth. In those Amphibia in which 

 the gills, or branchiae, last through life or are perennial (Perennibranchiata), there 

 are no eyelids, but they are found in some of the others. Nictitating membranes exist 

 in some, and the Frogs and Toads have a special muscle which drags the eye farther 

 within the orbit. All have simple organs of hearing, and in most there is a 

 labyrinth and three semicircular canals, and there is a fenestra ovalis, and a 

 gristly or cartilaginous auditory ossicle the columella, or stapes, whose 

 expanded end is fixed to the membrane of the fenestra. The worm-like kinds, the 

 tailed Amphibia, and one group of the Batrachia (Pelobatidce), have no tympanic 

 BRAIN OF FROG. cavity or membrane ; but in other Batrachia they exist, and the outer end of the 

 ll owacti*ry y swelMn| : ; '(c! stapes is connected with the tympanic membrane, there being cavities opening into 

 <tj r op'uc ThSi 1 amu. r (w) the throat also. The sound vibrations of the air or water are therefore conducted 

 Gi p and:(d) 8 origin" of from the outer tympanic membrane, through the single stapes (not through three 



Optic Nerves : () Audi- > r ' \ 



b3Fnm"< T ?i><rttiSr.J bones, as in the higher animals) to the internal ear. 



Med r una Ven o r bio e n : gat'a; I n the adults of the Amphibia, the whole alimentary canal is of a very simple 



character. The oesophagus is wide and short; the stomach is single, and consists of a 

 simple sac, which is globular in the land kinds, and is longer in the aquatic. The intestine is but slightly 

 convoluted, and the large and small parts do not differ much in size. The liver, gall-bladder, pancreas, 

 and spleen exist in all the Amphibia. A urinary bladder exists which opens into the vent, and the kid- 

 neys resemble those of fish more than those of higher Vertebrata, being persistent " Wolffian bodies '' 

 rather than true kidneys. These "bodies" are found with the kidneys in the young unborn Mammalia, 

 but they are of no use after birth, and the kidneys act alone. They persist in the Amphibia 

 and Fishes. 



In the adult Frog, Toad, Salamander, and many others, the air is taken in by the lungs, not by 

 the expansion of the chest by ribs and the consequent inrush of air to fill the space, as in the higher 

 animals, but by a swallowing process resembling that of the Tortoise (p. 248). The inflation of the 

 internal bag of the lung is produced by the creature first of all depressing its tongue and the hyoid 

 bone, and thus enlarging the cavity of the mouth, so that air rushes into it through the nostrils, the 

 mouth being shut. Then muscular contraction is exercised on the cavity and on the hyoid bone, so 

 that the air is forced through the air-tubes into the lungs. The escape of air back again, by the nostrils, 

 is prevented by their edges acting as a valve, and also by the tongue being pressed against them when 

 it exists ; and its entry by mistake into the oesophagus and stomach is prevented by closure of the 

 gullet spasmodically. Hence the way to suffocate a Frog is to place something in its mouth which 

 will keep the jaws apart, so that the air escapes and does not go into the lungs. The primary 

 necessity for this Tortoise-like breathing is from tlio absence of perfect and sternal ribs. 



In the case of the Amphibia with branchiae, or gills, the water, in passing over them, carries air 

 with it, and the oxygen in it is devoted to purifying the blood. But although these two kinds of respi- 

 ration are explicable, there is a difficulty in explaining the use of the rudimentary lungs in those 

 Amphibia which never shed their branchiae, such as the Siren and the Proteus. The construction of 

 their nostrils resembles those of fish, and the lungs are thin and extremely delicate ; moreover, they 

 are contracted near the gullet. Indeed, it does not appear that when the Siren is dying from having 

 impure water acting on its branchiae, that the lungs assist in the least in respiration. Bell long since 

 considered these bag- like "lungs" to be similar organs to certain air-bags in fish, which will be 



