DEVELOPMENT OF THE AETEEIES AND THE HEAET. 1031 



Primitive ventral aorta 



becomes converted into the lateral umbilical ligament, but a portion of the part which lies in the 

 pelvis minor remains pervious and from it springs the superior vesical artery. 



The Arteries of the Limbs. Our knowledge of the development of the arteries of the limbs 

 is still very deficient, but during the last few years some investigations have been made and 

 certain facts have been established. The veiy earliest stages of development have not yet been 

 seen in the human subject, but it is not probable that they differ, in any essential respects, from 

 those found in other mammals ; therefore it may be assumed that the upper limb is supplied, 

 at first, by a number of branches which spring from the sides of the primitive dorsal aortae. As 

 development proceeds the 

 number of the vessels is re- 

 duced until only one remains. 

 That loses its direct connexion 

 with the aorta and becomes 

 attached to the seventh seg- 

 mental artery forming the 

 ventral branch of that vessel, 

 and the lateral division of the 

 branch ; the ventral continua- 

 tion, past the lateral branch, 

 being, apparently, a later for- 

 mation. In the earlier stages 

 the portion of the artery which 

 lies in the free part of the limb 

 does not consist of a single stem 

 but of a plexiform series of 

 vessels which only gradually 

 become reduced to a stem. 

 When the stem is definitely 

 established it is divisible into 

 subclavian, axillary, brachial, 

 and volar interosseous portions. 

 The median, the radial, and 

 the ulnar arteries are of later 

 formation. 



In the case of the lower 

 limb the earliest known artery 

 is the primitive inferior 

 gluteal, which springs from 

 the secondary root of origin of 

 the umbilical artery. It is con- 

 verted into the inferior gluteal 

 and popliteal arteries of the 

 adult. The external iliac and 

 femoral arteries are parts of a 

 later formation which arises 

 from the cephalic aspect of the 

 secondary root of the umbilical 

 artery dorsal to the origin of the 

 inferior gluteal. This second- 

 ary vessel anastomoses distally, 

 at the level of the proximal 

 part of the popliteal fossa, with 

 the primitive inferior gluteal, 

 and, afterwards, the proximal 

 part of the primitive inferior 

 gluteal undergoes a certain 

 amount of atrophy. The de- 

 velopmental history of the 

 arteries of the leg and foot is Diagram showing the changes of form and external appearances at different 



stages. Modified from His's models. 



views ; 



Primitive ventral aorta 



Truncus arteriosus 

 Bulbus cordis 



Ventricle 



Atrium 



Sinus venosus 



Vitelline vein 



Truncus arteriosus 



Bulbus cordis 

 Atrium 



Atrio-ventricular 

 canal 



Ventricle 

 Vitelline vein 



Truncus arteriosus 



Bulbus cordis 



Atrium 



Position of orifice 

 of atrio- 



ventricular canal 

 Ventricle 



Vitelline vein 



Auricle of atrium 



Position of orifice 



of atrio 

 ventricular canal 



Right ventricle 



Bulbus cordis 



Atrium 



Atrio-ventricular 



canal 



Sinus venosus 



Ventricle 

 Vitelline vein 



Truncus arteriosus 



Bulbus cordis 



Atrium 

 Sinus venosus 

 Atrio-ventri- 

 cular canal 



Ventricle 

 Vitelline vein 



Auricle of atrium 

 Bulbus cordis 



Left ventricle 



FIG. 826. DEVELOPMENT OF THE HEART. 



III.B and IV.B are side 

 the other figures represent the heart as seen from the front. 



not definitely known. 



The Heart. The rudi- 

 ments of the heart are the 



caudal portions of the primitive ventral aortae, which lie in the ventral wall of the fore-gut and 

 the dorsal wall of the pericardium ; therefore the heart may be said to consist, at first, of two 

 tubes ; the two tubes fuse, and the heart is then a single median tube, separated by constrictions 

 into five compartments which, from the caudal to the cephalic end, are the sinus venosus, the 

 atrium, the ventricle, the bulbus cordis, and the truncus arteriosus. The constricted region between 

 the atrium and the ventricle is called the atrio-ventricular canal. 



At a later stage the longitudinal tube becomes folded on its long axis and at the same time 

 twisted. The caudal limb of the loop thus produced is formed by the sinus venosus, the atrium, 

 and part of the ventricle ; and the cranial limb by the remainder of the ventricle, the bulbus 

 cordis, and the short truncus arteriosus. " Subsequently, for a time, the ventricular and bulbar 



66 c 



