IXTJiUULCTJOX. 15 



.iiTaiiGjement is found; for in those binls the/. Iiallitcis splits u]. into three jmrts, .sti]i- 

 jilyins;- the second, third, ;ind fniirlli diL;its us irc/l as the lirsl one {/ni/lit.r), whilst the 

 /. per/urct/is is distributed to the third dij:;it alone. 



Another set of interesting niuseles are those lielonging to the organ of voice, in 

 connection with which they will he consi<lered. 



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

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

 nor the ta.Yonomic value of the modifications; we do not know what is essential, what 

 accessory, wliat original, and wliat derivative. For not only ilo we find extreme differ- 

 ences in the structure of the intestines between very closely allied forms in ca.ses where 

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

 cock (Ceiitrocerciis urnphdsiitiiKs), with its thin-walled stomach, in contradistinction 

 to the structure of the same organ in the i)ther members of tiie family, as first ]ioiiiteil 

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

 allied species, the liabits ;uii| food of which are not known to difTer at all. A striking 

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

 (Plotus anhiiuja and lei-ailltnUii), as demonstrated by Prof. Garrod. In the former, 

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

 like diverticulum, which ])rojects from the gizzard externally in a way ipiite unlike 

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

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

 Forbes remarked that " this very e.xtraordinary stomach is certainly, as far as yet 

 [ISSl] known, unique amongst birds." The African species has a stomach considei'- 

 ably diltVrent from that descril)ed above, as the i)roventriculus forms no gland-]ioucli, 

 but simply two se2)arate patches. A well-developed and hair-clad pyloric comj)art- 

 luenl is jiresent, as in the former, but "the hairy epithelium surrounding the pyloric 

 orifice is iiroduced into a considerable conical, hair-covered jirocess, projecting inti> the 

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



Similar differences occur also ann)ng the ]iigeons, of which the genus J'tifojuis h;is 

 the gizzard jirovided with '•\foitr crushing-pads, instead of two, as in all other birds, 

 including even Trero>i." Of the genus Carpophagu, two species, latruns and yolkith., 

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

 processes, like the si)ines of certain se.a-urchins, while no other species of the genus 

 are known to show .any trace of such a structure. 



The l)irds 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 s:iy "existing vertebrates," for there .seems to be 

 reason to suj>pose that the Pterosaurians, the remarkable extinct grou|> of flying 

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

 rouniling 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 sucli 

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

 blood from the heart, not two as in re]>tiles; but contrary to what takes jilace in mam- 

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

 of the carotid.s, which carry the arterial blood to the he.nl .and neck, since their 

 arrangement is w idely difTerent in diflerent birds. Without going into detail we may 

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



