7o6 



HUMAN ANATOMY. 



\ 



ricular appendages, rapidly increase, until they form the most conspicuous part of the 

 young heart (Fig. 673, C), embracing the upper part of the truncus arteriosus and 

 overlying the ventricle. 



Meanwhile the ventricular segment has assumed the most dependent and ventral 

 position, for a time appearing as a transversely expanding sac (Fig. 672, B) that in 

 form recalls a greatly dilated stomach, the truncus arteriosus joining the " pylorus," 

 and the contracted auricular canal suggesting the oesophagus. Soon, however, the 

 higher right end of the ventricular segment sinks to the level and gains the ventral 

 plane of the left end, the \'entricle in consequence losing in width but gaining in 

 height. A shallow longitudinal crescentic furrow, the later interventricular groove, 

 now appears on the surface of the ventricle and suggests the subdivision of this seg- 

 ment into right and left 

 Fig. 669. chambers, at the same 



time indicating the posi- 

 tion of the growing inter- 

 nal partition that leads to 

 this separation. 



Sections of the young 

 heart ( Fig. 670 ) show 

 the venous and ventricular 

 segments as widely com- 

 municating portions of the 

 sigmoid tube, the walls of 

 which are composed of the 

 myocardial and endothe- 

 lial layers. In somewhat 

 older embryos (Fig. 671), 

 the communication between 

 these divisions of the heart- 

 tube exhibits a slight con- 

 traction, marking the posi- 

 tion of the later auricular 

 canal, which becomes a nar- 

 row transverse cleft that 

 connects the primitive ven- 

 tricle with the auricular 

 chamber. The myocar- 

 dial layer of the heart- 

 wall, particularly in the 

 ventricle, also shows the 

 beginning of the trabeculae 

 that invaginate the endo- 

 thelial lining and event- 

 ually lead to the conspicu- 

 ous modelling of the 

 interior of the adult heart. 

 Frontal sections of the 

 young human heart (Fig. 674, A') show the commencing separation of the ventricular 

 and auricular chambers into right and left halves. This division is effected by the 

 formation of a vertical partition consisting of an upper auricular, a middle valvular, 

 and a lower ventricular pant, supplemented by the aortic septum that appears in the 

 truncus arteriosus and subdivides the latter into the pulmonary and systemic aortse. 

 The subdivision of the auricle, which anticipates that of the ventricle, begins in 

 the fourth week with the downward extension of a crescentic fold, the auriczilar sep- 

 tum, or septiim primiim, that gradually grows from the postero-superior wall of the 

 auricle towards the auricular canal and fuses with the partition that, as the septum 

 intermedium, is formed within the canal by local thickening of its anterior and pos- 

 terior lips. In this manner not only the common auricular chamber, but also the 

 transversely elongated auriculo-ventricular opening, is separated into a right and a 



First 

 , visceral 

 furrow 



First 

 aortic bow 



Anterior 

 cardinal 

 vein 



Dorsal 

 aorta 



Primitive 

 auricle 



Right duct 

 of Cuvier 



Posterior 

 cardinal 

 vein 



Maxillary 

 process 



Mandibular 

 process 



Truncus 

 arteriosus 



Primitive 

 ventricle 



Pericardial 



sac, cut 



Left umbil- 

 ical vein 



Left vitel- 

 line vein 



Right umbilical vein 



Right vitelline vein 



Reconstruction of upper part of human embryo of third 

 week (3.2 mm.), showin.a: relation of heart and blood-vessels. 

 X 50. {Drawn from His model.) 



