'694 



HUMAN ANATOMY. 



arch ; (5) the internal iliac artery through most of its course ; (6) in the female the 

 uterus and the ovaries. If the hand will enter the sigmoid flexure, most of the abdo- 

 men may be explored. 



Examination through the rectum by this method is distinctly dangerous trom 

 the risk of laceration of the gut. It is therefore not in much favor. 



DEVELOPMENT OF THE ALIMENTARY TRACT. 



Reference to the cross-section of a young mammalian embryo (Fig. 1428) shows 

 the early relation between the primitive gut and the yolk-sac, of which latter the 

 former is evidently a part. The longitudinal section of a very young human embryo 

 (Fig. 46, page 39) emphasizes the wide communication between the two. The 

 differentiation of the gut from the yolk-sac is accomplished by the approximation 

 and union of the two splanchnopleuric folds which consist of the entoblast internally, 

 continuous with that of the yolk-sac, and the visceral layer of the mesoblast exter- 

 nally. As the union of the splanchnopleurse proceeds, the gut-tube becomes closed 



FIG. 1428. 



Neural tube 



Amniotic sac 



Amiiion 



Vitelline vein 



Uodyovity / 



Myotome 



Notochord 

 \\^Primitive aorta 



Body-cavity 



Vitelline vein 



>\'n Open gut-tube Visceral mesoblast 



Transverse section of early rabbit embryo, showing differentiating gut-tube still communicating with vitelline 



sac. X 80. 



throughout its cephalic and caudal segments, between which, however, it remains 

 open and connected with the yolk-sac by a communication that rapidly narrows and 

 elongates into the vitelline or umbilical duct, a structure that for a considerable time 

 remains as a canal bearing the diminishing yolk-sac or umbilical vesicle at its outer 

 end. The primitive digestive tract, therefore, is closed both anteriorly and pos- 

 teriorly, and soon may In- divided into three segments : the fore-, mid-, and hind-gut. 

 Formation of the Mouth. The cephalic segment, the fore-gut, is somewhat 

 dilated at its anterior extremity, and there constitutes the fin'mitnr />h<u-\-n.\\ which at 

 IIIM i^ separated from a bay-like depression, the oral recess (jttomodentm}^ which 

 mi-.inuhile ha-, U <-n tormed by the downward flexure of the anterior cerebral vesicle 

 and tlie development <>f the visceral arches. The septum between the fore-gut and 

 the oral recess, the ^han'n^,al i,-ihran, < Fig. 1429), consists of the directly apposed 

 entoblast lining the primitive pharynx and the ectoblast continued from the surface, 

 no mesol.last intei \ mini;. The pharyngcal membrane very early ( probably about 

 the thirteenth or fourteenth day in man) becomes broken up by the formation of 

 holes and soon disappears, the primitive oral and pharyngeal spaces thereafter freely 

 communicating, 



