This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
A bulb or sacculus, formed by an inward reflexion of the cutis (fig. 404, b, e), and lined by a similar inflexion of the cuticle (f), contains in its fundus a vascular pulp (ggg), well supplied with large vessels and nerves (h.) It is from the surface of the pulps (g), exhibited upon a magnified scale at a, that the horny stem of the hair is gradually secreted; and its length of course increases in proportion to the accumulation of corneous matter continually added to the root.
(2348). Various are the appearances of epidermic appendages in every way analogous to hair both as relates to their composition and mode of growth, and widely different the uses to which they may be converted: the wool of the Sheep, the fur of the Rabbit, the spines of the Hedgehog, the quills of the Porcupine, the scaly covering of the Manis, and even the armour that defends the back of the Armadillo are all of them but modifications of the same structures, adapted to the altered conditions under which the creatures live. Even the horn upon the snout of the Rhinoceros is but an agglomeration of hairy filaments, formed upon a broad and compound pulp. The nails that arm our fingers and toes, the corneous sheath that invests the horns of the Ox and Antelope, nay, the hoofs of herbivorous quadrupeds are all epidermic secretions from the vascular cutis - or, in other words, are hairs, altered in their form and extent according to the exigencies of the case.
Fig. 404. Section of the lip of a young Lion. (After Hunter).
(2349). Widely different, however, are the so-called horns of the Deer tribe, which in reality consist of bone, and, being deciduous, have to be reproduced from year to year by a most peculiar and interesting process. No sooner does the return of genial weather again call forth the dormant reproductive energies of the system, than the budding antlers begin to sprout from the forehead of the Stag, and rapidly expand in their dimensions from day to day. On making a longitudinal section of the young horn, it is found to be continuous with the os frontis, having its outer surface covered with a vascular periosteal membrane derived from the pericranium, which in turn is protected by a fine velvety skin. Moreover, when a growing antler is injected minutely, and its earthy matter removed by means of an acid, vessels derived from the periosteum are found to traverse it in all directions, proving its identity with real bone. As growth goes on, the external carotid arteries, thus called upon rapidly to furnish a prodigious supply of materials, dilate in a remarkable manner, and soon the palm and the antlers of the horn have acquired their full dimensions.
No sooner is this accomplished than a prominent ring or burr is formed around the base, which, projecting outwards, compresses and soon obliterates the vessels that have hitherto supplied the growing defences. The circulation being thus put a stop to, the soft teguments and periosteum peel off in strips; and the bone, denuded of its covering, becomes a formidable weapon.
(2350). At the close of the breeding-season the removal of the horns is speedily effected: the connexion between their bases and the os frontis is gradually weakened by interstitial absorption, until at length a slight effort is sufficient to detach the branching honours of the Stag, and they fall off, leaving a broad cicatrix: this soon skins over; and the succeeding year calls forth a repetition of the process*.
(2351). The Cetacea form a very remarkable group among the hot-blooded Mammifers as relates to the external covering of their bodies.
* In a physiological point of view this rapid production of osseous matter is truly wonderful. The horns of the Wapiti Deer, thus annually reproduced, will weigh upwards of thirty pounds; and in the fossil Irish Elk the weight of these deciduous defences must have been greater than that of the entire skeleton.
No covering of hair or wool would have been efficient in retaining the vital heat under the circumstances in which these creatures live; and even if such clothing could have been made available, it would have seriously impeded their progress through the water. Another kind of blanket has therefore been adopted: - the cuticle is left perfectly smooth and polished, without any vestige of hair upon its surface; but, beneath the skin, fat has been accumulated in prodigious quantities; and, enveloped in this non-conducting material, the Whales are fully prepared to inhabit an aquatic medium, and to maintain their temperature even in the Polar Seas.
(2352). The skin of all quadrupeds contains innumerable secerning follicles, whereby lubricating fluids are continually furnished for the purpose of maintaining the surface in a moist or supple condition; but not unfrequently these glandular follicles are aggregated together in considerable numbers, so as to form secreting pouches. In many species of Stags and Antelopes, for example, large pouches of this description are found below the margin of the orbit, that furnish a secretion vulgarly regarded as the Stag's "tears." In most instances some of the cutaneous glands secrete a highly odorous material, especially in the vicinity of the parts of generation; and their secretion being most abundant during the rutting season, it is not without reason that these organs are looked upon as destined to attract the sexes, and perhaps to stimulate the sexual passions. The preputial glands, so called because they furnish an odoriferous fluid that lubricates the prepuce and glans of the penis in the male, and of the clitoris in the female, are of this kind *. For the most part, these are simple sebaceous follicles contained in the thickness of the prepuce; but occasionally they are replaced by true conglomerate glands, formed of lobes and lobules, and having but a single excretory duct, that opens upon the sides of the glans penis or clitoridis beneath the prepuce.