332 



AVES. 



ago that the extremities of the semilunar valves 

 in Birds were connected to small, firm, and 

 sometimes ossified styles imbedded in the fibrous 

 co;it 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, 

 (Jig. 169.) 



Arteries. the distribution of the arterial 

 system has been described in a general man- 

 ner by Cuvier, Tiedemann, and Nitzsch, and 

 has subsequently been very completely elu- 

 cidated by Barkow in the 12th Volume of 

 Meckel's Archives of Physiology ; where the 

 different varieties which various species of birds 

 present in the course of individual arteries 

 are laboriously pointed out. Tn our own dis- 

 sections we have been guided by the excellent 

 description long ago given by Dr. Macartney, 

 (Art. Birds, llees' Cyclopaedia) which we shall 

 here give verbatim, with some general remarks 

 and additional particulars afforded by the re- 

 searches of Barkow and our own dissections. 

 The description will be aided by the subjoined 

 beautiful figure taken from Barkow's Mono- 

 graph (Jig. 170). 



The arterial system in Birds is essentially 

 distinguished from that of Mammals by the 

 following differences : 



1st. The division of the aorta into three 

 principal branches, almost immediately at its 

 origin. 



2d. The course of the arch of the aorta over 

 the right instead of the left bronchus to become 

 the descending aorta. 



3d. The origin of the arteries of the posterior 

 extremities, which do not come off from a single 

 branch analogous to the external iliac of Mam- 

 malia, 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 apertures. 



The arteries of the systemic circulation 

 proceed, Macartney observes, "from a single 

 trunk, which arises from the left ventricle of 

 the heart. This trunk, the aorta, (1, fig. 170) 

 is so short that it is concealed by the other 

 parts on the basis of the heart, and is only 

 brought into view after the reflections of the 

 pericardium and the adjoining vessels are de- 

 tached by dissection. It is from thence that 

 as the parts are commonly beheld, there appear 

 to be three great arteries issuing together from 

 the middle of the heart, which are the primary 

 branches into which the aorta is divided. The 

 first branch is to the left side, and after it is 

 sent off, the trunk affects to turn over the au- 

 ricle before it gives the branch of the right 

 side; these two branches pass in a curved 

 manner from the heart towards the axilla in 

 the form of horns, and each is analogous to 

 the arteria innominata of the human subject, 

 so that instead of one there may be reckoned 

 two arteria innominata in Birds (t t, Jig. 167, 

 1 68). After these branches are parted with, the 

 arterial trunk (s, fg. 167, 168, 2, Jig. 170) is 

 continued over the auricles," and the right bron- 



Fig. 170. 



Arteries of the Trunk, Grebe. 



chus, " and, on reaching the back part of the 

 heart, becomes the descending aorta. 



" The arteria innominata (3) first sends off 

 the common trunk of the carotid and vertebral 

 arteries (4), which before its division gives off 

 one or two small branches ; one of these runs 

 down upon the lungs in company with the par 

 vagum, and appears to supply branches to the 

 aponeurosis of the lungs, and the air-cells at 

 the upper part of the thorax; the other branch, 

 aft^r supplying the lymphatic gland of the 

 neok 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 and integuments of the side of 

 the neck, its branches are distributed, anasto- 

 mosing with the superior cesophageal and tra- 

 cheal arteries. This branch is often not sent 

 off until the trunk divides into the vertebral and 

 carotid, in which case it comes from the latter 

 artery. Sometimes in the Duck, the supra- 

 scapular artery, which is usually divided from 

 the vertebral, is a branch of the common trunk." 



The carotid arteries (4, 4, Jig. 170, u u, Jig. 

 167) are frequently of unequal size; in the Dab- 

 chick the left is by much the largest; in the 

 Emeu we found it the smallest. " In the Com- 

 mon Fowl, each carotid, after parting from the 

 vertebral artery (6), proceeds to the middle of the 

 neck and soon disappears ; becoming covered by 

 the muscles of the anterior part of the neck, and 

 entering the canal formed by the inferior spinous 



