318 Dr Allen Thomson on the Vascular System 



the dilatation of a small vessel which adheres to the side of the 

 primitive ventricle. Rolando informs us that this vessel may 

 be perceived so early as at the 60th hour ; that it is connected 

 with the common auricle exactly opposite to the place where the 

 veins enter that cavity, and that, winding round the side of the 

 left ventricle, it reaches the bulb of the aorta, where it appears 

 to terminate. According to Rolando, this vessel is quite trans- 

 parent, and does not contain any blood at the 60th hour ; but, 

 towards the 68th or 70th hour, it is dilated in its middle part 

 by the influx of blood from the auricle, and it is then easily 

 seen applied closely to the base of the left ventricle, with which, 

 soon afterwards, it becomes firmly united by muscular fibres 

 that are deposited round both cavities. 



Prevost and Dumas, in their account of the formation of the 

 heart, not having been able to perceive the right ventricle be- 

 fore the end of the fourth day, have adopted the description of 

 Rolando, while at the same time they state that they have not 

 seen the appearances which he has described. 



The observations of Baer, on the other hand, lead him to be- 

 lieve, that the right ventricle is produced by the growth of a 

 partition in the interior of the primitive ventricle, by which it 

 is separated into two compartments. This partition, according 

 to Baer, commences at the apex or projecting part of the heart, 

 and, gradually increasing in size, extends itself from thence to 

 the upper or basilar part. The rudimentary partition may be 

 seen on the third day, or about the 60th hour, when it presents 

 the appearance of a dark stripe on the convex aspect of the 

 heart. On the fourth day, owing to the more rapid deposition 

 of muscular fibres, and the consequent opacity of the parietes of 

 the heart, the ventricular part seems externally to consist of a 

 single cavity only ; but, on examining its interior with care, the 

 division into right and left ventricles is sufficiently obvious : the 

 partition appearing like a fold arising from the inner wall of the 

 cavity. The same author has shewn that the partition between 

 the right and left ventricle remains for some time incomplete, and 

 allows these cavities to communicate with one another, as well 

 as with each of the auricles, along its free edge. Baer has ob- 

 bcrved that the constriction of the paries of the auricle, by which 



