HEART OF BIRDS. 189 



apex of the heart. The aorta is provided with three semilunar 

 valves, not -with two only as in Reptiles. The extremities of these 

 valves are connected to small, firm, and sometimes ossified styles 

 imbedded in the fibrous coat of the vessels.^ 



The arrangement of the muscular fibres of the ventricle in 

 Birds is such that the right ventricle appears to be formed by a 

 partial secession of the outer from the inner layers of the parietes 

 of the left ventricle at the anterior and right side of that cavity. 

 See the transverse section, fig. 92. 



§ 154. Arteries of Birds. — The arterial system of Birds 

 mainly differs from that of Mammals in the follomng points : — 

 The division of the aorta into three principal branches, almost 

 immediately at its origin : the course of the arch of the aorta 

 over the rio^ht instead of the left bronchus to become the descend- 

 ing aorta : the basilar artery being formed by the entocarotids, 

 not by the vertebral : the great length of the common carotid, 

 which is a single median trunk in some birds: and the origin 

 of the arteries of the posterior extremities, which do not come off 

 from a single branch, or ^ external iliac,' but from two arteries 

 which are detached successively from the aorta at a great distance 

 from each other, and pass from the pelvis by two separate aper- 

 tures. In these differences the closer affinity of Birds to Reptiles 

 is shoAvn. 



The aortic trunk, fig. 93, i, is so short that it is only brought 

 into view after the reflections of the pericardium and the adjoining 

 vessels are detached by dissection. The first branch is to the left 

 side and, after it is sent off, the trunk afifects to turn over the 

 auricle before it gives the branch of the right side ; thes^ two 

 branches pass in a curved manner from the heart towards the 

 axilla, and may be regarded as two arferice innominatce, fig. 90, t, t. 

 After these branches are parted with, the arterial trunk, ib. s, fig. 

 93, 2, is continued over the right bronchus, and, on reaching the 

 back part of the heart, becomes the ' descending aorta.' 



The arteria innominata, fig. 93, 3, first sends off the common 

 trunk of the carotid and vertebral arteries, ib. 4, which before its 

 division gives off one or two small branches ; one of these runs 

 down upon the lungs in company A\4th the par vagum, and ap- 

 pears to supply branches to the aponeurosis of the lungs, and the 

 air-cells at the upper part of the tliorax ; the other branch, after 

 supplying the lymphatic gland of the neck with several small 

 arteries, ascends upon the side of the oesophagus, to which, and 

 the inferior larynx, the divisions of the trachea, and to the parts 

 ' Home Clift, in vir. p. 331. 



