of Human Monstrosity. 243 



the aorta, and by the communicating branch on either side, fills the 

 arteries of the head. This third pair becomes the artery for the 

 neck and arm, and its communication with the bulb of the aorta 

 disappears also : the stream from the aorta then flowing in the 

 fourth and fifth pairs from the heart. The next that disappears 

 is the fifth branchial artery on the left side. Whilst this process 

 is taking place, the septum of the heart is forming, from the 

 apex upwards : the last part of the septum which is completed 

 being that immediately under the bulb of the aorta. It is a 

 consequence of this formation, that as the heart contracts it 

 sends the blood into the bulb, in two separate currents. These 

 currents surround each other in a spiral form as they advance 

 into the aorta, which arises indifferently from the two cavities of 

 the heart. The current from the right cavity, fills the fourth 

 branchial artery of the left side, and the fifth of the right side, 

 and these afterwards become the pulmonary arteries, with the 

 exception of small branches of communication with the root of 

 the aorta, which are the two ductus arteriosi of birds. From 

 the left side of the heart, the current of blood is directed to 

 the fourth branchial artery of the right side, which becomes the 

 descending aorta. This metamorphosis prevails in mammalia, 

 though with modifications which are not so well understood. 

 In the second month of the human embryo according to Bur- 

 dach, there are found two principal arterial stems, an ascending 

 stem to the head, a descending stem to the body. Of these, 

 the first appears to be the root of the third pair of branchial 

 arteries, with the subdivisions of those anterior to it, whose 

 roots have disappeared. Its blood comes from the left cavity of 

 the heart. The descending branch of the aorta comes from 

 the right cavity, and is composed of the remains of fourth and 



H II 2 



