COMPARATIVE ANATOMY. 



uected with the rest of the skeleton, than 

 the animals of the preceding classes. 



The cranium in several cartilaginous 

 fishes, (in the skate for instance) has a 

 very simple structure, consisting chiefly 

 of one large piece. In the bony fishes, 

 on the contrary, its component parts are 

 very numerous ; amounting to eighty in 

 the head of the perch. Most of the 

 latter have a more or less moveable un- 

 der jaw. 



Great variety. in the structure of the 

 teeth is observed in this class. Some ge- 

 nera, as the sturgeon, are toothless. Their 

 jaws, which ape distinct from the crani- 

 um, form a moveable part, capable of 

 being thrust for wards from the mouth, and 

 again retracted. 



Those fishes which possess teeth dif- 

 fer very much in the form, number, and 

 position of these organs. Some species of 

 sparus (as the S. probato-cephalus) have 

 front teeth almost like those of man ; they 

 are provided with fangs, which are con- 

 tained in alveoli. In many genera of fishes, 

 the teeth are formed by processes of the 

 jaw-bones covered with a crust of enamel. 

 In most of the sharks, the mouth is fur- 

 nished with very numerous teeth, for the 

 supply of such as may be lost. The white 

 shark has more than two hundred, lying 

 on each other in rows, almost like the 

 leaves of an artichoke. Those only which 

 form the front row have a perpendicular 

 direction, and are completely uncovered. 

 Those of the subsequent rows are, on the 

 contrary, smaller, have their points 

 turned backwards, and are covered with 

 a kind of gum. These come through the 

 covering substance, and pass forward 

 when any teeth of the front row are lost. It 

 will be understood from this description 

 that the teeth in question cannot have any 

 fangs. 



The saw-fish only (squalus pristis) has 

 teeth implanted in the bone on both sides 

 of the sword-shapen organ, with which its 

 head is armed. 



In some fishes the palate, in others the 

 bone of the tongue (as in the frog fish,) in 

 others (as in several of the ray-kind,) the 

 aperture of the mouth forms a continuous 

 surface of tooth. 



MOUTH, OESOPHAGUS, AND STOMACH. 



We have already shown the most im- 

 portant circumstances relating to the 

 mouth. Many species of the genus si- 

 mia, as well as the hamster, (marmota 

 cricetus) and some similar species of the 



marmot, are provided with cheek poucii- 

 es, in which the former, who live on trees, 

 place small quantities of food as a re- 

 serve : the hitter employ these bags to 

 convey their winter provision to their 

 burrows. 



The peculiar glandular and moveable 

 bag 1 , (bursa faucium,) which is placed 

 behind the palate, has hitherto been only 

 observed in the camels of the old world - 

 and it probably serves to lubricate the 

 throats of these animals, in their abode 

 in the dry sandy deserts which they inha- 

 bit. 



The oesophagus of quadrupeds is, dis- 

 tinguished from that of the human sub- 

 ject by possessing two rows of muscu- 

 lar fibres, which pursue a spiral course, 

 and decussate each other. In those car- 

 nivorous animals which swallow vora- 

 ciously, as the wolf, it is very large ; on 

 the contrary, in many of the larger her- 

 bivora, and particularly in such as ru- 

 minate, its coats are proportionably 

 stronger. 



No mammalia possess an uvula, except 

 man and the simia. 



In some herbivora the stomach has an 

 uniform appearance externally ; but it is 

 divided into two portions internally, ei- 

 ther by remarkable difference in the 

 two halves of its internal coat, as in the 

 horse, or by a valvular elongation of this 

 membrane, as in several animals of the 

 mouse-kind. This is also the case in the 

 hare and rabbit, where also the food in 

 the two halves of the stomach differs 

 very much in appearance, particularly if 

 the animal has been fed about two hours 

 before death. 



In these animals the left half of the sto- 

 mach is covered with cuticle, while the 

 other portion has the usual villous and se- 

 creting surface. The left portion of the 

 cavity may be regarded as a reservoir, 

 from which the food is transmitted to the 

 true digestive organ ; and the different 

 states in which the food is found in the 

 two parts of the cavity justify this sup- 

 position. Hence these stomachs form a 

 connecting link between those of rumi- 

 nating animals on one side, and those 

 which have the whole surface villows on 

 the other. 



On the whole internal surface of the 

 horse's stomach there are found, in vast 

 abundance, particularly in spring, the 

 larva: of two species of oestrus ; viz. the 

 oestrus equi (which Linnaeus called 

 oestrus bovis,) and the oe. haemorrhoidalis, 

 the true history of which has been eluci- 



