Animals are primarily classed in great divisions, based on the general characters which differentiate them most. These divisions, or _branches_, allow of their being so grouped that in each of them we find united the individuals whose general structure is uniform; and under the name of vertebrates are included man and the animals with which our studies will be occupied. The vertebrates, as the name indicates, are recognised by the presence of an interior skeleton formed by a central axis, the vertebral column, round which the other parts of the skeleton are arranged. The vertebrate branch is divided into classes: fishes, amphibians or batrachians, reptiles, birds, and mammals. The mammals--from the Latin _mamma_, a breast--are characterized by the presence of breasts designed for the alimentation of their young. Their bodies are covered with hair, hence the name _pilifères_ proposed by Blainville; and, notwithstanding that in some individuals the hairs are few, the character is sufficient to distinguish them from all other vertebrates. We find united in this class animals which, at first, seem out of place, such as the whale and the bat; and, from their external appearance alone, the former would appear to belong to the fishes, and the latter to birds. Yet, on studying their structure, we find that, not only do these animals merit a place in the class which they occupy, because they possess the distinctive characters of mammals; but, still further, their internal structure is analogous to that of man and of the other individuals of this class. Notwithstanding this similarity of structure, the whale is not without some points of difference from its neighbours the horse and the dog; therefore, in order to place each of these animals in a position suitable to it, mammals are divided into secondary groups called _orders_. The first of these orders includes, under the name _primates_, man and apes. The latter contain animals which approach birds in certain characters of their organism, forming a link between the latter and mammals. We find, in studying the regions of the body in some of the vertebrates, that, while they present differences from the corresponding regions of the human body, they also offer most striking analogies. We can, for example, recognise the upper limb of man in the anterior one of quadrupeds, in the wing of the bat, in the paddle of the seal, etc. It is, so to speak, those variations of a great plan which give such a charm to the study of comparative anatomy. The division of classes into orders, which we have just mentioned, being still too general, it was found necessary to establish subdivisions--more and more specialized--to which the names _families_, _genera_, _species_, and _varieties_ were given. CHAPTER I OSTEOLOGY AND ARTHROLOGY THE TRUNK The Vertebral Column We commence the study of the skeleton with a description of the trunk.