exist, with great variations, between the specialized one or two last tracheal and some of the first bronchial rings.
According to the position of the chief sound-producing membranes, three types of syrinx are distinguishable:—(1) Tracheo-bronchial, by far the commonest form, of which the two others are to a certain extent modifications. The essential feature is that the proximal end of the inner membranes is attached to the last pair of tracheal rings; outer tympaniform membranes exist generally between the 2nd, 3rd and 4th bronchial semi-rings. This type attains its highest development in the Oscines, but it occurs also in many other orders. (2) Syrinx bronchialis. The outer membranes are spread out between two or more successive bronchial semi-rings, a distance from the trachea which is, in typical cases, devoid of sounding membranes; some Cuculi, Caprimulgi, and some owls. (3) Syrinx trachealis. The lower portion of the trachea consists of thin membranes, about half a dozen of the rings being very thin or deficient. Inner and outer membranes may exist on the bronchi. The Tracheophonae among the Passeriformes, the possessors of this specialized although low type of syrinx, form a tolerably well-marked group, entirely neotropical. But indications of such a syrinx occur also in Pittidae, pigeons and gallinaceous birds (Gallidae), the last cases being clearly analogous.
Whilst the type of syrinx affords no help in classification, it is very different with its muscles. These—as indicated by their supply from a branch of the hypoglossal nerve, which descends on either side of the trachea—are, so to speak, a detached, now mostly independent colony of glosso-pharyngeal muscles. Omitting the paired tracheo-clavicular muscles, we restrict ourselves to the syringeal proper, those which extend between tracheal and bronchial rings. Their numbers vary from one pair to seven, and they are inserted either upon the middle portion of the bronchial semi-rings (Mesomyodi), or upon the ends of these semi-rings where these pass into the inner tympaniform membrane (Acromyodi). The former is morphologically the more primitive condition, and is found in the overwhelming majority of birds, including many Passeriformes. The acromyodian type is restricted almost entirely to the Oscines. Further, according to these muscles being inserted only upon the dorsal, or only upon the ventral, or on both ends of the semi-rings, we distinguish between an-, kat- and diacromyodi. But the distinction between such Acromyodi and the Mesomyodi is not always safe. For instance, the Tyranninae are anacromyod, while the closely allied Pipras and Cotingas are katacromyod; both these modifications can be shown to have been derived but recently from the weak meso- and oligomyodian condition which prevails in the majority of the so-called Oligomyodi. On the other hand, the diacromyodian type can have been developed only from a strong muscular basis which could split into a dorsal and a ventral mass; moreover, no Passeres are known to be intermediate between those that are diacromyodian and those that are not.
Attempts to derive the anacromyodian and the katacromyodian from the diacromyodian condition are easy on paper, but quite hopeless when hampered by the knowledge of anatomical facts and how to use them. There remains but one logical way, namely, to distinguish as follows:—(1) Passeres anisomyodi, in which the syrinx muscles are unequally inserted, either on the middle or on one end of the semi-rings, either dorsal or ventral. This type comprises the Clamatores. (2) Passeres diacromyodi, in which some of the syrinx muscles are attached to the dorsal, and some to the ventral ends, those ends being, so to say, equally treated. This type comprises the Oscines. Both types represent rather two divergent lines than successive stages, although that of the Clamatores remains at a lower level, possessing at the utmost three pairs of muscles, whilst these range in the Oscines from rarely two or three to five or seven.
This way of using the characters of the syrinx for the classification of the Passeriformes seems simple, but it took a long time to accomplish. Joh. Müller introduced the terms Polymyodi and Tracheaphones, Huxley that of Oligomyodi; Müller himself had, moreover, pointed out the more important characters of the mode of insertion, but it was Garrod who invented the corresponding terms of Acro- and Mesomyodi (= Tracheophones + Oligomyodi). (For further historical detail, see Ornithology). After W. A. Forbes had investigated such important genera as Philepitta and Xenicus, P. L. Sclater, A. Newton and R. B. Sharpe divided the Passeres respectively into Oscines, Oligomyodae, Tracheophonae and Pseudoscines (= Suboscines); Oligomyodae, Tracheophonae and Acromyodae; Oscines, Oligomyodae, Tracheophonae and Atrichiidae. Ignoring the fact that some Oligomyodae are meso- and others acromyodian, they tried to combine two irreconcilable principles, namely, mere numbers against quality.
Bibliography.—M. Baer, “Beitr. z. Kenntniss d. Atemwerkzeuge bei den Vögeln,” Zeitschr. wiss. Zool. lxi. 1896, pp. 420-498; Campana, Physiologie de la respiration chez les oiseaux. Anatomie de l’appareil pneumatique ... (Paris, 1875); A. H. Garrod, “Major Divisions of Passerine Birds (syrinx, &c.),” P.Z.S., 1876, pp. 506-519; and “On the Conformation of the Thoracic Extremity of the Trachea in the Class Aves,” P.Z.S., 1879, pp. 357-380; J. Müller, Stimmorgane der Passerinen, Müller’s Arch. (1847); and Abh. Akad. Wiss. (Berlin, 1845-1847), translation by F. J. Bell, Oxford, 1878; H. Strasser, “Luftsäcke der Vögel,” Morph. Jahrb. iii., 1877, pp. 179-227; C. Wunderlich, “Unterer Kehlkopf der Vögel,” Nov. Act. Leop. Carol., 1884; Ph. C. Sappey, Recherches sur l’appareil respiratoire des oiseaux (Paris, 1847); W. A. Forbes, “Contributions to the Anatomy of Passerine Birds (syrinx),” P.Z.S., 1880, pp. 380-386, 387-391; 1881, pp. 435-737; 1882, pp. 544-546, 569-571; W. Yarrell, “Observations on the tracheae of Birds,” Trans. Linn. Soc., 1827, pp. 378-391.
