SENSE ORGANS.] MAMMALIA 367 in a great degree to the size of the cerebral hemispheres. In very small species of all groups, even the Primates, they are absent, and in the largest species of groups so low in the scale as the Marsupials and Edentates they are found. They reach their maximum of development in the Cetacea. The researches of palaeontologists, founded upon studies of casts of the interior of the cranial cavity of extinct forms, have shown that, in many natural groups of mammals, if not in all, the brain has increased in size, and also in complexity of surface foldings, with the advance of time, indicating in this, as in so many other respects, a gradual progress from a lower to a higher type of develop ment. The twelve pairs of cranial nerves generally recognized in vertebrates are all usually found in mammals, though the olfactory nerves are excessively rudimentary, if not altogether absent, in the Toothed Whales. The spinal cord, or continuation of the central nervous axis, lies in the canal formed by the neural arches of the vertebrae, and gives off the compound double-rooted nerves of the trunk and the extremities corresponding in number to the vertebra?, through the interspaces between which they pass out to their destination. The cord is somewhat enlarged at the two points where it gives off the great nerves to the anterior and- the posterior extremities, -which from their interlacements soon after their origin are called respectively the brachial and lumbar plexuses. The ganglionic or sympathetic portion of the nervous system is well developed, and presents few modifications. The sense of touch is situated in the skin generally, but is most acute in certain regions more or less specialized for the purpose by the presence of tactile papillae, such as por tions of the face, especially the lips and end of the snout, and the extremities of the limbs when these are used for other purposes than mere progression, and the under surface of the end of the tail in some Monkeys. The " vibrissae" or long stiff bristles situated on the face of many mammals are rendered extremely sensitive to touch by the abundant supply of branches from the fifth nerve to their basal papillae. In Bats, the extended wing membranes, and prob ably also the large ears and the folds and prominences of skin about the face of some species, are so sensitive as to receive impressions even from the different degrees of resist ance of the air, and so enable the animals to avoid coming in contact with obstacles to their nocturnal flight. The organs of the other special senses are confined to the head. Taste is situated in the papillae scattered on the dorsal surface of the tongue. The organ of smell is present in all mammals except the Toothed Whales. It consists of a ramification of the olfactory nerves over a plicated, moist, mucous membrane, supported by folded plates of bone, placed on each side of the septum nasi in the roof, or often in a partially distinct upper chamber of the nasal passage, so arranged that, of the air passing into the lungs in inspira tion, some comes in contact with it, causing the perception of any odorous particles with which it may be charged. Many mammals possess intense powers of smelling cer tain odours which others are quite unable to appreciate, and the influence which this sense exercises over the well- being of many species is very great, especially in indicating the proximity of others of the same kind, and giving warning of the approach of enemies. The develop ment and modification of the sense of smell is probably associated with that of the odorous secretion of the cutaneous glands. The organ of sight is quite rudimentary, and even con cealed beneath the integument, in some burrowing Rodents and Insectivores, and is most imperfectly developed in the Platanista, or Freshwater Dolphin of the rivers of India. In all other mammals the -eyeball has the structure characteristic of the organ in the higher Vertebrata, con sisting of parts through which the rays of light are admitted, regulated, and concentrated upon the sensitive expansion of the optic nerve lining the posterior part of the ball. A portion of the fibro-vascular and highly pigmented layer, the choroid, which is interposed between the retina and the outer sclerotic coat, is in many mammals modified into a brilliantly coloured light-refiecting surface, the tapetum lucidum. There is never a pecten or marsupium, as in the Sauropsida, nor is the sclerotic ever supported by a ring of flattened ossicles, as is so frequently the case in the lower vertebrated classes. The eyeball is moved in various directions by a series of muscles the four recti, two obliqui, and, except in the higher Primates, a posterior retractor muscle called choanoid. It is protected by the lids, generally distinctly separated into an upper and a lower movable flap, which, when closed, meet over the front of the eye in a more or less horizontal line ; but sometimes, as in the Sirenia, the lids are not distinct, and the aperture is circular, closing to a point. In almost all mammals below the Primates, except the Cetacea, a " nictitating membrane" or third eyelid is placed at the inner corner of the eyeball, and works horizontally across the front of the ball within the true lids. Its action is instantaneous, being apparently for the purpose of cleaning the front of the transparent cornea, a function which is unnecessary in animals whose eyes are habitually bathed in water, and which in Man and his nearest allies is performed by winking the true eyelids. Except in Cetacea the surface is kept moist by the secretion of the lacrymal gland, placed under the upper lid at its outer side, and the lids are lubricated by the Harderian and Meibomian glands, the former being situated at the inner side of the orbit, and especially related to the nictitating membrane, the latter in the lining membrane of the lids. The organ of hearing is enclosed in a bony capsule Hearing, (periotic) situated in the side of the head, intercalated between the posterior (occipital) and the penultimate (parietal) segment of the skull. It has, in common with other vertebrates, three semicircular canals and a vestibule, but the cochlea is more fully developed than in Sauropsida, and, except in the Monotremes, spirally convoluted. The tympanic cavity is often dilated below, forming a smooth rounded prominence on the base of the skull, the tympanic bulla. The three principal ossicles, the " malleus," " incus," and " stapes," are always present, but variable in characters. In the Sirenia, Cetacea, and Seals they are massive in form, being in the first-named order of larger size than in any other mammals. In the Cetacea the malleus is ankylosed to the tympanic ; in other mammals it is connected only with the membrana tympani. The stapes in the lower orders Edentates, Marsupials, and Monotremes has a great tendency to assume the columnar form of the correspond ing bone in Saurvpsida, its two rami entirely or partially coalescing. 1 The tympanic membrane (drum of the ear) forms the outer wall of the cavity. In the foetal state it is level with the external surface of the skull, and remains so permanently in a few mammals, as the American Monkeys ; but commonly, by the growth of the squamosal bone, it becomes deeply buried at the bottom of a bony tube (meatus auditorius externns), which is continued to the surface of the skin in a fibrous or fibre-cartilaginous form. In Whales, owing to the thickness of the subcutaneous adipose tissue, this is of great length, and is also extremely narrow. In most aquatic and burrowing animals the meatus opens upon the surface by a simple aperture, but 1 The modifications of these bones are fully described by A. Doran, " Morphology of the Mammalian Ossicula auditus," Trans. Linn. Soc.,
ser. 2, vol. i. pp. 371-497, pi. lviii.-lxiv., 1878.