ORGANIC DF.VELOPrMENT. ,^;4") 



pulsion, or heart, the construction of which is now 

 commenced, at a central point, by the folding 

 inwards of a lamina of the middle membrane, 

 forming first a simple groove, but, after a time, 

 converted, by the union of its outer edges, into a 

 kind of sac, which is soon extended into a longi- 

 tudinal tube.* The next object is to brin^j; this 

 tube, or rudimental heart, into communication with 

 the neighbouring vascular trunks ; and this is 

 eifected by their gradual elongation, till their cavi- 

 ties meet, and are joined; one set of trunks (the 

 future veins,) first uniting with the anterior end of 

 the tube ; and then another set (the future arteries,) 

 joining its other end. The addition of this central 

 tube to the vessels previously formed completes the 

 continuity of their course ; so that the uniform 

 circulation of the blood is established in the direc- 

 tion in which it is ever after to flow ; and we may 

 now recognise this central organ as the heart, 

 which, under the name of the punclnm sniieiis, tes- 

 tifies by its quick and regular pulsations that it 

 has already begun to exercise its appropriate func- 

 tion. It is long, however, before it acquires the 

 form which it is permanently to retain ; for from 

 being at first a mere lengthened tube, presenting 

 three dilatations, which are the cavities of the 

 future auricle, ventricle, and bulb of the aorta, it 

 assumes in process of time a rounded shape, by the 

 folding of its parts, the whole of whicli are coiled, 

 as it were, into a knot; by which means the dif- 

 ferent cavities acquire relative situations more 



* The discovery of this fact is due to Pander. See also the 

 works of Rolando, Wolflf', Prevost and Dumas, and Serres. 



VOL. II. N N 



