204 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



The'arteries and veins thus far described are the chief longitudinal trunks of the embryo. 

 From them, branches, usually segmentally arranged, extend to various parts of the body. 

 The branches of the main longitudinal vessels are classified into three kinds: the median visceral 

 or splanchnic branches, unpaired branches to and from the digestive tract; the lateral visceral 

 branches, paired branches to and from the urogenital organs; and the parietal or somatic 

 branches, paired vessels to and from the body wall. This arrangement is most obvious in 

 the arteries (Fig. 54). It is more or less persistent in the adult, chiefly in the posterior part 

 of the body. The vessels to the paired appendages are simply enlarged somatic branches. 

 5. The origin of the portal systems. It has already been explained that a portal system 

 is a portion of the venous system, the constituent veins of which instead of entering the heart 

 pass into a network of capillaries from which the blood is re-collected by a systemic vein. In 

 other words, in a portal system, a network of capillaries is interposed in the path of a vein or 

 veins. There are two portal systems, the hepatic portal system, in which the interposed 

 capillaries are located in the liver, and the renal portal system, in which they are in the kidney. 



The origin of each of these sys- 

 tems may be given briefly. 



a) The hepatic portal system: 

 This arises as follows. When 

 the transverse septum develops 

 at the posterior end of the 

 heart, the two vitelline veins 

 must naturally pierce the sep- 

 tum on their way to the heart. 

 At this region the liver buds 

 out from the small intestine 



lateral visceral 

 branch 



dorsal aorta 



peritoneum 



vertebra 



somatic branch 

 kidney 



median visceral 

 branch 



intestine 

 body wall 



FIG. 54.- Diagram of a cross-section through a vertebrate 

 embryo, to show the segmental branches of the aorta. (After 

 McMurrich's Development of the Human Body, copyright by 

 P. Blakiston's Son and Company.) 



and extends into the transverse 

 septum. As it grows, the liver 

 gradually uses up the sub- 

 stance of the transverse sep- 

 tum and fills the available space in the septum. The liver substance thus comes to 

 surround the proximal ends of the two vitelline veins (Fig. 55,4 and B}. At first the vitelline 

 veins pass through the liver into the heart, but soon they begin to break up into smaller and 

 smaller vessels (Fig. 556") in the liver until the liver is occupied by a network of capillaries (of 

 the kind known as sinusoids) which permeate the liver substance. Thus, the circulation in 

 the vitelline veins passes from the yolk sac and digestive tract to the liver, passes through a 

 capillary network in the liver, and from the liver to the heart in the remaining proximal portions 

 of the vitelline veins (Fig. 55!)). The latter are now called the hepatic veins. Posterior to 

 the liver by ringlike unions between the two vitelline veins (Fig. 5$C and D), a single vessel, the 

 hepatic portal vein, is formed, and caudad of these unions the right vein disappears, leaving 

 the left vein with its tributary, the subintestinal vein, now named the mesenteric vein, to form 

 the chief vein of the digestive tract. This arrangement insures that all of the venous blood 

 from the digestive tract caudad of the cardia must pass through a capillary network in the 

 liver before it can reach the heart. 



b) The renal portal system: This develops as follows. At first the caudal vein opens into 

 the subintestinal vein, forming a loop around the anus as in Figure 5$C and D. Later the 

 posterior cardinal veins grow posteriorly and connect with the caudal vein. The union of 

 the caudal vein with the subintestinal vein is then broken as in Figure 55^, the latter vein then 

 becoming a tributary of the hepatic portal system. There next develops between the kidneys 

 a vein, at first single, later paired the subcardinal vein (Fig. SsF). This connects with the 

 caudal vein and posterior ends of the posterior cardinal veins. The blood flows from the tail 



