CUTANEOUS SYSTEM. 



223 



milk they yield. The antlers of the stag, after re- 

 maining nearly a year, are cast off and soon replaced. 

 Immediately before this occurs the carotid artery is 

 seen to throb, and all the vessels of the skin are 

 turgid, the heat of the parts being much augmented. 

 A tubercle is then observed below the skin, which 

 gradually increases in size, as it becomes converted 

 into bone. The cartilage being ossified, shoots up- 

 ward in the form of a lengthened cylinder, which 

 soon divides into branches. The skin being carried 

 along with and investing the horn, forms what is called 

 its velvet coat, but when fully formed, it shrivels and 

 peels off, and the antler presents then a smooth and 

 bare surface. The rapidity with which the vessels 

 form the antler is remarkable ; an antler weighing 

 thirty pounds is sometimes completely formed in the 

 space of fourteen days. After being fully developed, 

 the vessels at the base continue to deposit additional 

 osseous matter, which is called the burr. The accu- 

 mulation of this substance at length encroaches on the 

 arteries, and by diminishing their capacity for con- 

 taining blood, gradually cuts off the supply of nourish- 

 ment. The antler is then nourished only by its own 

 internal vessels. These at length shrink and become 

 obliterated, and the horn then adheres to the head 

 only as a foreign body. In the mean time the 

 absorbent vessels under the base of the horn, scoop a. 

 groove between the living and the dead matter, so 

 that the antler, deprived of support, falls upon the 

 slightest concussion. Thus are these graceful appen- 

 dages to the figure of this animal annually cast and 

 reproduced. 



BEAKS, NAILS, CLAWS, SPURS and HOOFS, are of 

 the same composition as horn, consisting principally 

 sf gelatine with only a trace of earthy matter. The 

 horny substance of the beak differs from horn in its 

 colour not being so fixed. The colour of the bills of 

 many birds varies with the season, and changes also 

 after death : the colour of the horns of cattle is per- 

 manent. Nails differ from horn only in not being 

 tubular, but deposited in flat plates. They occur in 

 the toes of quadrupeds, birds, reptiles, and pass into 

 claws. Spurs occur chiefly in gallinaceous birds ; 

 they are softened by heat, and may be easily bent 

 into particular shapes. Hoofs are peculiar to herbi- 

 vorous animals, forming a solid base and protection 

 to the foot. 



FEATHERS, which are the characteristic appendages 

 to the cutaneous system of birds, form a covering 

 which not only protects them from the inclemencies 

 of the weather, but winch, when large, as in the wings 

 and tail, serve as instruments of motion. The struc- 

 ture of a feather is exceedingly curious. Each feather 

 consists of a tube or barrel, from which proceeds the 

 shaft or stock, on each side of which are affixed barbs 

 or flat filaments tapering to a point. The tube or 

 barrel takes its rise like hair in the tissue immediately 

 below the cutis vtra, and passes by a tubular opening 

 through the other layers of the skin, which are so 

 folded downwards and inwards as to form a sheath, 

 which invests the bulb or vascular part of the feather. 

 The tube or barrel is a hollow, horny, transparent 

 cylinder, consisting of two layers, an external and 

 an internal. The external is a circular plate, exhibit- 

 ing linear impressions which run round the barrel in 

 a transverse direction ; the internal consists of fibres 

 running longitudinally. It is largest in anserme and 

 gallinaceous birds, as the turkey, swan, goose, &c., 

 for which reason the quills of these birds are best 



adapted for making pens. By being formed into a 

 hollow cylinder, the barrel of the feather is rendered 

 stronger, lighter, and more capable of resisting flexion, 

 than it otherwise would be. Its cavity indeed in 

 some birds of flight, as the condor, eagle, and hawk, 

 is continued some way into the shaft, and is, like the 

 bones of the skeleton, filled with air, which enters 

 into it during respiration through a minute orifice at 

 the extreme point or end of the quill. Within the 

 barrel is a dry jointed membranous substance, which 

 is the remains of the vascular pulp which originally 

 contributed to the growth and nourishment of the 

 feather. The shaft which is prolonged from the 

 barrel consists of a cuticular layer, disposed likewise 

 in a circular and longitudinal direction, the central 

 portion of which is filled with a pithy substance like 

 cork. Its outer surface is slightly convex, its inner 

 nearly flat, with a groove running along the middle. 

 From each side of the shaft proceed the barbs, which 

 are placed with their flat sides towards each other, 

 whereby a power of resistance is derived which 

 enables them when flying to encounter the impulse 

 and pressure of the atmosphere. 



They derive this power of resistance, says Dr. 

 Roget, from the flattened shape of these filaments, 

 which allows them to bend less easily in the direction 

 of their flat surfaces than in any other, in the same 

 way that a slip of card cannot easily be bent by a 

 force acting in its own plane, though it easily yields 

 to one at right angles to it. Now it is exactly in the 

 direction in which they do not bend that the filaments 

 of the feather have to encounter the greater resistance 

 and impulse of the air; it is here that strength is 

 wanted, and it is here that strength has been bestowed. 

 But this is not all : in birds of flight and water birds, 

 the barbs are.by their adhesion to each other, rendered 

 firm, and this is effected by a very curious mechanism. 

 When viewed under the microscope, a number of 

 minute fibrils or processes are seen arranged along 

 the margin of the laminae, so curved as to interlace 

 with or clasp one another whenever they are brought 

 within a certain distance. Those fibrils which pro- 

 ceed along the side of the laminae in the direction 

 from the quill and to the extremity of the feather are 

 long, flexible, and bent downwards ; those which 

 proceed from the adjacent lamina, from the extremity 

 of the feather towards the quill end, are shorter, 

 firmer, and turn upwards, so that when the long fibrils 

 are forced far enough over the short ones, their 

 crooked parts fall into the cavity made by the crooked 

 parts of the other, just as the latch that is fastened to 

 a door enters into the cavity of the catch fixed to the 

 door-post, and there hooking itself fastens the door. 

 These little teeth, like processes or barbules, are 

 wanting in feathers not intended for flight ; hence 

 they are not found in the feathers of the ostrich, nor 

 do they interclasp each other in the manner above 

 described in the feathers of the peacock's tail, nor in 

 those placed around the ear of the owl. The feathers 

 of nocturnal birds of prey have the barbs covered 

 with long silky down, whence, it has been observed, 

 arises the slow and silent flight of these birds. 



The mode in which feathers grow or become deve- 

 loped has been attentively examined by several natu- 

 ralists, especially by M. Cuvier, whose memoir in the 

 Memoires du Museum, torn, xiii., comprehends almost 

 all that is known on this intricate subject. On quit- 

 ting the egg, the body of every bird is found covered, 

 excepting the under part of the belly, with a sort 



