A sacral region is not defined in Cetacea and Sirenia where the ilia either fail to reach the backbone, or else are absent. The caudal series may be reduced to 3-5. vertebrae in Man and the higher Apes, but is generally numerous. The largest number known, 46, occurs in Manis macrura among Edentata. The two first cervical vertebrae articulate one with the other, and with the skull by synovial joints: the remaining vertebrae by fibro-cartilaginous discs, in the axis of which is found a remnant of the notochord, the nucleus pulposus. The centra of the vertebrae are usually flat, but in the Ungulata those of the cervical region in particular are more or less opisthocoelous. The ribs are divided into a vertebral and sternal section, the latter sometimes cartilaginous, sometimes ossified. Some of the posterior ribs may lose their connection with the sternum, and also with the vertebrae, and in the latter case are known as floating ribs, e. g. in Cetacea. The sternum undergoes, at a certain stage of growth, transverse segmentation into a series of sternebrae which may or may not remain separate.
It is divisible into three regions - a praesternum or manubrium sterni, with which the clavicles and first pair of ribs articulate; a mesosternum composed of a variable number of sternebrae with a pair of ribs articulating between every two adjoining sternebrae, the last sterne-bra, however, sometimes, e. g. in Man, giving attachment to more than one pair of ribs; and a xiphisternum, sometimes cartilaginous, sometimes ossified, representing a sternal region in which the original connection with ribs has been aborted. The fore-limbs are never absent; the hind-limbs are wanting in Cetacea and Sirenia, or reduced to rudiments of a femur, and in the Greenland Whale of a tibia as well. The scapula has the true anterior border or spine placed on the external surface of the bone, and the apparent anterior border is a new development. The coracoid is a small process which forms a portion of the glenoid cavity but fails to reach the sternum. The clavicle, originally continuous with the acromion or free extremity of the spine, articulates with the praesternum and may either be represented by ligament at each end, e. g. many Carnivora and Rodentia, or be absent altogether, e. g.
Ungulata, Cetacea. The interclavicle is fused with the praesternum; it may be partially converted into ligament or atrophied away completely. The ilium slopes downwards and backwards from its articulation with the sacrum. The pubes meet in a ventral symphysis with rare exceptions (e. g. the Mole, Talpa); the ischia on the contrary have no symphysis, or only just touch one another. There is a well-developed heel, or os calcis, formed by the growth of the fibular tarsal bone. The digits are limited to three phalanges in addition to the metacarpal and metatarsal bones, except in the hand of some Cetacea, where the second and third fingers have a larger number. The bones of Mammalia, with the exception of those of the skull, of the sternebra, and save in a few cases of the carpus and tarsus, possess epiphyses or separate caps of bone to their free extremities or articulating surfaces.
The cerebral hemispheres are the largest part of the brain: their surface is often convoluted, and they are connected by a system of transverse commissural fibres, the corpus callosum, as well as by longitudinal fibres, the fornix. Their ventricles are large and form an anterior and a descending cornu with the addition in the higher Primates of a posterior cornu. The olfactory lobes are usually small, and are absent in toothed Whales: the olfactory nerves very numerous, and perforating the ethmoid bone in small bundles so as to give it a sieve-like aspect (the cribriform plate). The anterior commissure is small. The pineal gland has no connection with the dura mater or the skull; is small, and its base alone contains nervous substance. The optic lobes are small, solid, and divided by a transverse fissure into four lobes - the corpora quadragemina. The lateral lobes of the cerebellum are large and connected by a pons Varolii, or set of ventral transverse fibres. The angle, or bend, between the medulla oblongata and spinal cord is generally considerable. The brain of the gigantic extinct Mammalia from the American Eocene was remarkably small.
The cast of it passes with ease through the neural canal of the vertebrae, and is Lacertilian in aspect, with large olfactory and small cerebral lobes. The sympathetic cord accompanies the vagus nerve in the neck, and has never more than three cervical ganglia. There are no sensory hairs to the cells of the olfactory epithelium. The eye is rudimentary in Plataiiista among Cetacea: rudimentary or absent in certain burrowing Rodentia and Insectivora. There is often a tapetum lucidum external to the retina, composed of fine parallel waved connective tissue fibres as in Ungulata, or of cells as in Carnivora. There is, except in Primates, a suspensory or choanoid muscle to the eye internal to the circle of recti muscles; and in Carnivora it is broken up into four muscles. The nictitating membrane is present except in Primates and Cetacea, but has no special muscle. The eyelids are reduced in Sirenia to a circular fold which contracts to a point. The Cetacea have no lacrymal glands. In the inner ear the cochlea is spirally convoluted. The external aperture of the ear is a simple aperture in most aquatic and burrowing Mammals, but in others it has a well-developed and characteristic pinna.
For the ear-bones, see p. 361.
A few Mammals are edentulous as the Ant-eaters (Myrmecophaga, Cyclothurus, and Manis); or there may be transitory teeth which never cut the gum (Baleen Whales). Teeth are limited to the prae-maxillary, maxillary, and mandibular bones. They are implanted in sockets by one or more roots, and are attached to the socket by a fibrous alveolo-dental membrane, and anchylosis is said to occur only with the incisors of certain Shrews (Insectivora). The teeth are either simple and alike in shape (homodont), or differ from one another (heterodont), and are then termed in a complete series incisors, canines, praemolars and molars. When their number is limited, as is usually the case, the full dentition includes 44 teeth, that is to say 11 teeth on each side above and below, viz. 3 incisors, 1 canine, 4 praemolars, and 3 molars, as in the Pig among Ungulata, and the genera Gymnura, Myogale and the Mole (Talpa) among Insectivora. But in living Mammals the number generally falls below this maximum. In homodont dentitions, however, the number is often great, e. g. 100 in Priodon among Edentata, 200 in Delphinus among Cetacea. There are two sets of teeth, a milk and a permanent, in the majority of Mammals, hence termed Diphyodonts, as opposed to Monophyodonts, which have but one set.