7. Digestive System.
For a general account of the digestive organs, see Alimentary Canal. Here only a few peculiar features may be mentioned.
The young pigeons are fed by both parents with a peculiar stuff, the product of the strongly proliferating epithelial cells of the crop, which cells undergo a cheese-like fatty degeneration, and mixed with mucus, perhaps also with the proventricular juice, make up a milk-like fluid. Should the young die or be removed during this period, the parents are liable to die, suffering severely from the turgid congestion of the hypertrophied walls of the crop.
The male of the hornbills, Bucerotinae, feeds his mate, which is imprisoned, or walled-up in a hollow tree, during the whole time of incubation, by regorging his food. This bolus is surrounded, as by a bag, by the cast-up lining of the gizzard. Since this process is repeated for many days the habitual reaction of the stomach well-nigh exhausts the male. A graphic account of this is given in Livingstone’s travels.
The hoactzin, Opisthocomus, feeds to a great extent upon the leaves of the aroid Montrichardia or Caladium arborescens. The crop is modified into a large and very rugose triturating apparatus, while the gizzard, thereby relieved of its function, is reduced to the utmost. The large and heavy crop has caused a unique modification of the sternal apparatus. The keel is pushed back to the distal third of the sternum, whilst the original anterior margin of the keel is correspondingly elongated, and the furcula fused with the rostral portion.
In the ostrich, Struthio, the craze of overloading the stomach with pebbles which, when triturated into sand, are not voided, has brought about a dislocation, so that the enormously widened and stretched space between proventriculus and gizzard forms a bag, directed downwards, whilst the gizzard itself with part of the duodenum is rotated round its axis to more than 100°. A similar rotation and dislocation occurs in various petrels, in correlation with the indigestible sepia-bills, &c., which these birds swallow in great quantities. In Plotus, the snakebird, the pyloric chamber of the stomach is beset with a mass of hair-like stiff filaments which permit nothing but fluid to pass into the duodenum. The gizzard of various birds which are addicted to eating hairy caterpillars, e.g. Cuculus canorus and trogons, is often lined with the broken-off hairs of these caterpillars, which, penetrating the cuticle, assume a regular spiral arrangement, due to the rotatory motion of the muscles of the gizzard.
8. Cloaca and Genital Organs.
The cloaca is divided by transverse circular folds, which project from its inner walls, into three successive chambers. The innermost, the coprodaeum, is an oval dilatation of the end of the rectum, and attains its greatest size in those birds whose faeces are very fluid; it serves entirely as the temporary receptacle of the faeces and the urine. The next chamber, the urodaeum, is small, and receives in its dorso-lateral wall the ureters and the genital ducts; above and below this chamber is closed by circular folds, the lower of which, towards the ventral side, passes into the coating of the copulatory organ when such is present. The urodaeum serves only as a passage, the urine being mixed with the faeces in the chamber above. The third or outermost chamber, the proctodaeum, is closed externally by the sphincter ani; the orifice is quite circular. It lodges the copulatory organ, and on its dorsal wall lies the bursa Fabricii, an organ peculiar to birds. It is most developed in the young of both sexes, is of unknown function, and becomes more or less obliterated in the adult. Only in the ostrich it remains throughout life, being specialized into a large receptacle for the urine, an absolutely unique arrangement. A true urinary bladder, i.e. a ventral dilatation of the urodaeum, is absent in all birds. It is significant that the whole type of their cloaca much resembles that of the Crocodilia and Chelonia, in opposition to that of the Lacertilia.
The penis, and its much reduced vestige of the female, is developed from the ventral wall of the proctodaeum. It occurs in two different forms. In the Ratitae, except Rhea, it consists mainly of a right and left united half (corpora fibrosa), with a deep longitudinal furrow on the dorsal side, and much resembles the same organ in crocodiles and tortoises. It is protruded and retracted by special muscles which are partly attached to the ventral, distal end of the ilium. Another type exists in Rhea and in the Anseriformes, greatly specialized by being spirally twisted and partly reversible like the finger of a glove. This is mainly due to the greater development of an unpaired, median portion, analogous to the mammalian corpus spongiosum, which is much less prominent in the Ratitae; the muscles of this type are derived solely from the anal sphincter. In other Carinatae, e.g. tinamous and storks, the penis is very much smaller and simpler, with every appearance of a degenerated organ. In the great majority of birds it has disappeared completely and the primitive way of everting the cloaca is resorted to.
Both right and left testes are functional. They become greatly