Old male gorillas are gigantic, extremely massive animals which run on the ground on all fours upon bent knuckles. Neverthe less the locomotor skeleton as a whole is surprisingly close to that of man, except in certain parts such as the ilium and the great toe, in which the brachiating features are conspicuous.
Biped Type.—This is exemplified by man and the adaptations are treated elsewhere (see MAN, EVOLUTION OF).
V olplaning Types.—Certain active climbing types with long limbs, leaping boldly from the trees skim downward easily by holding extended their patagium, a fold of skin stretching from the neck outward and from the arms to the legs and sometimes from the legs to the tail. Such an adaptation has been acquired independently among marsupials in the flying phalangers, among rodents in the flying squirrels and in the anomalures, and also in the colugos ("flying lemurs"), an isolated type remotely related perhaps to the tree-shrews and lemurs. No marked skeletal spe cializations except the lengthening of the limbs distinguish the volplaning types from their arboreal relatives.
Flying Types.—The Chiroptera (q.v.) or bats are the only mammals to achieve true flight. Their wings are enormously elongated hands and arms covered with thin skin. The hind legs are weak and mostly used for suspension. It is supposed that bats have been derived from skimming forms in which the web of skin extended between the long fingers.
In the oldest Amphibia there was a gradual regional differentia tion of the backbone as we pass from the neck to the tail; in the mammal-like reptiles, especially in the cynodonts, regional differentiation is pronounced though not sudden. The ribs of the cervical or neck region are short, but still have shafts, while at least in the higher mammals these shafts are lost. The cynodonts were progressing in the direction of the mammals in the fact that their lumbar ribs were suturally connected with the sides of the vertebrae as in some very young marsupials. In many more ad vanced mammals the lumbar ribs are completely replaced by transverse processes which grow out from the sides of the verte brae. Similarly in the sacral region the cynodonts and less spe cialized mammals had free sacral ribs, whereas in more specialized mammals the rib elements of the sacrum are replaced by trans verse processes.
In the older mammal-like reptiles the condyle was median and ball-like as in typical reptiles, but in the cynodonts the lateral parts grew outward while the median part retreated so that a double condyle almost of mammalian type was attained.
The mammalian atlas-axis complex is a contrivance of great functional and morphologic intricacy, the purpose of which is to provide a wide range of movement combined with automatic checks, to prevent sudden stresses from dislocating the joint and squeezing the spinal cord. One of the most essential morphologi cal features of the atlas-axis complex of cynodonts and mammals is that the paired occipital protuberances are received into the upper or neural arches of the atlas or first vertebra and that the centrum or body of the atlas becomes a buffer which in the adult is closely united to the centrum of the second vertebra or axis, of which in adults it forms the odontoid process. Meanwhile the neural spine of the second vertebra becomes enlarged vertically to give origin to some of the powerful muscles that raise the head.
In the dorsal vertebrae the stout neural spines act as levers for the powerful spinal muscles in extending the back, while the trans verse processes and centra serve as bases for the movable ribs enclosing the heart, lungs and diaphragm.