16 NATURAL HISTORY OF BIRDS. 



The comparatively smaller number of birds possess the original arrangement of two 

 distinct carotids, one right and one left, since in all the Passeres and a number of 

 other groups the left only is preserved, which, however, branches off before reaching 

 the head, thus performing the duty of both carotids. So radical this difference may 

 seem at first sight, so unreliable are the characters furnished by it as indicating relation- 

 ship, that it is altogether out of the question to use it as a means of primary division. 

 For, while it is true that all Passeres that is to say, all that have been examined, 

 and many are still to be investigated-- have only the more specialized arrangement 

 indicated by the presence of the left carotid only, we find in other groups nearly 

 related forms, with one or two carotids, as, for instance, among the auks, the parrots, 

 and the herons. In nearly all birds the crural artery is derived from the sciatic, 

 and the chief vein of the legs, the femoral ; and only in a few passerine forms, the 

 Pipras and the Cotingas, is the artery of the thigh formed by the femoral artery. 

 During incubation the vessels of the abdominal wall dilate enormously, forming the 

 so-called brood-organ. The blood corpuscles of birds are, on the average, of a size 

 twice those of man, and the shape of the red ones is oblong as are those of reptiles, 

 while in most mammals they are round. 



Very characteristic, though not absolutely peculiar to birds, as we have seen above, 

 is their pneumacity, several of their bones being hollow, and connected by openings 

 with air-sacs, which again communicate with the lungs ; by this, air is distributed all 

 through the body, even to the interior of the bones. The enormous importance of 

 this feature to creatures destined to inhabit the air will be readily understood when 

 we learn that a bird with a specific gravity of 1.30 may have this reduced to only 1.05 

 by pumping itself full of air. The lungs themselves are two rather large sacs wedged 

 in around the vertebrae and the heads of ribs, not free, nor enclosed in a pleura, as in 

 mammals. The voice of birds is generally thought not to be formed in the larynx, as 

 it is in mammals, but in a separate, and to the class quite peculiar, " lower larynx," the 

 so-called syrinx, usually situated at the lower end of the trachea, or between it and 

 the bronchi, though the correctness of this view concerning the formation of the 

 voice has been recently seriously questioned. The syrinx consists of a modification of 

 the cartilaginous and coalescent rings, forming a tympanic chamber, in the middle of 

 which occurs a vertical membranous fold, the free edge of which is called the semilunar 

 membrane, while on each side is attached another free-edged membrane ; the voice is 

 formed by the air causing these membranes to vibrate when forced out through the 

 slits between the central and the lateral membranes. Intrinsic muscles run from the 

 trachea to the bronchial rings, and are supposed to serve in varying the tension of the 

 membrane. The peculiar arrangement of these muscles, and their importance to 

 systematic ornithology, will be more fully treated of under the introduction to the 

 order Passeres. The syrinx is not absent in any known bird, though somewhat rudi- 

 mentary in some Struthious birds, and still more so in some of the Cathartidse. 



The anatomical investigations of later years have added very little to our knowledge 

 of the neural system of birds and of the organs of sense, having been directed mostly 

 to those features which seemed to promise greater results in the study of the affinities, 

 the morphological development, and the systematic arrangement, thus leaving nothing 

 of general interest to be added to what is contained in the ordinary text-books. 



There is another question which is just now occupying the studies and thoughts 

 of ornithologists, and which therefore cannot be passed by in the present work, namely 

 the question of the migration of birds. 



