56 



divided into two; but in single-hearted animals this primitive heart 

 is undivided. There exists, they say, a transitory bulb, divided in 

 mammals and birds, and single in batracians and fishes. The per- 

 manent heart appears first beneath the transitory left ventricle as a 

 permanent left ventricle. The permanent right ventricle forms 

 later, beneath the transitory right ventricle. The transitory 

 bulb is a part of the heart, different from the permanent bulb 

 of the aorta, which is formed later. There are two primitive 

 aortge (as was observed by Serres). The permanent one is formed 

 between these, which disappear, and the descending portion of the 

 permanent aorta results from two short vessels which leave the bran- 

 chial sinuses at the place where the primitive aorta are detached, 

 and blend in front of the median line. The portion of the permanent 

 aortas, which curves on leaving the bulb, joins the descending aorta 

 beneath the junction of its roots; and not, as is believed, at the level 

 of the third branchial arch. And in the transitory bulb are formed 

 two vessels which join to form the branchial vessel, from which the 

 arteries of the branchial arches part. 



16. Theory of Respiration. — Prof. J. W. Draper* thus describes 

 the cause of the circulation: 



Water will rise, in a capillary tube, to a certain height. Break 

 the tube below this point and the water will rise to the top, but not 

 flow over. Now suppose a rapid evaporation to ensue from the 

 upper end of the tube, and the water from below will continue to 

 rise and supply the tube. The same will occur if the liquid is com- 

 bustible and burns, as in a lamp. 



From these facts he deduces the following principle : That though 

 ordinary capillary attraction cannot determine a continuous flow of 

 liquid through a tube, yet there are many causes which may tend 

 to produce that result. "If a given liquid occupies a capillary tube, 

 or a porous or parenchymatous structure, and that, whilst in that 

 tube or structure, changes happen to it which tend continually to 

 diminish its attraction for the surface with which it is in contact, 

 movement will ensue in a direction from the changing to the changed 

 fluid." 



The principle so stated he thus applies to the different systems of 

 circulation in the body: 



1st. To the Systemic Circulation. — The arterial blood contains 



* London, Dublin, and Edinburgh Magazine, January, 1S1G. 



