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The Brachial Arterial Arches in Birds. [June 16,. 



vessel, runs towards the head in the adult bird, upon the dorsal aspect 

 of the alimentary canal, and distributes intervertebral branches as it 

 goes. Rathke believed that this artery was originally ventral in 

 position, and passed gradually round the oesophagus in the course of 

 growth until it "finally reached the dorsal aspect ; but the author 

 points out that while the artery does change its position somewhat,, 

 it is to a much more limited extent than Rathke believed. In the 

 chick of the third or fourth day, the main vessel for the supply of the 

 head is the dorsal continuation from the third arch, the ventral 

 vessel being small. The examination of the further development by 

 sections and dissections makes it evident that the ventral vessel 

 dwindles in importance, and becomes finally a small branch passing 

 from the subclavian to the ventral aspect of the trachea, while the 

 dorsal prolongation becomes the sole supply of the head. At the 

 close of the sixth day the dorsal connexion between the ends of the 

 third and fourth arches is still present, and it is continuous with the- 

 common carotid, which may be easily followed to the head as a dorsal 

 vessel, and its external branches, as well as its internal, are therefore 

 to be regarded as derivatives of a dorsal stem. In the adult the 

 common carotid lies in the middle line of the neck in contact with its- 

 fellow of the opposite side, but in the embryo the vessels of opposite- 

 sides are at some little distance from one another ; by the seventh day,, 

 however, they have approached one another so as to be almost in con- 

 tact. This change, however, is not, as Rathke supposed, the passage 

 of a ventral vessel to a dorsal position, but a slight alteration in the 

 line of a vessel already dorsal. 



When the carotid system of different groups of vertebrate animals 

 is examined, it is found that in those forms where, on account of the 

 preservation of the dorsal connexion between the third and fourth 

 arches, the continuous dorsal longitudinal vessel can be traced, the 

 branches for the supply of both internal and external aspects of the 

 head arise from this dorsal vessel. In these forms the ventral pro- 

 longation supplies only the tongue. This is the case in most lizards. 

 As the higher stages are reached the disappearance of the portion of 

 the dorsal vessel between the ends of the third and fourth arches 

 makes a comparison of the vessels uncertain, but the author has dis- 

 covered in the crocodile, and as an abnormality in a guillemot, solid 

 cords stretching between the common carotids and aortse on the dorsal 

 aspect of the alimentary canal. In these cases it is seen that the 

 greater part of the common carotid is to be regarded as the dorsal 

 prolongation from the third arch towards the head. The ventral 

 vessel in birds and crocodiles is not, therefore, the external carotid, as^ 

 Rathke has it, but an artery running upon the trachea and supplying 

 branches to the muscles on the ventral surface of the neck. 



