662 



REPRODUCTION. 



cavity. They consequently become twisted, the two umbilical arteries 

 winding round the vein in a spiral direction. 



The direction of the spiral is not always the same. Prof. McLane 

 has recorded observations in regard to this point upon 260 umbilical 

 cords at term, partly in private practice and partly at the Nursery 

 and Child's Hospital, New York. Of this number, in 138 cases the 

 direction of the spiral was from left to right; in 112 cases, from right 

 to left; and in the 10 remaining instances it was doubtful, the twist 

 being too imperfectly marked for decision. This gives nearly the fol- 

 lowing percentage as the result of all the observations : 



DIRECTION OF THE TWIST OF THE HUMAN UMBILICAL COED. 



From left to right 

 From right to left 

 Indeterminate . 



53 per cent. 



43 " 

 4 " 



100 



There is, accordingly, no great preponderance in frequency of the 

 twist in either direction. Two cases of twins are included in the 



above list; in the first of 



FIG. 213. 



in 



which both umbilical cords 

 turned from right to left ; in 

 the second, one of them 

 turned from right to left, the 

 other from left to right. In 

 two instances, the cord pre- 

 sented turns in opposite di- 

 rections in different parts of 

 its length. 



The gelatinous matter de- 

 posited between the amnion 

 and chorion gradually disap- 

 pears over the greater part 

 of these membranes, but ac- 

 cumulates in the umbilical 

 cord in considerable quantity, 

 surrounding the vessels with 

 an elastic envelope, which 

 protects them from injury. 

 It is covered by an extension 

 of the amnion, which is con- 

 tinuous with the integument of the abdomen, and invests the cord 

 with an uninterrupted sheath throughout its length. 



The cord also contains the stem of the umbilical vesicle. The situa- 

 tion of this vesicle is always between the chorion and the amnion. 

 Its pedicle gradually elongates with the growth of the umbilical cord ; 

 and the vesicle, which generally disappears soon after the third month, 



PREGNANT HUMAN UTERUS AND ITS CONTENTS, about 

 the end of the seventh month ; showing the relations 

 of the cord, placenta, and membranes. 1. Decidua 

 vera. 2. Decidua reflexa. 3. Chorion. 4. Amnion. 



