The contour of the vertebral column in the standing pose in the side view differs widely in accordance with the habits. In scampering types with short legs the column is usually strongly arched in the mid-dorsal region. This culminates in the short footed carnivorous types such as weasels. In long-limbed running types, especially those with a long neck or a heavy head, the backbone may bear long stout spines on the neck and forepart of the back, to which are attached the heavy ligaments of the neck and the deeper muscles of the occiput.
A profound difference between a typical reptile and a typical mammal is that in the former the great muscles on the lower side of the tail act powerfully in pulling the hind limbs backward in running. In mammals, on the contrary, the tail muscles are greatly diminished and of slight importance in locomotion.
Modifications of the jaws and teeth in the mammals, in adapta tion to different food habits, have a profound effect upon the form of the skull as a whole, since the jaws usually form the greater part of the bony face, while the jaw muscles cause the upgrowth of ridges on the top and sides of the braincase and condition in many ways the form of the base of the skull.
On the whole the mammalian braincase has advanced beyond that of primitive reptiles in the following features : (I) The elimination of several dermal bones of the circum orbital and occipito-temporal series, the tabulars, supratemporals, postorbital, postfrontal, prefrontals; (2) The widening-out of the brain-trough through the widening of the brain ; the change of the reptilian epipterygoid into the mammalian alisphenoid; (3) The elaboration of the turbinate bones, scroll-like out growths from the median cartilaginous septum of the nasal chamber ; (4) The consolidation of the bony elements surrounding the inner ear into a single dense bone, the periotic, and the fusion of this with the squamosal; (5) The shifting of the inner ear from the side to the base of the skull.
The mammalian nervous system, which reaches unprecedented complexity in man, can scarcely be understood apart from other regulating devices or control systems, which greatly condition its activities. In all vertebrates the ductless or endocrine glands (q.v.) play an important part in the production and maintenance
of the specific and individual patterns of growth and behaviour, since they pour into the blood-stream certain hormones which give the chemical impetus to particular changes in the direction of growth. (See ENDOCRINOLOGY, COMPARATIVE.) On the whole there is a marked contrast, however, between chemically and nervously determined functions. Chemical regu lation (as from the ductless glands) tends to be rigid and deter minate. Nervous regulation, on the other hand, has tended toward flexibility, modifiability, choice of several courses of action. Mammals, especially man, have achieved extraordinary adapta bility to wide ranges of environmental changes largely through the greater flexibility and range of response of their nervous system.
The nervous system of mammals (more fully treated in the article BRAIN), like that of other vertebrates, comprises the fol lowing elements: (I) paired organs of smell, sight, balance, hear ing ; numerous organs of touch and taste and "somesthetic" sense (organs which convey sensations of bodily posture and movement); (3) "motor nerves," which release the activities of glands and muscles; (4) innumerable connecting tracts, relay stations and control systems of amazing intricacy.
In general the mammalian brain differs from that of lower vertebrates in the fact that the upper part and sides of the end brain have grown outward into sack-like expansions, which form the neopallium or cerebral cortex of mammals. In the lower mammals this new part of the brain is but moderately developed and the localization of functional areas is at best incipient but in the higher mammals, including man, the neopallium becomes enormously complex, dominating the entire organism and tend ing to differentiate the cortical "centres" described in the article BRAIN. Meanwhile the thalamus and the "brain stem" have like wise become extremely complex, developing a bewildering maze of connections with other parts of the brain. The result of these expansions and complications is that the old primary control systems pass under the dominance of the newer and higher cen tres. (See C. J. Herrick, Brains of Rats and Men, and F. Tilney and H, A. Riley, The Brain from Ape to Man.) In a typical placental mammal the minute fertilized egg is developed internally and the embryo is nourished by an out growth of its own "allantoic bladder" which becomes appressed to the wall of the maternal uterus, finally uniting with the latter, at least along certain zones. Thus the embryo is enabled to draw nourishing blood directly from the mother (see VERTEBRATE EMBRYOLOGY). At birth the placenta is pulled away from the uterine wall but excessive uterine hemorrhage is normally stopped automatically by appropriate chemical substances in the maternal blood.