976 SPECIAL PHYSIOLOGY. 



conjoined veins, is turned backwards towards the vertebral column. 

 The saccular heart is now marked off, by two constrictions, into three 

 chambers, of which the first, or the one nearest the veins, soon exhibit- 

 ing two superficial bulgings, corresponds with the future auricles ; the 

 second, also quickly showing an external line of subdivision, or notch, 

 forms the ventricles, and the third, lying in front of and above the 

 others, forms the bulbus arteriosus. Each of these quickly becomes 

 subdivided. Thus, a septum, commencing on the anterior part, oppo- 

 site to an external groove in its ventricular portion, grows upwards 

 and backwards from the apex to the base of the heart, and ultimately 

 divides it into the right and left ventricles. Some time after this, the 

 septum between the auricles begins to appear, growing both from above 

 and below, like two folds with crescentic margins; the edge of the 

 upper fold, as it grows, keeps to the right side of the lower fold, so 

 that the passage from one auricle to the other, named the foramen ovale, 

 is more or less oblique. The septum is completed at the period of 

 hatching of the chick, and of the birth of the Mammalian embryo. 

 Lastly, the bulbus arteriosus becomes subdivided longitudinally into 

 the pulmonary artery and the aorta, each remaining connected with 

 its respective ventricle. The primitive heart, whilst it is still merely 

 a straight tube, already manifests its proper function, its walls con- 

 taining sarcous cells of round, oval, and fusiform shape, and distinctly 

 contractile, the contractions soon being seen to proceed, in regular 

 succession, from the hinder, or venous, to the anterior, or arterial end. 

 Then, and even much later, when the heart is beginning to acquire its 

 characteristic shape, its contractile walls present no distinct muscular 

 fibres, but only large contractile nucleated cells, which are at first 

 roundish, but afterwards become developed into spindle-shaped, or even 

 forked, contractile fibre cells. The valves are developed, in their re- 

 spective positions, from the inner surface of the heart, commencing 

 ve,ry early. 



The arterial system of the embryo is developed by the formation, 

 and subsequent metamorphoses, of five arches on each side, which are 

 formed in succession from the original arterial trunk at the anterior 

 end of the heart, all, however, not being present . at the same time. 

 The arch first formed, consisting of the two branches into which the 

 primitive arterial trunk divides, and which, curling backwards, unite 

 along a part of their course to form the descending aorta, has already 

 been described. Behind this first arch, a second, third, fourth, and 

 fifth arterial arch are formed on each side, the first arch being obliter- 

 ated before the last one appears ; all in turn coalesce as they pass 

 backwards, like the returning part of the first pair, to assist in forming 

 the descending aorta. This is the condition which persists in Fishes, 

 by which the trunks of their characteristic branchial vessels are formed 

 (p. 692). In the higher Vertebrata, this state is very rapidly changed, 

 and its several phases of development correspond with the adult con- 

 dition of the Amphibia, and, in some respects, of Reptiles and Birds 

 as well. These changes fully explain the mode of origin of all these 

 varieties, from a common Vertebrate embryonal type. The upper 

 primitive arterial arches, or primary arches, are said to occupy the 



