ANATOMY OF THE TORTOISE. 



247 



hard palate, and joins the palatine bone on each side, to form a very solid piece, behind which is 

 the opening of the inner nostril. There is a post-frontal bone, which, with the squamosal of the 

 ear-bone, occupies the upper part of the temporal region, and the last-named bone is placed at the 

 side of the ear-capsule and above the quadrate bone. The pterygoid bones of the base of the skull 

 and hard palate are behind the nostril, and are united together, and with the quadrate bones also, 

 There is no transverse bone. 



All these bones, most of which, but not all, are comparable with those of the Birds, are united 

 together immovably, and the solidity of the skull is great. In the Turtles, and in some of the 

 Terrapins, there is a false roof to the skull, produced by a flattening of the parietal ridge and its 

 union in front with the post-frontal bones and behind with the squamosal. It is this which gives 

 the large and solid appearance to the head. The skulls are nearly all face and outside, and the 

 brain-cavity is very small. 



On examining the mouth of the Tortoise a sharp edge of horn is felt; and it is noticed that 

 the mandible is very solid, and that it unites with the head-bones far back, and after the fashion 

 of the Bird rather than of the Mammal. There are traces of all the different bones which make up 

 the lower jaw, on each side, in the Birds, to be found in the Chelonia, but they all become solidly 

 united at an early age. In the Tortoises there is union 

 at the chin, and the dentary bone resembles that of the 

 Bird, but the Matamata of North Brazil is said to have 

 the front bones of the lower jaw separate. 



The jaw-joint, which moves on the skull on each 

 side, is fitted into a wide and large quadrate bone, which 

 is fixed to the skull, the mammalian method of articula- 

 tion not being seen, but that of the Bird being closely 

 followed. Teeth are absent in the group, but in some 

 kinds the horny substance of the jaws is irregular and 

 saw-like, and the front may be produced into a kind 

 of beak. 



Sight is evidently keen in most Chelonians, and 

 the eye is a very elaborate organ, provided with eye- SKULL OF TORTOISE. 



lids, a nictitating membrane, and well-developed tear *"" %t^l"a?f4c,^^^ 

 glands. There is much perfection of the movement of the 



eye-ball, and there are six powerful muscles, and four smaller ones, which embrace the optic nerve and 

 are expanded over the ball behind. There is the same considerable convexity of the cornea, and 

 the smaller convexity of the rest of the front of the eye in the Chelonians which was noticed in the 

 Birds, and singularly enough the plates forming a bony ring of separate pieces seen in some of 

 these last are present generally in some of the Chelonians. The iris is circular in the Order. 



The internal ear has the same important parts as in the Birds, and there are a tympanic 

 cavity, a rudimentary cochlea, and three semicircular canals. The Eustachian tube of the higher 

 animals is represented, and particles of a calcareous nature, called otoliths, are found in the sac- 

 like cochlea. The vibrations of the tympanic membrane, which is visible at the side of the skull, 

 are transmitted to the internal ear by one little ossicle instead of by three, as in the Mammalia. 

 It is said that some of the gigantic Tortoises are deaf. 



The sense of taste must be reduced to the utmost in the Chelonia, and the tongue is covered 

 with a thick rugged membrane, smooth in the Turtles, but beset with pointed papilla? in the Great 

 Tortoises. A number of glands exist beneath the membrane, and supply the mouth with a moisten- 

 ing secretion, and the tongue is supported by a hyoid bone, which is cartilaginous, and sends a little 

 bone into the centre of its substance the lingual bone. 



The hyoid bone is of great importance in the Chelonians, for it not only has to do with the 

 tongue, but it also assists materially in the mechanism of breathing. Mammals and Birds breathe 

 by having air drawn into their lungs by means of the expansion of their ribs and the movement 

 of their chest by muscles, and by expelling it through the reverse movements, and by the elasticity 

 of the lung itself. Now, Chelonians have no movable ribs, and the muscles are either wanting or are 



