329 Dr Allen Thomson on the Vascular System 



large proportionally to the rest of the body, and projecting from 

 the fore part of the breast. The ventricle, strong and thick, 

 resembled somewhat in its form that of the adult. As yet it 

 consisted of a single cavity, but the septum which divides the 

 right from the left ventricle had begun to be formed (Fig. 9). 



The farther changes which the heart of animals belonging to 

 the class Mammalia undergoes have been investigated by Meckel*, 

 principally in the human embryo ; and by my friend Mr Owen f , 

 in the embryo of the pig and sheep. From their observations, it 

 appears, that, in the human embryo of about four weeks old, and 

 at a corresponding period in other mammiferous animals, the sep- 

 tum begins to be formed between the ventricles, and the auricle 

 is slightly divided into two cavities. The right ventricle (Fig. 

 10. *'), is at first very small, and is situated at the base, and on 

 the right side of the left ventricle. The bulb of the aorta (<'), 

 appears like a large vessel arising from the ventricle, and com- 

 municating with both its compartments. The auricles are pro- 

 portionally very large, and are situated behind the ventricle (r r^). 



Some time after the septum of the ventricles begins to be 

 formed in the interior, there appears a corresponding notch on 

 the exterior, which, as it gradually deepens, renders the apex 

 of the heart double, (n. Figs. 11, 12, & 14). As development 

 proceeds, the ventricular part of the heart becomes longer, and 

 the capacity of its right compartment proportionally increases. 

 The notch between the right and left ventricles continues to 

 become deeper till about the eighth week, when the two ventri- 

 cles are quite separated from one another, except at their bases, 

 where they are attached (Fig. 15.), and where, by the deficiency 

 of the internal septum, these cavities still communicate with one 

 another, as in the chick on the fourth day. It is an interesting 

 fact, that this state of separation is to be found permanent in 

 some adult mammalia, as in the Dugong. 



After the eighth week, the. septum of the ventricle is com- 

 pleted, so that they no longer communicate with one another : 

 the external walls of these cavities at the same time become at- 



• Meckel's Archiv. b. ii. p. 402 ; and Journal Complement, tom. i. 

 •f The Assistant Curator of the Hunterian Museum, London, who has 

 kindly made me acquainted with the results of his observations. 



