'lie other kind of stomach which we referred to above as possessing a peculiar structure, and acting on a different principle from that of the human species, is the muscular stomach of certain classes of birds. Birds are not pro. vided with teeth, or with any apparatus which can directly serve for the process of mastica tion ; yet many of them feed upon hard sub stances, which cannot be acted upon by the gastric juice, until they have undergone sonic process, by which they may be comminuted or ground down into a pulpy mass. This is effected by the inglnvies, the craw or crap, and the ventriculus bulbosus or gizzard. The first 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 cesophagus, 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 mammahia. W e 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. An account of thenaturenf the substances usually employed as fimd.—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 principles and their ultimate elementsolthough 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 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 he 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.