COMPARATIVE ANATOMY, 



A deviation from the natural struc- 

 ture, which is completely unparalleled, 

 occurs in the stomach of the cuckoo. 

 The gizzard of the bird is covered, 

 internally, with an abundance of short, 

 bristle, and spiral hairs, lying close toge- 

 ther. 



The structure of the stomach differs 

 most widely in the different orders and 

 genera of this class. It appears merely 

 as a thin membranous bag, in several of 

 those which feed on flesh and insects, 

 when compared with the thick mus- 

 cular globes of the granivorous genera. 

 But there are in both many interme- 

 diate links between these extremes, and 

 at the same time considerable analogies 

 in the structures, which are apparent- 

 ly the mosi opposite. This is particular- 

 ly observeable in the course of the muscu- 

 lar fibres, and in the callous structure arid 

 appearance of the internal coat ; in which 

 point*, many of what are called membra- 

 nous stomachs have a great resemblance 

 to those of the gallinx. 



Both parts, but particularly the mus- 

 cular, are very strong in the gizzard 

 (ventriculus bulbosus) of gi\*n;vorous 

 birds. We find here, instead of a mus- 

 cular coat, four immensely thick and 

 powerful muscles, viz. a large hemi- 

 spherical pair at the sides (laterales,) and 

 two smaller ones (intermedii) at tht two 

 ends of the cavity. All the four are dis- 

 tinguished, by the unparalleled firm- 

 ness of their texture, and by the pecu- 

 liar colour, from all the other muscles of 

 the body. 



The internal callous coat must be con- 

 sidered as a true epidermis ; since, like 

 that part, it becomes gradually thicker 

 from pressure and rubbing. It forms 

 folds and depressions towards the cavity 

 of the stomach ; and these irregularities 

 are adapted to each other on the oppos- 

 ed surface. The cavity of this curious 

 stomach is comparatively small ; its 

 lower orifice is placed very near the 

 upper. Every part of the organ is, 

 indeed, calculated for producing very 

 powerful trituration ; and this is still 

 further promoted by the well-known in- 

 stinctive practice of granivorous birds, of 

 swallowing small hard stones with their 

 food. 



The end and use of swallowing these 

 stones have been very differently ex- 

 plained. Caesalpinus considered it rather 

 as a medicine than as a common assist- 

 ance to digestion ; Boerhaave, as an ab- 

 sorbent for the acid of the stomach ; 

 Redi, as a substitute for teeth ; accord- 

 ing to Whytt, it is a mechanical irritation, 



adapted to the callous and insensible na- 

 ture of the coats of the stomach ; Spallan- 

 zani rejected all supposition of design or 

 object, and thought that the stones were 

 swallowed from mere stupidity. There 

 seems not much sagacity to be discover- 

 ed in this opinion, when we consider 

 that these stones are so essential to the 

 due digestion of the corn, that birds 

 grow lean without them, although they 

 may be most copiously supplied with 

 food. This paradoxical opinion has, 

 however, been already refuted by J. 

 Hunter and G. Fordyce. Blumenbach 

 thinks that the stones kill the grain, 

 and deprive jt of its vitality, which 

 otherwise resists the action of the diges- 

 tive powers. Thus it lias been found that 

 if the oats and barley given to horses be 

 previously heated, the animal only re- 

 quirv:> half the quantity, and thrives 

 e quaiiy well. 



Kept iies and birds have their nostrils 

 terminated by two longitudinal slits on 

 the palate ; they have no velum palati, 

 nor epiglottis. 



The oesophagus of the serpent kind is 

 of immense magnitude ; for these reptiles 

 swallow animals larger than themselves, 

 which are retained for a considerable 

 time in the tube, and descend into the 

 stomach by degrees, where they are 

 slowly subjected to the action of the gas- 

 tric juice. The whole process sometimes 

 occupies many days, or even weeks. 

 There is hardly any distinction between 

 oesophagus and stomach. 



From the peculiar formation of the 

 nose of fishes, and from their respir- 

 ing by means of gills, their fauces have 

 no connection with any nasal cavity, or 

 glottis. 



The oesophagus is of great width in 

 fishes ; and is distinguished with difficul- 

 ty in many cases from the stomach. 

 These animals swallow their food whole, 

 without subjecting it to any mastication ; 

 and if the stomach will not hold the 

 whole, a part remains in the oesophagus, 

 until that which has descended lower is 

 digested. The alimentary canal is gene- 

 rally very short ; sometimes extending 

 straight from the mouth to the anus with 

 very little dilatation, as in the lamprey 

 (petromyzon marinus.) 



The Crustacea, and some insects, are 

 furnished with organs of mastication of 

 similar structure. Their mouth is form- 

 ed of two or more pairs of jaws placed 

 laterally. These move from without in- 

 wards, and vice versa ; whereas those of 

 red-flooded animals move from above 

 downwards, and back again. The parts 



