1868.] 



Valves in the Heart of the Chick. 



331 



and examined by strong transmitted light, and were afterwards mounted 

 in glycerine jelly. 



The new facts observed demonstrate — 



(1) The manner in which the trunciis artet'iosus drvides into two vessels, 

 which is different from that commonly supposed to occur. 



(2) The close connexion between this process of division, and the forma- 

 tion of the semilunar valves of the aorta and pulmonary artery, and their 

 place of origin and mode of development. 



The following is a brief account of the manner in which the division of 

 the tnincus arteriosus takes place. It should be said that about the third 

 day of incubation, just before the division begins, the somewhat spirally 

 twisted truncus arteriosus is everywhere smooth, and free from ridges 

 on its interior, and ends abruptly in the three pairs of branchial arteries 

 which then exist. These are the third, fourth, and fifth pair. There is 

 no valvular apparatus at its branchial end, but next the ventricle the defi- 

 ciency of valves seems to be supplied by a considerable development of the 

 elastic wall of the truncus arteriosus on its two opposite sides, so that the 

 ventricular aperture, which is at first circular, becomes slit-shaped. The 

 two lips of the slit seem to prevent in great measure the reflux of blood 

 into the ventricle, before the semilunar valves are sufficiently developed to 

 do so. 



The division of the vessel commences about the 106th hour of incuba- 

 tion, at rather less than one-fifth of the whole period of incubation, which 

 is 21 days. 



It begins at the branchial end of the truncus arteriosus by the ex- 

 tension into it of a plane septum growing horizontally downwards into 

 the vessel from the terminal arterial wall between the openings of the 

 fourth and fifth pair of branchial arteries. Its lower margin is forked, so 

 that it extends further along the sides than along the centre of the vessel, 

 and it is inclined a little obliquely across the vessel, sloping downwards 

 from left to right. The little channel in front of this septum leads to the 

 third and fourth pair of branchial arteries, and is the rudimentary aorta ; 

 the channel behind it leads to the fifth pair of branchial arteries, and is 

 the rudimentary pulmonary artery. 



At the same time, or slightly before this, the canal of the vessel just be- 

 low the septum becomes constricted by the formation — 



(1) On its anterior and left surface, of two flattened prominences, sepa- 

 rated by a groove. These are the rudiments of the anterior semilunar 

 valve of each artery. 



(2) On its anterior and right surface, of a flattened ridge, extending 

 obliquely across the vessel nearly opposite to the anterior valve rudiments, 

 afterwards becoming prominent and pyramidal in the centre, and extending 

 gradually down the posterior surface of the vessel. The right and left ends 

 of this ridge are the rudiments of the inner semilunar valves of each artery. 

 As these growths enlarge, the forked septum grows downwards into the 



