It is always divisible into a portion dorsal to the articulation of the limb known as the scapula and a portion ventral to it, the coracoid. A clavicular process grows forwards from the junction of the two regions, or in higher forms from the scapula. The corresponding portions of the pelvic girdle are the ilium (dorsal), the pubes, and ischium (ventral); the pubes perhaps homologous with the clavicle. All these parts ossify independently. A supra-scapula or vertebral border may either remain cartilaginous at the dorsal end of the scapula or ossify separately from it; and an epi-coracoid may be similarly formed from the ventral or sternal edge of the coracoid which is in contact with the costal sternum except in the Eu- and Meta-theria among Mammalia. The anterior edge of both scapula and coracoid may, when large, be partially converted into membranous spaces with intervening processes, prae- and meso-scapula; prae- and meso-coracoid. The two clavicles in some Amphibia and in higher types unite ventrally in the embryo, and from their point of union is differentiated an interclavicle or episternum. This episternum is free in Reptilia when it is present.
It unites in Aves with the costal sternum forming the keel (?). In Mammalia except Prototheria, where it is large and free, it fuses with the manubrium sterni, and then the clavicular region of this bone either persists, aborts, or is transformed into ligament. The origin of the epipubic cartilage, which often exists projecting forwards from the median anterior edge of the pubes, is unknown. The two pubes and ischia unite ventrally, and the line of union is termed the symphysis.
The limbs articulate with cup-shaped cavities - the glenoid cavity for the fore- and the acetabulum for the hind-limb - in all classes higher than Pisces. In the latter tubercles may take the places of cavities. In Amphibia and upwards the fore-limb is divisible into a series of segments pre-formed in cartilage, but all ossifying - viz., a humerus, articulating with the shoulder-girdle; a radius and ulna, articulating with the humerus and with each other; followed by a carpus, consisting of (typically) a radiale = scaphoid, intermedium = lunar, and ulnare = cuneiform, constituting a proximal row; a centrale; and then a distal row composed of carpale 1 = trapezium, 2 = trapezoid, 3 = os magnum, carpalia 4 and 5, or 4 + 5 = unciform, to which articulate the digits typically 5 in number, composed each of a proximal metacarpal, and a distal series of phalanges. The corresponding parts in the hind-limb are femur; tibia, and fibula; tarsus composed of tibiale, intermedium, or both together = astragalus, fibulare = calcaneum; centrale = navicular; tarsale 1 = ento-cuneiform, 2 = meso-cuneiform, 3 = ecto-cuneiform, tarsalia 4 and 5, or 4 + 5 = cuboid; and 5 digits each with a meta-tarsal and phalanges.
There are great modifications observable in the hand and foot of different groups of Vertebrata1. In addition to the paired limbs, the Ichthyopsida possess a system of azygos fins, either temporarily or permanently. An account of them is given under the general introduction to that division.
In addition to the above-named bones, ossifications appear frequently in tendons. Such bones are known as sesamoids. The most familiar example is the patella or knee-pan.
1 The account given in the text is the one ordinarily accepted. In some Urodele Amphibia the centrale is double. A digit external to the great toe also occurs in some of them, and some Anuran Amphibia. Baur (Z. A. viii. 1885) has come to the conclusion that the astragalus of Mammalia represent one of the two centralia + the intermedium, the tibiale being represented by a sesamoid, or forming part of the navicular ( = the other centrale). The unciform and cuboid represent the fifth carpal and tarsal respectively. Traces of a finger external to the thumb, and of a toe external to the great toe, are found in some Mammals, the sesamoid of the abducens pollicens representing the former, an extra ossicle seen in many Carnivora the latter. Cf. Id. on Archegosaurus, Z. A. ix. 1886.
The nervous system is divisible into central and peripheral parts. The former is constituted by the brain and spinal cord; the latter by the nerves and sympathetic system. The brain consists in the embryo of three hollow vesicles - a fore-, mid-, and a hind-brain. The first of these eventually gives origin to the two olfactory lobes (rhinencephala), the two cerebral hemispheres (prosencephala), whilst a remnant of the original vesicle persists as the thalamencephalon or vesicle of the thalami optici, from which arise the two hollow outgrowths converted afterwards into the optic nerves, retina and retinal pigment layer. The pineal gland (epiphysis cerebri) is continuous with the roof of the thalamencephalon. It has been supposed to represent either the region of closure of the neural folds (cf. Cephalochorda,) or else, an unpaired eye1. The floor of the thalamencephalon is prolonged into a hollow infundibulum, to the apex of which is attached the pituitary body (hypophysis cerebri = conarium), derived in part from the ectoderm of the stomodaeum, in part from mesoblast.
It is supposed to represent either the sub-neural gland of Urochorda (Balfour, Julin) or an abortive pair of gill-clefts (Dohrn). The floor of the mid-brain (mesencephalon) becomes crura cerebri; its roof simple in Protopterus and Siredon ( = Axolotl) forms two hollow optic lobes or corpora bigemina; or in Mammalia four solid corpora quadrigemina. The notochord in the embryo extends as far forwards as the mid-brain. At one period the whole fore-brain is bent or deflexed more or less in the different Vertebrate classes, and the mid-brain thus forms the anterior extremity of the body. This bend is known as the cranial flexure. The hind-brain is constricted into two lobes - the cerebellum in front, and the medulla oblongata behind. The cerebellum of the adult is the roof, the pons Varolii, the floor of the first lobe. The medulla oblongata is the floor of the second lobe. Its original roof widens and thins away at the same time, and is eventually represented only by an epithelial layer. The two ridges which carry the roots of the nerves are thus widely separated from the middle line. The primitive cavities of the brain-vesicles persist as the ventricles.