AMPHIBIA. 



101 



ments of the spinal cord are observed to take 

 place, and the contraction of the coccygeal ver- 

 tebrae into a single linear bone, is accompanied 

 by a corresponding diminution in the length of 

 that part of the spinal marrow, which at length 

 only extends, in the form of a small filament, 

 into the anterior third of that bone. 



The inferior condition of the brain which has 

 been described as existing in the tadpole of the 

 higher species, is permanent in the proteus and 

 other perennibranchiate genera ; so that the 

 brain of the animal just named bears a very 

 obvious resemblance to that of the larva of the 

 aquatic salamander or triton. 



VIII. The organ of vision. The eye differs 

 considerably in its form and magnitude in dif- 

 ferent genera of the amphibia, and without any 

 very apparent relation to either their habits or 

 their circumstances. In the frogs and some 

 others they are remarkably large and prominent ; 

 in the salamanders they are comparatively small, 

 though from their at least equally aquatic 

 habits, this difference might perhaps have 

 scarcely been anticipated, and in the coecilia, as 

 the name imports, the eyes are scarcely if at all 

 visible. In the latter animal the same object 

 has doubtless been intended by this absence of 

 vision, as in the mole and many other ani- 

 mals, whose common subterranean mode of 

 life would render the possession of acute 

 sight not only generally useless, but an extreme 

 inconvenience on their occasional appearance 

 above the surface. 



In some points of their structure the eyes of 

 the amphibia are not remotely related to tho.se 

 of the fishes ; as, for instance, in the flattened 

 anterior surface of most of them, arising from 

 the small supply of the aqueous humour, and 

 in the depth of the crystalline. In some of 

 the lower forms, there can scarcely be said to 

 be a true orbit, the eyes being fixed as it were 

 in the integuments, and surrounded by a mass 

 of minute veins, intermixed with extremely 

 small branches of nerves. Rusconi states that in 

 the proteus he was not able to discover muscles, 

 nor even the optic nerve ; though on carefully 

 and gently raising the hemispheres of the brain 

 a minute nervous filament was seen going to- 

 wards the foramen which serves for the passage 

 of the ophthalmic artery ; but whether this 

 was the optic nerve or not, appears a matter of 

 entire doubt. In fact, the structure of the eye 

 in this animal, on the whole, is very imperfect. 



In the frog, on the contrary, the eye is fully 

 developed, and all the essential parts of its 

 structure sufficiently conspicuous. The globe 

 of the eye is large and projecting ; the scle- 

 rotic is considerably solid and tough, and semi- 

 transparent ; the cornea is large, and though 

 somewhat flattened, is much less so than in 

 fishes, or in the lower forms of the elass. The 

 inner surface of the choroid is extremely black, 

 and the external of a silvery whiteness. The 

 ciliary processes have not with certainty been 

 discovered in these animals, unless, as Altena 

 suggests, a little tubercular mass, occupying 

 nearly their situation, and closely connected 

 with the edge of the choroid and with the cap- 



sule of the crystalline, may be a modification 

 of this structure. The iris is covered on its 

 posterior surface with pigmentum nigrum ; the 

 anterior having a shining metallic lustre, pre- 

 cisely similar to that which we see in fishes. 

 The contractility of the pupil asserted by 

 Carus is denied by Altena and others. The 

 retina is thick, and covers the whole internal 

 surface to the capsule of the crystalline. The 

 vitreous humour is, in proportion, abundant, 

 and the lens is large and of a spheroidal shape, 

 consisting of numerous concentric laminae, en- 

 closing a nucleus of extreme density, exhibiting 

 a close relation to the state of this part in 

 fishes. There are in the frog three palpebrse ; 

 or perhaps, with greater strictness of analogy, 

 it might be said that there are two palpebrae, 

 and a sort of expansion of the inferior, serving 

 as a membrana nictitans. The superior pal- 

 pebra is small, and is not possessed of any 

 degree of mobility; the inferior is broad, ex- 

 panded, and semitransparent. It has an in- 

 ternal membranous expansion, which has just 

 been alluded to, and which is capable of cover- 

 ing the whole eyeball. 



IX. The organ of hearing. The function 

 of hearing exists in very different degrees in 

 the different groups of amphibia. The aquatic 

 habits to which the lower forms are confined 

 by their branchial respiration, would render an 

 acute perception of sonorous impulses as unne- 

 cessary as it would be incompatible with the 

 dense medium in which they live ; and we find 

 in this sense, as in every other function of the 

 body, the most perfect concord existing be- 

 tween the habits of the animal and its structural 

 arrangements. The pisciform aquatic genera 

 of this class, therefore, are found to possess as 

 near an affinity to the fishes in the structure of 

 the organ in question as in most others ; and in 

 this they are also imitated by the tadpole state 

 of the higher reptiliform groups, the adult 

 condition of which exhibits a much more ad- 

 vanced development of the acoustic organ. In 

 the proteus and the allied genera, there is 

 neither a tympanic cavity, nor membrana tym- 

 pani ; it consists of a large cavity hollowed as 

 it were out of the temporal bone, at the bottom 

 of which cavity is the sacculus with its creta- 

 ceous body ; the fenestra oval is is closed by a 

 bony lamina, the representative of the stapes. 

 Behind the sacculus are the membranous semi- 

 circular canals. The whole organ is covered 

 externally by the integuments, without any pos- 

 sible communication with the atmosphere. 



In the frog, on the other hand, the whole 

 structure is more complicated. The sacculus, 

 which is membranaceous, is filled with the 

 cretaceous matter, which is here semifluid, 

 having the appearance of cream. The semi- 

 circular canals are contained within the sub- 

 stance of the temporal bone. The ossicula 

 auditus are three, united, and contained within 

 the tympanum, which they traverse, and are 

 attached to the membrana tympani, a broad 

 round membrane, perfectly superficial, and very 

 distinct from the surrounding integument. The 

 cavity of the tympanum is not capacious. It 



