THE SKIN AND FEATHERS. 



the last ring of the windpipe. A bridge in the middle, formed by 

 the doubling of the inner membrane of the windpipe, divides it 

 into slits or clefts, and its borders, set in motion by the air. passing 

 out, produces the voice. The second cleft in the throat (rima glot- 

 tidis) performs, in a great many birds, the same office as the 

 reed in the clarionet, while the first cleft (rima) acts like the vent- 

 age or small hole of the instrument, by giving utterance to the note. 

 On each side of the lower larynx lie muscles, from one to five in 

 number, which, by their action, may change the larynx. These 

 muscles are lacking in only a few birds ; in others, especially in 

 the singing birds, there are five pairs of these muscles. Besides, 

 on both sides of the windpipe, there are some long muscles, which 

 begin in the lower larynx, and, in some birds, run up to the ears, 

 serving, by their action, to lengthen or shorten the windpipe. Very 

 curious is the course of the windpipe in some birds. It does not 

 always pass down the lower part of the neck directly into the in- 

 terior of the thorax ; but, in some cases, it passes before into the 

 comb or ridge of the breastbone, and forms, with the outer pectoral 

 muscles, a more or less deep nose, turning backward and upward, 

 and then passing down into the thorax. 



The Digestive Apparatus in birds is very differently con- 

 structed from that in mammals, as the former have no teeth. 

 Birds have salivary glands ; but a mixing of the food with saliva 

 hardly takes place, as the food is not masticated before it is swal- 

 lowed. The food, in the case of a great many birds, passes first 

 into a widened part of the throat, called a crop, where it remains 

 and is prepared for digestion. In the case of other birds, the food 

 passes directly into the membraneous stomach — a widening of the 

 lower part of the throat with numerous glands, and always thinner 

 than the third stomach or gizzard. It is never lacking in any bird, 

 and is very large in those that do not possess a crop. 



The Gizzard is variously constructed. In birds that live par- 

 tially or exclusively on other animals, it usually consists of a thin, 

 skinny sack ; but in birds feeding on hard grain and seeds, it re- 

 sembles a sort of grinding-mill, being composed of two semi-glob- 

 ular masses of thick and powerful muscles, covered on their flat 

 inside with a strong leathery skin, and working over each other 

 like a pair of mill-stones. This action is aided by sharp-cornered 

 grains of sand, or little pebbles, which the instinct of the bird has 

 prompted it to swallow for this purpose. In this way, the hardest 

 seeds or grains are, in a very short time, ground down to a fine 

 pulp. 



The Beak. — Though the beak never performs mastication 

 proper, yet its use varies in different birds. It is used to divide 

 flesh, to crack nuts, to separate grain from husks, and by the aid 

 of the tongue, to shell it like the Canary. Some birds make an 

 approach to a sort of mastication, as the Buntings, which, by means 

 of a knob in the middle of the palate, bruise the hard seeds before 

 they are swallowed. 



The Viscera. — In birds, the larger intestine is wholly absent. 

 A small rudimentary portion of it is found in the Ostrich. The 

 rectum in birds widens toward the termination into a sort of cham- 

 ber (cloaca), into which enter the two urinary tubes, the seminal 

 tubes, and those of the ovarium. The milt or spleen is proportion- 

 ally small ; the abdominal salivary gland is large ; the liver is divided 

 into several lobes, hard, grainy, and of considerable size ; the 

 gall-bladder is also large, and the kidneys long, broad, and lobed. 



Organs of Generation. — Some birds have a distinct penis ; 

 all of them testes and seminal tubes. The testes are found in the 

 cavity of the abdomen, lying in the upper part of the kidneys. In 

 the time of mating they acquire considerable size, but soon after- 

 ward shrink to small globular bodies, and are, in some birds, 

 hardly visible. The seminal tubes run in a zigzag way along the 



urinary tubes to their termination, gradually widening and forming 

 little bladders at their termini. The grape-shaped ovaries lie on 

 the upper end of the kidneys, and consist of a multitude of small, 

 globular bodies, varying in number from one hundred to five hun- 

 dred. The ovi-director is a long intestine-like skin, entering by one 

 mouth into the abdominal cavity, and by another into the cloaca. 



The Skin and Feathers. 



The Skin of birds resembles in its formation that of mammals. 

 Like the latter it consists of three distinct layers — the epidermis, 

 the mucous net, and the cutis. The cutis is thin and full of folds, 

 but becomes thicker on the feet, consisting on the soles and toes of 

 horny scales, and undergoing a similar transformation in the bill. 

 The cutis varies in thickness. In some birds, it is very thin ; in 

 others, thick and hard, but it is always full of vessels and nerves, 

 and is often found with a thick layer of fat on the inner side. The 

 mucous net lies between the cutis and epidermis, and is filled with 

 a liquid (mucus). The epidermis consists of countless little gland- 

 ular cells, of which the lower layers only are active, and filled with 

 mucus. The layers of cells nearer the surface are more or less 

 dried up, causing the cells to lose their round form and appear 

 flattened, while the upper or outside layer constantly peels off. It 

 is in the mucous net that the little feathers find their birthplace ; but 

 the larger ones pass through the cutis into pocket-shaped bags, as can 

 be seen in the tail feathers and the larger feathers of the wings. 



Explanation of Plate C. (Picus Auratus.~) 



Fig. i. Upper or back side; the dotted places represent the covert, the blank places 

 the uncovered parts, a. Is the salivary glands, so remarkably developed in Wood- 

 peckers. Fig. 2. Represents the same bird from the lower side or belly. 



The Feathers of birds are similar to the hair, spines, or scales 

 of mammals, but vary in the different members of the same class, 

 as well as on the different parts of the same bird. We distinguish 

 in the feather, the trunk and the barb or vane; and in the trunk, 

 the quill and the shaft. The quill is the part to which the feather 

 is fastened in the skin ; it is of a transparent, cylindrical formation. 

 Higher up it changes into a four-sided form, filled with a porous " 

 marrow, containing in its middle the so-called " saul," a row of 

 little cells inserted into each other, and containing the nourishment 

 of the feather. The " saul" is united or joined, above and below, 

 to the quill. 



The upper part of the shaft is arched, and covered with a smooth 

 horny mass ; the lower part is divided into halves by a longitudinal 

 groove. On the shaft is the vane or beard, composed of a double 

 series of layers, or thin parallel plates on each side of the shaft. 

 Toward the shaft of the feather, these layers are broad, and of a 

 semicircular form, for the sake of strength, and for the closer 

 placing of them, one against the other, when in action; but toward 

 the outer part of the vane, these layers grow slender and ta ering, 

 so as to become lighter. On their wider side, they are thin and 

 smooth ; but the upper, outer edge is parted into two hairy edges, 

 each side having a different set of hairs, rather broad at the bot- 

 tom, but slender and bearded above. In this way the hooked beards 

 of one layer always lie next the straight beards of the next, lock- 

 ing into and holding each other, but offering no resistance or hin- 

 drance to the flight of the bird. Beneath these layers of feathers 

 is the "down," similarly constructed, but more irregular and more 

 tender. It preserves the bird from cold, to which it would other- 

 wise have been greatly exposed. 



All the feathers of a bird are divided into the outer feathers and 

 the down. The former are subdivided into feathers of the body, 

 coverts, wing and tail feathers. As previously observed, the wings 

 of a bird, zootomically speaking, correspond with human arms and 

 hands ; but the human hand consists of four fingers and a thumb, 

 while on the hand of a bird (see Plate B, n, o) there are only 

 three fingers and a thumb, and these are all only rudimentary. 



