AMPHIBIA. 101 



ments of the spinal cord are observed to take sule of the crystalline, may be a modification 



place, and the contraction of the coccygeal ver- of this structure. The iris is covered on its 



tebrae into a single linear bone, is accompanied posterior surface with pigmentum nigrum ; the 



by a corresponding diminution in the length of anterior having a shining metallic lustre, pre- 



that part of the spina! marrow, which at length cisely similar to that which we see in fishes, 



only extends, in the form of a small filament, The contractility of the pupil asserted by 



into the anterior third of that bone. Carus is denied by Altena and others. The 



The inferior condition of the brain which has retina is thick, and covers the whole internal 

 been described as existing in the tadpole of the surface to the capsule of the crystalline. The 

 higher species, is permanent in the proteus and vitreous humour is, in proportion, abundant, 

 other perennibranchiate genera ; so that the and the lens is large and of a spheroidal shape, 

 brain of the animal just named bears a very consisting of numerous concentric laminae, en- 

 obvious resemblance to that of the larva of the closing a nucleus of extreme density, exhibiting 

 aquatic salamander or triton. a close relation to the state of this part in 



VIII. The organ of vision. The eye differs fishes. There are in the frog three palpebrae ; 

 considerably in its form and magnitude in dif- or perhaps, with greater strictness of analogy, 

 ferent genera of the amphibia, and without any it might be said that there are two palpebrae, 

 very apparent relation to either their habits or and a sort of expansion of the inferior, serving 

 their circumstances. In the frogs and some as a membrana nictitans. The superior pal- 

 others they are remarkably large and prominent ; pebra is small, and is not possessed of any 

 in the salamanders they are comparatively small, degree of mobility; the inferior is broad, ex- 

 though from their at least equally aquatic panded, and semitransparent. It has an in- 

 habits, this difference might perhaps have ternal membranous expansion, which has just 

 scarcely been anticipated, and in the ccecilia, as been alluded to, and which is capable of cover- 

 the name imports, the eyes are scarcely if at all ing the whole eyeball. 



visible. In the latter animal the same object IX. The organ of hearing. The function 



has doubtless been intended by this absence of of hearing exists in very different degrees in 



vision, as in the mole and many other ani- the different groups of amphibia. The aquatic 



mals, whose common subterranean mode of habits to which the lower forms are confined 



life would render the possession of acute by their branchial respiration, would render an 



sight not only generally useless, but an extreme acute perception of sonorous impulses as unne- 



inconvenience on their occasional appearance cessary as it would be incompatible with the 



above the surface. dense medium in which they live ; and we find 



In some points of their structure the eyes of in this sense, as in every other function of the 

 the amphibia are not remotely related to those body, the most perfect concord existing be- 

 of the fishes ; as, for instance, in the flattened tween the habits of the animal and its structural 

 anterior surface of most of them, arising from arrangements. The pisciform aquatic genera 

 the small supply of the aqueous humour, and of this class, therefore, are found to possess as 

 in the depth of the crystalline. In some of near an affinity to the fishes in the structure of 

 the lower forms, there can scarcely be said to the organ in question as in most others ; and in 

 be a true orbit, the eyes being fixed as it were this they are also imitated by the tadpole state 

 in the integuments, and surrounded by a mass of the higher reptiliform groups, the adult 

 of minute veins, intermixed with extremely condition of which exhibits a much more ad- 

 small branches of nerves. Rusconi states that in vanced development of the acoustic organ. In 

 the proteus he was not able to discover muscles, the proteus and the allied genera, there is 

 nor even the optic nerve ; though on carefully neither a tympanic cavity, nor membrana tym- 

 and gently raising the hemispheres of the brain pani ; it consists of a large cavity hollowed as 

 a minute nervous filament was seen going to- it were out of the temporal bone, at the bottom 

 wards the foramen which serves for the passage of which cavity is the sacculus with its creta- 

 of the ophthalmic artery ; but whether this ceous body ; the fenestra ovalis is closed by a 

 was the optic nerve or not, appears a matter of bony lamina, the representative of the stapes, 

 entire doubt. In fact, the structure of the eye Behind the sacculus are the membranous semi- 

 in this animal, on the whole, is very imperfect. circular canals. The whole organ is covered 

 In the frog, on the contrary, the eye is fully externally by the integuments, without any pos- 

 developed, and all the essential parts of its sible communication with the atmosphere, 

 structure sufficiently conspicuous. The globe In the frog, on the other hand, the whole 

 of the eye is large and projecting ; the scle- structure is more complicated. The sacculus, 

 rotic is considerably solid and tough, and semi- which is membranaceous, is filled with the 

 transparent ; the cornea is large, and though cretaceous matter, which is here semifluid, 

 somewhat flattened, is much less so than in having the appearance of cream. The semi- 

 fishes, or in the lower forms of the elass. The circular canals are contained within the sub- 

 inner surface of the choroid is extremely black, stance of the temporal bone. The ossicula 

 and the external of a silvery whiteness. The auditus are three, united, and contained within 

 ciliary processes have not with certainty been the tympanum, which they traverse, and are 

 discovered in these animals, unless, as Altena attached to the membrana tympani, a broad 

 suggests, a little tubercular mass, occupying round membrane, perfectly superficial, and very 

 nearly their situation, and closely connected distinct from the surrounding integument. The 

 with the edge of the choroid and with the crip- cavity of the tympanum is not capacious. It 



