INTRODUCTION. 15 



arrangement is found ; for in those birds the/, hallucis splits up into three partsj sup- 

 plying the second and fourth digits as well as the first one (hattux), whilst the 

 f. perforans is distributed to the third digit alone. 



Another set of interesting muscles are those belonging to the organ of voice, in 

 connection with which they will be considered. 



It would take us too much space, should we account for all the various modifications 

 of the digestive system, especially because we do not yet understand its development 

 nor the taxonomic value of the modifications ; we do not know what is essential, what 

 accessory, what original, and what derivative. For not only do we find extreme differ- 

 ences in the structure of the intestines between very closely allied forms in cases where 

 the disagreement can be accounted for by the difference in the diet, as in the sage- 

 cock ( Centrocercus urophasianus\ with its thin-walled stomach, in contradistinction 

 to the structure of the same organ in the other members of the family, as first pointed 

 out by Mr. Ridgway, but radical structural differences obtains often in two closely 

 allied species, the habits and food of which are not known to differ at all. A striking 

 example is the structure of the stomachs of the American and the African anhingas 

 (f lotus anhinga and levaillantii), as demonstrated by Prof. Garrod. In the former, 

 the proventriculus, instead of forming a zone or path, is developed into a special sac- 

 like diverticulum, which projects from the gizzard externally in a way quite unlike 

 that of any other bird. Moreover, the pyloric compartment develops a covering of 

 hairs, a peculiarity only found in one other bird, viz., the turkey buzzard, and Prof. 

 Forbes remarked that "this very extraordinary stomach is certainly, as far as yet 

 [1881] known, unique amongst birds." The African species has a stomach consider- 

 ably different from that described above, as the proventriculus forms no gland-pouch, 

 but simply two separate patches. A well-developed and hair-clad pyloric compart- 

 ment is present, as in the former, but " the hairy epithelium surrounding the pyloric 

 orifice is produced into a considerable conical, hair-covered process, projecting into the 

 second stomach, and evidently acting as a valve to close the pylorus when necessary." 



Similar differences occur also among the pigeons, of which the genus Ptilopus has 

 the gizzard provided with "four crushing-pads, instead of two, as in all other birds, 

 including even Treron" Of the genus Carpophaga, two species, latrans and goliath, 

 have the epithelial lining of the gizzard developed into a number of bony conical 

 processes, like the spines of certain sea-urchins, w r hile no other species of the genus 

 are known to show any trace of such a structure. 



The birds are the first class of existing vertebrates with a complete double circu- 

 lation, a four-chambered heart, with two entirely separate halves, and a blood of a 

 temperature considerably higher than that of the surrounding atmosphere, ranging as 

 it does from 100 to 112 Fahr. We say "existing vertebrates," for there seems to be 

 reason to suppose that the Pterosaurians, the remarkable extinct group of flying 

 reptiles, also had hot blood, and we said " considerably higher 'than that of the sur- 

 rounding atmosphere," because there are well-known examples of fishes and reptiles, 

 the temperature of which is higher than the medium they live in, though not to such 

 a degree as in birds and mammals. Only a single permanent aortic trunk carries the 

 blood from the heart, not two as in reptiles ; but contrary to what takes place in mam- 

 mals it is the right aortic arch which remains. Of special interest is the arrangement 

 of the carotids, which carry the arterial blood to the head and neck, since their 

 arrangement is widely different in different birds. Without going into detail we may 

 say that the chief difference consists in the absence or presence of the right carotid. 



