12 



DIGESTION. 



of these is a large membranous bag, analogous 

 to the paunch of the ruminants, into which the 

 food, without any previous alteration, is re- 

 ceived from the oesophagus, and where it is 

 macerated in the usual manner by the conjoined 

 action of heat and moisture. 



The gizzard is of much smaller dimensions 

 than the crop, composed of four muscles, two 

 of which are of a flattened form and of very 

 dense texture, lined internally with a firm cal- 

 lous membrane, and capable of an extremely 

 powerful action. These constitute the main 

 part of the parietes, the two other muscles being 

 much smaller, and situated at the extremities, 

 serving, as it would appear, merely to com- 

 plete the cavity.* The gizzard is so connected 

 with the crop, that the food, after due macera- 

 tion, is allowed to pass by small successive 

 portions between the two larger muscles ; by 

 their contraction they are moved laterally and 

 obliquely upon each other, so that whatever is 

 placed between them is completely triturated. 

 The force of these muscles, as well as the 

 impenetrability of their investing membrane, is 

 almost inconceivably great, so that, according 

 to the experiments of Spallanzani and others, 

 not only are the hardest kinds of seeds and 

 grains reduced to a perfect pulp, but even 

 pieces of glass, sharp metallic instruments, 

 and mineral substances, are broken down or 

 flattened, while the part still remains unin- 

 jured.f The action of both the crop and the 

 gizzard must be regarded as at least essentially 

 mechanical, mainly adapted for the purposes 

 of maceration and trituration, and as compen- 

 sating for the saliva and teeth of man and the 

 greatest part of the mammalia. We are able 

 in this case to observe the connexion between 

 the habits of the animals and the peculiarities 

 of their organs more clearly than with regard 

 to the ruminants, for we can always perceive 

 an intimate relation between the food of the 

 different kinds of birds and the structure of 

 their stomach. 



II. Anaccount of the nature of the substances 

 usually employed as food. All the articles that 

 are employed in diet may be arranged under 

 the two primary divisions of animal and vege- 

 table, according to the source whence they are 

 derived. Those in which the distinctive cha- 

 racters are the most strongly marked differ both 

 in their proximate principle's and their ultimate 

 elements, although in this, as in most other cases, 

 there are many intermediate shades. The ulti- 

 mate elements of vegetables are oxygen, hy- 

 drogen, and carbon, to which, in some cases, 

 a portion of nitrogen is added. Animal sub- 

 stances contain all these four ingredients, the 

 carbon being in less quantity than in vege- 



* Grew, ubi supra, p. 34 ; Blumenbach, ubi 

 supra, $ 99 ; Peyer, Anat. Ventr. Gall., in Man- 

 get, Bibl. Anat. t. i. p. 172; Hunter on the Ani- 

 mal (Economy, p. 198-9; Clift, in Phil. Trans, for 

 1807, pi. 5, fig. 1 ; Home's Lect. v. ii. pi. 49, 62 ; 

 and the art. AVES. 



t Spallanzani, Dissert, i. 5 . . 8, and 10. . 22 ; 

 see also Acad. del Cimento, p. 268,9 ; Borelli, De 

 motu anim. t. ii. prop. 189 ; Redi, Esperiense, p. 89 

 et seq. ; Grew, ch. 8 ; the art. " Birds" in Rees ; 

 and " Aves" by Mr. Owen, in the present work. 



tables, while the hydrogen, and still more the 

 nitrogen, are generally in much greater quan- 

 tity. There are various circumstances which 

 seem to prove that either species of diet is 

 alone competent to the support of life, although 

 each of them is more especially adapted to 

 certain classes of animals. This, it is pro- 

 bable, depends both upon the chemical and 

 the mechanical nature of the substances in 

 question, but perhaps more upon the latter 

 than the former, for we find that the processes 

 of cookery, which act principally upon mecha- 

 nical principles, render various substances per- 

 fectly digestible, which the stomach could not 

 act upon before they had undergone these 

 operations. We also find that animals, which, 

 in their natural state, have the strongest in- 

 stinctive predilection for certain kinds of food, 

 may, by a gradual training and the necessary 

 preparation of the articles employed, have their 

 habits entirely changed, without their health 

 being in any degree affected. 



There is, however, a circumstance in the 

 structure of the animal, which clearly points 

 out a natural provision for the reception of one 

 species of food in preference to the other, viz. 

 the comparative capacity of the digestive or- 

 gans. It may be concluded that, in all cases, 

 the aliment must undergo a certain change 

 before it can serve for the purpose of nutrition, 

 and that this change will occupy a greater 

 length of time, and that a greater bulk of 

 materials will be requisite, according as the 

 nature of the food received into the stomach is 

 more or less different from the substance into 

 which it is to be afterwards reduced. Hence, 

 as a very general rule, we find that the diges- 

 tive organs of carnivorous animals are less 

 capacious than those of the herbivorous, and 

 that even in the latter there is a considerable 

 difference, according as the food consists of 

 seeds and fruits or of the leaves and stems of 

 plants. 



There are indeed certain circumstances in the 

 habits of some of the carnivora which require 

 organs of considerable capacity, as, for ex- 

 ample, those beasts of prey who take their 

 food at long intervals, being supplied, as it 

 were, in an occasional or incidental manner, 

 so that it becomes necessary for them to lay up 

 a considerable store of materials, and to take 

 advantage of any opportunity which presents 

 itself of replenishing the stomach. The anato- 

 mical structure of the human digestive organs 

 indicates that man was intended by nature for 

 a mixed diet of animal and vegetable aliment, 

 but with a preponderance towards the latter ;* 

 and it appears in fact that, while a suitable 

 combination of the two seems the most condu- 

 cive to his health, and to the due performance 

 of all his functions, either species is alone 

 competent to his growth and nutrition.^ 



* Cuvier, Regne Animal, t. i. p. 86 ; Lawrence's 

 Lect. p. 217 et seq. ; see also the elaborate dis- 

 sertation of Richter, De victus animalis antiq. &c. 



t Haller, El. Phys. xix. 3. 2 . . 4 ; these sections 

 contain a very full account of the different kinds of 

 diet employed by different nations or individuals. 

 We have a number of curious facts of this kind in 



