Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/370

Rh 348 animal, or upon different animals, assume various forms, and are of various sizes and degrees of rigidity, as seen in the delicate soft velvety fur of the Mole, the stiff bristles of the Pig, and the spines of the Hedgehog and Porcupine, all modifications of the same structures. These differences arise mainly from the different arrange ment of the constituent elements into which the epithelial cells are modified. Each hair is composed usually of a cellular pithy internal portion, containing much air, and a denser or more horny cortical part. In some animals, as Deer, the substance of the hair is almost entirely composed of the medullary or cellular substance, and it is conse quently very easily broken ; in others the horny part pre vails almost exclusively, as in the bristles of the Wild Boar. In the Three-toed Sloth (Bradypus) the hairs have a central horny axis and a pithy exterior. Though gene rally nearly smooth, or but slightly scaly, the surface of some hairs is strongly imbricated, notably so in some Bats, while in the Two-toed Sloth (Choloepus) they are longi tudinally grooved or fluted. Though usually more or less cylindrical or circular in section, they are often elliptical or flattened, as in the curly-haired races of men, the terminal portion of the hair of Moles and Shrews, and conspicuously in the spines of the Rodents Xerus and Platacanthomys. Hair having a property of mutual cohesion or &quot;felting,&quot; which depends upon a roughened scaly surface and a tendency to curl, as in domestic Sheep (in which animals this property has been especially culti vated by selective breeding), is called &quot; wool.&quot; In a large number of mammals hairs of one kind only are scattered pretty evenly over the surface, but in many there are two kinds, one longer, stiffer, and alone appear ing on the surface, and the other shorter, finer, and softer, constituting the under fur, analogous to the down of birds. In most cases hairs of a different character from those of the general surface grow in special regions, forming ridges or tufts on the median dorsal or ventral surface or elsewhere. The tail is very often completed in this way by variously disposed elongated hairs. The margins of the eyelids are almost always furnished with a special row of stiffish hairs, called cilise or eyelashes, and in most mammals specially modified hairs, constituting the vibrissge or whiskers, endowed, through the abundant nerve supply of their basal papillae, with special tactile powers, grow from the lips and cheeks. In some mammals the hairy covering is partial and limited to particular regions ; in others, as the Hippopotamus and the Sirenia, though scattered over the whole surface, it is extremely short and scanty ; but in none is it reduced to so great an extent as in the Cetacea, in which it is limited to a few small bristles confined to the neighbourhood of the lips and nostrils, and often only present in the young or even foetal condition. Some kinds of hairs, as those of the mane and tail of the Horse, appear to persist throughout the life-time of the animal; but more generally, as in the case of the body hair of. the same animal, they are shed and renewed periodically, generally annually. Many mammals have a longer hairy coat in winter, which is shed as summer comes on ; and some few, which inhabit countries covered in winter with snow, as the Arctic Fox, Variable Hare, and Ermine, undergo a complete change of colour in the two seasons, being white in winter, and grey or brown in summer. The several species of Cape Mole (Chrysochloris), the Desmans or Water Moles (Myngale and Potamogale velox are remarkable as being the only mammals whose hair reflects those iridescent tints so common in the feathers of tropical birds. The principal and most obvious purpose of the hairy covering is to protect the skin against external influences, especially cold and damp. Its function in the hairless [TEGUMENTAKY STRUCTURES. Cetacea is supplied by the specially modified and thickened layer of adipose tissue beneath the skin called &quot; blubber.&quot; True scales, or flat imbricated plates of horny material, Scales, covering the greater part of the body, so frequently occur- &c - ring in reptiles, are found in one family only of mammals, the Manidx or Pangolins; but these are also associated with hairs growing from the intervals between the scales or on the parts of the skin not covered by them. Similarly imbricated epidermic productions form the covering of the under surface of the tail of the flying Rodents of the genus Anomalurus ; and flat scutes, with the edges in apposition, and not overlaid, clothe both surfaces of the tail of the Beaver, Eats, and others of the same order, and also of some Insectivores and Marsupials. The Armadillos alone have an ossified exoskeleton, composed of plates of true bony tissue, developed in the derm or corium, and covered withscutes of horny epidermis. Other epidermic appendages are the horns of Ruminants and Rhinoceroses, the former being elongated, tapering, hollow caps of hardened epidermis of fibrillated structure, fitting on and growing from coni cal projections of the frontal bone, and always arranged in pairs, while the latter are of similar structure, but without any internal bony support, and situated in the median line. Callosities, or bare patches covered with hardened and thickened epidermis, are found over the ischial tuberosities of many apes, the sternum of camels, on the inner side of the limbs of the Equidse, the grasping under surface of the tail of prehensile-tailed monkeys, &c. The greater part of the skin of both species of one-horned Asiatic Rhinoceros is immensely thickened and stiffened by increase of the tissue both of the derm and epiderm, con stituting the well known jointed &quot; armour-plated &quot; hide of those animals. With very few exceptions, the terminal extremities of Nails, the digits of both limbs are more or less protected or armed claws, by epidermic plates or sheaths, constituting the various K forms of nails, claws, or hoofs. These are wanting in the Cetacea alone. A perforated spur, with a special secreting gland in connexion with it, is found attached to the hind leg of the males of the two species of Monotremata, the Ornithorhynchus and Ecliidna, Besides the universally distributed sebaceous glands Odour- connected with the pilose system, most mammals have secretin special glands situated in modified portions of the integu- lan(ls - ment, often involuted to form a shallow recess or a deep sac with a narrow opening, situated in various parts of the surface of the body, and which secrete odorous substances, by the aid of which individuals appear to recognize one another, and which probably afford the principal means by which wild animals are able to become aware of the pre sence of other members of the species, even at great dis tances. Although the commencement of the modifications of portions of the external covering for the formation of special secretions maybe at present difficult to understand, the principle of natural selection will readily explain how such organs can become fixed and gradually increase in development in any species, especially as there would probably be a corresponding modification and increased sensibility of the olfactory organs. Such individuals as by the intensity and peculiarity of their scent had greater power of attracting the opposite sex would certainly be those most likely to leave descendants to inherit and in their turn propagate the modification. To this group of structures belong the suborbital gland or &quot; crumen &quot; of Antelopes and Deer, the frontal gland of the Muntjak and of Bats of the genus Phyllorhina, the submental gland of the Chevrotains and of Taphozous and some other Bats, the post-auditory follicle of the Chamois, the temporal gland of the Elephant, the lateral glands of the Musk-Shrew, the lumbar gland of the Peccary, the inguinal