(1622). Secondly, we find appended to the cranial or cephalic portion of the spine, a set of bones disposed symmetrically, and forming the framework of the face: these bones, it is true, have by many Continental writers been regarded as constituting additional vertebras, the parts of which are still recognizable, although amazingly modified in shape, so as to enclose the different cavities wherein the senses of vision and smell, as well as the organs of mastication, are situated. We shall not, however, waste the time of the student by considering in this place the as yet unsettled and vague opinions of transcendental anatomists upon this subject; it is sufficient for our purpose merely to indicate the facial bones as appendages to the cranial vertebrae, avoiding for the present further discussion concerning them.

(1623). Another most important addition to the central axis of the skeleton is obtained by the provision of lateral prolongations, derived from the transverse processes of the vertebrae, which form a series of arches largely developed at certain points, so as more or less completely to embrace the principal viscera, and give extensive attachment to muscles serving for the movements of the body.

(1624). The first set of arches is appended to the lateral portions of the cranial vertebrae, and the bones thus derived enter largely into the composition of the respiratory apparatus. In Man this important portion of the skeleton is reduced to a mere rudiment, distinguished by the name of the os hyoides; and in the human subject its relations and connexions with the surrounding parts are so obscurely visible, that the student is scarcely prepared to witness the magnitude and importance of the hyoid framework in other classes, or the amazing metamorphoses which, as we shall afterwards see, it undergoes.

(1625). Behind the hyoid apparatus, other arches are attached to the transverse processes of the spinal vertebrae, called ribs; and the study of these appendages to the spine is one of the most interesting points in the whole range of osteology. In Fishes, wherein respiration is effected entirely by the movements of largely-developed hyoid bones, the ribs are mere immoveable derivations from the transverse processes of the vertebrae, and serve exclusively for the attachment of muscles. In Reptiles, respiration is still accomplished by the os hyoides; and the ribs, thus performing a secondary office, become convertible to different uses, and assume various forms and proportions. In the Amphibious Reptiles, the most nearly approximated to Fishes, they either do not exist at all, as being needed neither for respiration nor locomotion, or they are represented by minute and almost imperceptible rudiments appended to the extremities of the transverse processes of the vertebrae. In Serpents, the ribs are wanted for locomotion, and are accordingly developed from the head nearly to the tail, forming a series of strong arches, articulated at one extremity with the vertebral column by a very complete joint, but at the opposite extremity they are loose and unconnected.

In proportion, however, as the hyoid bones, with the larynx, of which they form an important part, become converted into a vocal apparatus (as they gradually do), the ribs, assuming more complete development in a certain region of the spine, and being augmented by the addition of a sternal apparatus, form a complete thoracic cavity, and thus become the basis of those movements of the body which in hot-blooded animals are subservient to respiration.

(1626). The next additions required to complete the skeleton are two pairs of locomotive limbs, representing the legs and arms of Man. Infinitely diversified as are these members both in form and office, they are, when philosophically considered, found to be constructed after the same type. Both the anterior and posterior limbs, when fully organized, consist of similar parts, most of which are met with in the limbs of the human skeleton. Three bones constitute the shoulder, called respectively the scapula, the clavicle, and the coracoid bone. Three bones in like manner sustain the hinder extremity - the ilium, the ischium, and the pubis; and these evidently represent individually the corresponding pieces found in the shoulder, but differently named. The formation of the limbs is likewise strictly parallel: a single bone articulates with the osseous framework of the shoulder, or of the hip, called in one case the humerus, in the other the femur; two bones form the arm - the radius and ulna; and two likewise enter into the composition of the leg - the tibia and fibula.

The hand and foot are each supported by a double series of small bones, forming the carpus of the one, and the tarsus of the other; and in like manner consist of similar pieces, five in number, called the metacarpal or metatarsal bones, and of the phalanges, or joints of the fingers and toes.

(1627). A perfect or typical skeleton must therefore be supposed to consist of all the before-named portions: namely, 1, the cranial and spinal vertebrae; 2, the face; 3, an elaborately-formed hyoid framework; 4, the ribs; 5, a sternal system of bones, constituting, in conjunction with some of the ribs, a thorax; and 6, of four locomotive extremities, made up of the parts above enumerated as entering into their composition. Seldom, indeed, is it that the student will find even the majority of these portions of the osseous apparatus coexistent in the same skeleton; but, whatever forms of animals may hereafter present themselves for investigation, let the above description be taken as a general standard of comparison, and let all variations from it be considered as modifications of one grand and general type.

(1628). We must, however, proceed one step further in this our preparatory analysis of the skeleton, and, instead of regarding the individual pieces of the osseous framework of an adult animal as so many simple bones, be prepared to find them resolvable into several distinct parts or elements, all or only a part of which may be developed in any given portion of the osseous system.