HARDWICKE'S SCIENCE -GOSSI P. 



267 



The deep portion of the skin, the "true skin," is a I 

 fibrous network ; the superficial part is a cellular layer, I 

 called the cuticle. At the junction between the two is 

 the "basement membrane," the seat of active growth, 

 often thrown into ridges or papillce. Thecells, orepithe- 

 lium, of which the cuticle is composed, are formed 

 hei-e. They are at first soft and gelatinous, but as 

 they approach the surface, they become flattened hard 

 homy scales, and they are at length cast off. 



In amphibians, the skin is naked ; in fishes and 

 reptiles, covered with scales ; in birds, with feathers ; 

 in mammalia, with hair. The scales of fishes are 

 classed in four groups, placoid or plate-like, as in the 

 shark ; ganoid or enamelled, as in the sturgeon ; 

 cycloid or circular, as in the salmon ; and ctenoid or 

 comb-like, as in the perch. Each scale is embedded in 

 a sac of the cutis. The cycloid and ctenoid scales, 

 which are met with in the fishes with a bony skeleton, 

 are of a homy texture. The placoid and ganoid 

 scales are found in those fishes which have a car- 

 tilaginous vertebral column. The scales are hard, 

 and composed either of bone, or of dentine, like that 

 of teeth ; the ganoid scales are also covered with a 

 shining enamel. The scales of the Crocodile are 

 plates of bone covered with a homy epidermis ; the 

 bony plates are curiously pitted, as if the top of the 

 little finger had been pushed into them when soft. 



Hairs consist of elongated and flattened epithelial 

 scales. They spring from a single 'large papilla at 

 the bottom of a sheath, — an inversion of the basement 

 membrane, which may be compared to the finger of 

 a glove pushed inside out, all but the tip, the latter 

 representing the papilla. In the Hedge-hog and Por- 

 cupine, the hairs are very large, strong, and pointed, 

 forming the well-known quills ; they are hollow 

 internally, and divided into a number of chambers by 

 transverse and radiating partitions. The feathers of 

 Birds are simply large pinnate or bipinnate hairs ; 

 they are developed from a large papilla or pulp ; 

 this is vascular in the young state, but in the full- 

 grown feathers the base of the quill contracts around 

 the root of the papilla, so as to cut off its supply of 

 blood and stop further gi-owth. The human nail 

 consists of compact layers of flattened epithelial 

 scales resting on close rows of fine papillee. The hoofs 

 of Ungulates and the claws of Carnivorous Animals 

 are modified nails, and consist of compacted epithe- 

 lium ; that of the Horse springs from a layer of 

 papillee, which form flat lamina, like the leaves of a 

 book ; this layer is plentifully supplied with blood- 

 vessels and nerves, forming an important organ of 

 sense, by which the Horse makes himself acquainted 

 with the nature of the ground on which he treads. 

 Horns are of three classes ; the antlers of the Stag 

 consist wholly of bone ; they are covered at first with 

 a soft skin, which dries and peels off; they are shed 

 and reproduced every year, the horn of each year 

 having one branch more than that of the year before. 

 The horn of the Rhinoceros is wholly an epidermic 



structure, resembling in microscopic characters the 

 hoof of the Horse ; it is not shed. The hom of the 

 Cow consists externally of hard epidermis, but it has 

 a bony core springing from the skull ; between the 

 two portions there is a layer of soft vascular tissue, 

 which bleeds profusely when injured. 



Horn, in its chemical nature, is allied to gelatine 

 and chondrine ; it contains sulphur. It is quite 

 insoluble in water, but has the property of becoming 

 soft when heated. It strongly rotates the plane of 

 polarization of light, hence sections of hom are 

 among the most brilliant of polariscope objects. 



The horny plates or strainers of the Whale, the so- 

 called whalebone, are epidermic organs, and in structure 

 may be compared to large compound masses of com- 

 pacted hairs, the ends of which alone are free, forming 

 a fibrous fringe. 



Teeth are found in most Vertebrates, except 

 Birds and Turtles, in whom the jaws take the 

 form of a horn-covered bill. In the higher Vertebrates 

 the teeth are confined to the jaw-bones, but in many 

 Fishes they are found on the palate as well, and indeed 

 in some, as the Skate, the scales over the general sur- 

 face of the body exactly resemble teeth both in form 

 and stmcture. In Fishes and the lower Reptiles the 

 teeth are united to the jaw and indefinite in number, 

 being continually shed and reproduced. In the 

 Crocodile and the Mammalia the teeth are implanted 

 in sockets in the jaw. In most Mammalia we find 

 four classes of teeth, incisors, canines, prsemolars, and 

 molars. There are two sets, a temporary or milk set 

 and apermanent set. Each tooth hasabody, an exposed 

 crown, and one or more fangs implanted in the jaw. 

 In the centre of the tooth is a cavity containing a soft 

 pulp plentifully supplied with blood vessels and nerves 

 through a minute hole at the apex of the fang. Teeth 

 are highly sensitive ; not only are they, when diseased, 

 the seat of acute pain, as most of us have probably 

 experienced to our sorrow, but they are important 

 organs of touch ; a gritty particle, however minute, 

 getting between the teeth, is at once detected. 



The structure of teeth is somewhat complicated. 

 In each tooth there are three difTerent hard tissues,, 

 the dentine or ivoiy, the enamel, and the cement. The 

 ivoiy constitutes the main bulk of the tooth ; it is 

 composed of fine parallel wavy branched tubules ; in 

 the centre of the tooth there is a cavity, containing a 

 soft vascular pulp, slender prolongations of which 

 are continued into the dentinal tubules. The enamel 

 forms a hard cap covering the exposed crown of the 

 tooth ; it consists of prismatic wavy striped fibres 

 arranged perpendicularly to the surface ; it is intensely 

 hard. The cement covers the fang, that part of the 

 tooth which is embedded in the jaw bone ; it is of a 

 yellowish colour, -and softer than the dentine and 

 enamel ; under the microscope it shows lacunae 

 and canaliculi like those of bone: in old people 

 it becomes thicker, and is traversed by Haversian 

 canalf. 



