DEVELOPMENT OF THE EMBRYO. 619 



great depurating organs, the Placenta and the foetal Liver, it is in the condi- 

 tion of arterial blood ; but, being mixed in the vessels with that which has 

 been returned from the trunk and lower extremities, it loses this character in 

 some degree by the time that it arrives in the Heart. In the right Auricle, 

 which it then enters, it would be also mixed with the venous blood conveyed 

 by the descending Cava; were it not that a very curious provision exists to 

 prevent (in great degree, if not entirely) any such further dilution. The Eusta- 

 chian valve has been found, by the experiments of Dr. J. Reid,* to serve the 

 purpose of directing the arterial blood, which flows upwards from the ascend- 

 ing Cava, through the Foramen Ovale, into the left Auricle, whence it passes 

 into the Ventricle: whilst it also directs the Venous blood that has been 

 returned by the descending Cava, into the right Ventricle. When the Ven- 

 tricles contract, the Arterial blood which the left contains is propelled into the 

 ascending Aorta, and supplies the branches that proceed to the head and upper 

 extremities, before it undergoes any admixture ; whilst the Venous blood, con- 

 tained in the right Ventricle, is forced through the Pulmonary artery and 

 Ductus Arteriosus, into the descending Aorta, mingling with the arterial cur- 

 rent which that vessel previously conveyed, and passing thus to the^trunk and 

 lower extremities. Hence the Head and superior extremities, whoPe develop- 

 ment is required to be in advance of that of the lower, are supplied with blood 

 nearly as pure as that which returns from the Placenta; whilst the rest of the 

 body receives a mixture of this, with what has previously circulated through 

 the system ; and of this mixture a portion is transmitted to the Placenta, to be 

 renovated by coming into relation with the maternal fluid. At birth, the 

 course of the current is entirely changed by its diversion into the Lungs ; 

 which takes pla&e immediately on the first inspiration. The Ductus Venosus 

 and Ductus Arteriosus soon shrivel into ligaments; the Foramen Ovale 

 becomes closed by its valve ; and the circulation, which was before carried on 

 upon the plan of that of the higher Reptiles, now becomes that of the complete 

 Bird or Mammal. It is by no means unfrequent, however, for some arrest of 

 development to prevent the completion of these changes ; and various malfor- 

 mations, involving an imperfect discharge of the function, may hence result.! 

 767. The Alimentary Canal has been shown to have its origin in the Yolk- 

 sac or Umbilical Vesicle ; being a portion pinched off (as it were) from that 

 part of it which is just beneath the Spinal Column of the Embryo ( 760). 

 At first it is merely a long narrow tube, nearly straight, and communicating 

 with the Umbilical Vesicle at about the middle of its length ; thus it may be 

 regarded as composed of the union of two, an upper and a lower division. 

 At first, neither Mouth nor Anus exists ; but these are formed early in the 

 second month, if not before. The tube gradually manifests a distinction into 

 its special parts, ^Esophagus, Stomach, Small Intestine, and large Intestine ; 

 and the first change in its position occurs in the Stomach, which, from being 

 disposed in the line of the body, takes an oblique direction. The curves of 

 the large and small intestines present themselves at a later period. It is at 

 the lower part of the small Intestine, near^ts termination in the large, that the 

 entrance of the Omphalo-Enteric duct exists ; and a remnant of this canal i 

 not unfrequently preserved throughout life, in the form of a small pouch or 

 diverticulum from that part of the intestine. The various Glandular struc- 

 tures connected with the alimentary canal, originate in diverticula from its 

 walls, in the manner already described in regard to the Liver ( 655). The 

 Lungs and Respiratory apparatus are formed in like manner, as diverticula 

 from the (Esophagus ( 526). 



* Edinb. Med. and Surg. Journal, voK xliii. 



j- See Principles of General and Compafative Physiology, chap. vi. 



