DEVELOPMENT OF THE PRINCIPAL VEINS. 151 



internal carotid, as stated by Rathke, and the ventral prolongation of the trancus arteriosus 

 forms, not the external carotid, but a small branch from the subclavian, or innominate 

 artery, to the front of the trachea. These observations have not been as yet extended to 

 mammals. 



Zimmermann has described, both in the rabbit and in the human embryo, an arterial arch 

 between the aortic and pulmonary arches. If this is of constant occurrence it must be 

 reckoned as the fifth arch, and the pulmonary will become the sixth. 



The histogenetic changes involved in the development of the blood-vessels are described in 

 the chapter on Histology. 



The first vessels appear, as has been already stated, in the mesoblast of the vascular area ; 

 the lamina of mesoblast in which they are formed is sometimes distinguished as the vascular 

 lamina. They are said (by His and others) to grow inwards from the vascular area, but the 

 manner in which the principal arteries and veins of the body are first developed is not clear, 

 beyond the fact that they are at first merely endothelial tubes. The muscular tissue of tho 

 primitive aortsB is derived from the lower part of the proto-vertebra (E. Muller). 



DEVELOPMENT OF THE PBINOIPAL VEINS. 



In this subject also the description given by His of the condition and changes 

 of the veins in the human embryo will be followed, although it differs in certain 

 particulars from that which has usually been received. 



In the early embryo, before the development of the allantois, two vitelline or 

 omphalo-meseraic veins, right and left, bring back the blood from the vascular area 

 upon the yolk sac, and unite to form a common trunk, which is continued as the 

 sinus venosus into the auricular extremity of the rudimentary heart. 



At the commencement of the placental or allantoic circulation (fourth week in 

 man) two umbilical veins are seen coming from the placenta and opening into the 

 sinus near the vitelline veins (fig. 168). Into this also opens on either side a 

 transverse vein, the duct of Cuvier or superior vena cava, which is formed by the junc- 

 tion of the primitive jugular vein, bringing blood from the head, and the cardinal 

 vein, which returns the blood from the Wolffian bodies, the vertebral column and 

 the body walls (figs. 179, 180). The trunk or sinus into which all these veins pour 

 their blood is now transversely disposed, immediately below the diaphragm, and 

 forms the saccus reunions of His, which has been already alluded to (p. 139). 



The vitelline or omphalomeseraic veins enter the abdomen along the vitelline 

 duct and ascend at first along the front of the alimentary canal, but higher up they 

 are seen on either side of that tube (duodenum and stomach). Here transverse 

 communications form between the two veins, two in front of and one behind the 

 duodenum, so that this is encircled by two vascular rings (figs. 180, 183). Above 

 these venous circles the direct communication with the sinus becomes lost, the inter- 

 mediate venous vessel or either side becoming broken up within the substance of the 

 liver (which has by this time developed around them) into a vascular network, the 

 middle part of which becomes capillary. 



The vessels which pass from the upper venous ring to the capillary network are 

 known as venw advehentes, they become the branches of the portal vein ; those 

 which pass from it into the sinus are the ven& revehentes, they become the hepatic 

 veins. 



The lower communication between the vitelline veins takes the form of a com- 

 plete longitudinal fusion of the two vessels, at least for some distance. This fused 

 part receives veins from the intestine and stomach, and becomes the commencement 

 of the portal vein. 



The umbilical veins are for a long time double within the abdomen, although 

 Jiey have fused within the umbilical cord into a single trunk. They diverge from 

 this and pass up to the sinus on either side in the somatopleure, just where this is 

 becoming bent round into the amnion. After a time, however, it is found that this 

 direct communication with the sinus is partially interrupted by the development of a 



