DKl'KLnI'MKXT nF THE FCETUS. 10'. 



front — the cardiac cavity, in the interior of wliich the heart is deve- 

 loped. This organ is at first a small mass of cells, the innermost of 

 which separate in order to leave a little space, and to create blood 

 globules. As soon as it appears, the heart contracts and relaxes alter- 

 nately, the movements being very slow, though they gradually become 

 accelerated. Towards the twelfth day the organ has the shape of a 

 cylindrical contractile tube. From its anterior part arise two branches, 

 the aortic arches, which at first proceed towards the head of the em- 

 bryo, but afterwards are bent backward and downward. These arches 

 join to form the simjlc aorta, which in its turn divides into two branches 

 — the subrertebral or common aortie — which run parallel along the 

 lower surface of the embryo, giving off four or live branches, the 

 omphalo-meseraic arteries, which ramify in the germinative area and 

 end in a vein — the terminal sinus. From this ramification and sinus 

 arise two vessels, the omphalo-vicscraic veins, which return to the 

 posterior extremity of the heart. The circulation in the umbilical 

 vesicle is very ephemeral in several species, while it is as persistent in 

 others ; varying, of course, with the duration of the vesicle itself. 



The heart, from being merely a cylindi"ical tube, undergoes consider- 

 able modifications before it arrives at its complete development, passing 

 through all the different forms which characterise the organ in the 

 various vertebrate animals. The first change consists in an inflexion, 

 by which the inferior part becomes the superior; then it dilates at three 

 distinct points — one anterior and superior, at the origin of the aorta, 

 named the aortic bulb ; a middle one, the ventricular cavity ; and a 

 posterior, the auricular cavity. The constriction between the auricle 

 and ventricle is named Hallcr's passage {d&troit). 



These two cavities are single only for a brief period ; the ventricular 

 first divides into two — a division marked externally by a fuiTOw which 

 is visible on the surface of the heart of a foetal Lamb at the nineteenth 

 day, and the twenty-fifth in the Foal. This furrow corresponds to an 

 interventricular septum which arises insensibly at the bottom of the 

 ventricles and reaches the auricles, where it concurs to form the 

 auricula-ventricular orifices. The borders of the openings are furnished 

 with a slightly salient lip, which at a later stage develops, and consti- 

 tutes the mitral and tricuspid valves. The heart now contains three 

 cavities — two ventricles and an auricle, but it is soon to have a fourth ; 

 for the auricle becomes partitioned into two portions, and externally this 

 division is marked by anotlicr depression, corresponding to the septum 

 formed in the cavity. It is to be remarked, however, that the partition 

 remains incomplete during the wliole period of uterine life, being per- 

 forated by an opening — the Joranwn ovale or foramen of Botal. The 

 aortic bulb now contracts and forms two vessels — the aorta and pul- 

 monary artery. 



The arteries are developed, partly at the expense of the primary 

 circulation, and partly in the vascular layer of the embryo. The aortic 

 arches, situated at the inner face of the first two pharyngeal arches, 

 form five new vessels, w-hich are placed within the other arches ; all 

 these, however, do not exist at the same time, some becoming atrophied 

 while the others are forming. For instance, the first two completely 

 disappear ; the third forms the carotids ; the fourth, the axillary artery 

 and the arch or cross of the aorta ; the fifth is atrophied on the right, 

 but on the left forms the pulmonary artery, the ductus arteriosus, and 

 the aorta. The latter is continued along the spine by the fusion of the 



