AVES. 



321 



The differences in the structure of the gizzard 

 resolve themselves into the greater or less extent 

 of the tendons, and the greater or less thick- 

 ness of the muscular coat, and of the lining 

 membrane. 



In the Raptores the gizzard (d, jig. 156) 

 assumes the form of a mere membranous cavity, 

 in accordance with the animal and easily di- 

 gestible nature of their food. The muscular 

 coat is extremely thin; the fibres principally 

 radiate from small tendons (e, fg. 156), and 

 there are some longitudinal fibres beneath the 

 radiating or external layer. 



In the Rasores and lamellirostral Natatores 

 it exhibits the structure to which the term giz- 

 zard can be more appropriately applied. The 

 muscular fibres are distinguished by their 

 unparalleled density of texture and deep 

 colour, and are arranged in four masses ; two 

 are of a hemispherical form, and their closely- 

 packed fibres run transversely to be connected 

 to very strong anterior and posterior tendons 

 (e, Jig. 157, 162); they constitute the sides 

 of the gizzard, and are termed the digastric 

 muscles or ' musculi laterales' (d, Jig. 161, 

 162) : between these, at the end of the gizzard, 

 are the two smaller and thinner muscles called 

 ' musculi intermedii' (f,Jig. 162). There are 

 likewise irregular bands placed about the cir- 

 cumference of the gizzard. 



Fig. 161 shows the relative thickness of the 

 musculi laterales in the gizzard of a Swan, and 

 Jig. 162 that of the musculi intermedii and 

 tendon. 



Fig. 162. 



Gizxard of a Swan. 



The internal coat of the gizzard (c, h, Jig. 162) 

 is extremely hard and thick, and being of a 

 horny or cuticular nature, it is liable to be 

 increased by pressure and friction, and as it is 

 most subject to these influences at the parts of 

 the gizzard opposite the musculi laterales, two 

 callous buttons are there formed, (g, g, fig. 162). 

 It is here that the fibrous structure of the lining 

 membrane can be most plainly seen : and it 

 is worthy of observation that the fibres are not 

 perpendicular to the plane of the muscles but 



VOL. i. 



oblique, and in opposite directions, on the two 

 sides. Elsewhere the cuticular lining is dis- 

 posed in ridges and prominences (//, jig. 161, 

 162), which vary in different birds, but are 

 pretly constant in the same species. Carus* 

 has recently figured the gizzard of a Petrel 

 ( Procellaria glaciulis), the lining membrane 

 of which is disposed in a pavement of small 

 square tubercles, like the gastric teeth of some 

 Mollusca. 



The cavity of the gizzard is so encroached 

 upon by the grinding apparatus, that it is 

 necessarily very small, the two horny callosities 

 having their internal flat surfaces opposed to 

 one another, like ' millstones.' A crop is as 

 essential an appendage to this structure as the 

 ' hopper' to the mill ; it receives the food as 

 it is swallowed, and supplies it the gizzard in 

 small successive quantities as it is wan ted .f 



Between the stomach of the carnivorous 

 Eagle, and that of the graminivorous Swan, 

 there are numerous intermediate structures, but 

 it is necessary to observe that the animal or 

 vegetable nature of the food cannotalwaysbe pre- 

 dicated of from the different degrees of strength 

 in the gizzard. Hard-coated coleopterous in- 

 sects, for example, require thicker parietes for 

 their due comminution than pulpy succulent 

 fruits. 



In the subgenus Euphones, among the Tana- 

 gers, the muscular or pyloric division of the 

 stomach is remarkably small and not sepa- 

 rated from the duodenum by a narrow pylorus. J 



The parietes of the gizzard, like those of other 

 muscular cavities, become thickened when 

 stimulated to contract on their contents with 

 greater force than usual. In the Hunterian 

 collection this fact is well illustrated by pre- 

 parations of the gizzard of the Sea-gull in the 

 natural state, and that of another Sea-gull which 

 had been brought to feed on barley. The 

 digastric muscles in the latter are more than 

 double the thickness of those in the Sea-gull 

 which had lived on fish. 



The immediate agents in triturating the food 

 are hard foreign bodies, as sand, gravel, or peb- 

 bles. The well-known habit in the granivorous 

 birds of swallowing stones with their food has 

 been very differently explained. Blumenbach 

 observes that ' Cresalpinus considered it rather 

 as a medicine than as a common assistance to 

 digestion ; Boerhaave, as an absorbent for the 

 acid of the stomach ; Redi, as a substitute for 

 teeth; Whytt, as a mechanical irritation, adapt- 

 ed to the callous and insensible nature of the 

 coats of the stomach.' Spallanzani rejected all 

 supposition of design or object, and hazarded 

 the stupid observation that the stones were 

 swallowed from mere stupidity. 



* Tabulae AnatomiamComparativam illustrantes, 

 fol. pars iv. 1835. 



f Thus we find in Parrots, where the gizzard is 

 remarkably small, that a crop is present. A like 

 receptacle exists also in the Flamingo, in which the 

 gizzard is small but strong. 



J Cavus ut supra, tab. vi. fig. \f. 



See Home, Comp. Anatomy, vol. i. p. 271, and 

 Hunter, Animal (Economy, p. 221, \vhere it is re- 

 lated that a similar change was effect id by changing 

 the food of a tame Kite. 



