ANATOMY OF A BIRD. 251 



by their vibration produce the notes of the human voice, are altogether and always absent from the 

 larynx ; in other words, the vocal organ is not the larynx, but an organ seated at a lower level, and 

 known as the syrinx. This instrument may, further, be formed in the trachea alone (as in some 

 American Passerines), or in the bronchi alone (as in Steatornis), or at the point at which the tracheal 

 and bronchial tubes pass into one another (as in the majority of singing birds). 



The last-mentioned, or bronchio-tracheal syrinx, consists of the following parts; (i.) a tympanic 

 chamber formed by the union of some of the lower rings of the trachea ; (ii.) a membranous septum 

 separating from one another the tracheal orifices of the two bronchi ; (iii.) on either side a tympaniform 

 membrane, formed on the inner side of the uppermost bronchial rings ; in consequence of this these 

 bronchial rings are not complete circles ; their mucous membrane is developed into a fold which bounds 

 one side of a cleft which is formed by the presence on the other side of the above-mentioned tympani- 

 form membrane. The air which passes through these bronchial clefts sets in vibration the membranes 

 which bound them, while the character of the note is affected by the position of the bronchial half-rings, 

 and the length of the column of air in the trachea. These rings have their positions changed by five 

 lateral muscles, which act on their ends, and so rotate, them. The principle variations in the characters 

 of the muscular supply of the organ of the voice were long ago worked out by Johannes Miiller, the 

 famous German anatomist and physiologist. 



It is also to this observer that we owe our first information with regard to the bronchial syrinx 

 of Steatornis ; the anatomy of this animal was also investigated by the late Prof. Garrod, who gave 

 the following account of its vocal apparatus : " Each semi-syrinx, as it 

 may be termed, is formed on the same principle as that of the combined 

 organ in most of the non-singing birds. Taking for description that of 

 the left side, it is found that the thirteenth bronchial ring is complete, 

 though considerably flattened from side to side ; the fourteenth is not 

 complete in the middle of its upper surface ; it is a little longer from 

 before backwards than the one above, and not so long as the one 

 following it. The fifteenth is only a half ring, its inner portion being 

 deficient ; it is slightly convex upwards, and articulates, both at its 

 anterior and posterior ends, with the fourteenth incomplete ring and the 

 sixteenth half-ring. The sixteenth half-ring is concave upwards, and so 

 forms an oval figure in combination with the one above, which is filled 

 with a thin membrane to form part of the outer wall of the bronchus. 

 There is a membrane also between the ends of these and the succeeding 

 half-rings, which completes the tube of the bronchus internally." (After 



The ducts from the urinary organs open to the exterior through the 



cloaca, into which, as already mentioned, the digestive tube also opens. The chief point with regard 

 to the urinary secretion of birds is the fact that it is semi-solid, and that it contains a quantity of the 

 substance known as uric acid. The kidneys are placed some way back and near the cloaca ; they 

 are set on either side of the spinal column, between the transverse processes of the sacral vertebrae, 

 and are generally divided into three portions of greatly varying size. On their inner edge are given 

 off the ureters, which pass on each side to enter separately into the before-mentioned cloaca. 



The right ovary of birds is always atrophied, and it is in rare cases only that rudiments of it are 

 found (namely, in the diurnal Raptores). The oviduct is a coiled canal, the lower portion of which 

 has strong, muscular walls, while internally the characters of its surface vary according to the 

 substance which the glands of different regions add to the descending egg. The right oviduct is not so 

 completely atrophied as is the ovary of the same side. This duct opens into the cloaca through 

 which the egg passes to reach the outer world ; as further development is so largely independent of 

 the mother, the female organs offer no peculiarities of arrangement, or complexities of structure. 



All birds lay eggs, or, in other words, the bom young are not carried about by the mother 

 till the time of birth. The advantage of this to a flying animal is so obvious that we may pass 

 at once to describe the egg of a common fowl The shell, which consists of organic matter and lime- 

 salts, is found to be formed of two layers; it is in the outer one only that pigment is found. 

 Both layers are traversed by canals, through which air can pass only when the shell is dry; 



