508 THE SYSTEMIC VEINS. 



branches ; the hepatic branch is greatly enlarged and usually furnishes almost the whole of 

 the arterial supply to the liver ; while its anastomotic or pyloric branch is but small. The 

 splenic artery, which occupies an intermediate position, is also primitively gastric ; its ascend- 

 ing division forms the vasa brevia, and its descending division the left gastro-epiploic artery, 

 while the splenic and pancreatic branches are collateral offsets to the glands formed in the 

 mesogastrium. 



The superior mesenteric artery, developed from the embryonic vitelline or omphalo- 

 mesenteric artery, is the artery of the midgut, and supplies the portion of the bowel formed 

 from the primary intestinal loop. The inferior mesenteric artery is similarly the trunk for 

 the supply of the hind gut. The dorsal anastomoses between the branches of these trunks 

 are largely developed, and the number of successive arches that are formed is in direct relation 

 to the length of the suspensory peritoneal fold and the consequent mobility of each region of 

 the intestine. 



3. The suprarenal, renal and spermatic arteries form a group intermediate in position 

 between the parietal and visceral arteries, but more closely related to the former in accordance 

 with the position in which the organs supplied by these vessels are developed. Originally 

 more numerous, they have become reduced in number like the visceral arteries ; and the 

 occurrence of supernumerary renal arteries may be regarded as a reversion to the multiple, 

 possibly a metameric condition. The uterine and vaginal arteries probably belong also to this 

 group (Mackay). 



4. The mediastinal and phrenic arteries are more closely allied to the splanchnic 

 system of vessels, and the bronchial arteries may be referred to the same group. 



(On this subject see W. J. Walsham, "Abnormal Origin and Distribution of the upper 

 seven right Intercostal Arteries, with remarks," Journ. Anat., xvi, 1882 ; W. His, " Anat. 

 menschl. Embryonen," iii, 1885 ; A. Froriep, " Zur Entwickelungsgeschichte der Wirbelsaule," 

 &c. (Segmental arteries of neck), Arch. f. Anat., 1886 ; A. Macalister, " The Morphology of 

 the Arterial System in Man," Journ. Anat., xx, 1886 ; J. Yule Mackay, " The Arterial System 

 of Vertebrates homologically considered," Proc. Phil. Soc., Glasgow, xviii, 1887, and in 

 " Memoirs and Memoranda in Anatomy " ; H. St. John Brooks, ' : Arterial Trunks indicating 

 Archaic or Unusual Courses of Nerve-Trunks in the Limbs," Roy. Acad. Med. Ireland, Jan. 4, 

 1889 ; F. Hochstetter, " Ueber die urspriingliche Hauptschlagader der hinteren Gliedmasse des 

 Menschen und der Saugethiere, nebst Bemerkungen iiber die Entwicklung der Endaste der 

 Aorta abdominalis," and " Ueber die Entwicklung der Art. vertebralis beim Kaninchen," &c., 

 Morph. Jahrb., xvi, 1890 ; A. H. Young, " On the Termination of the Mammalian Aorta, 

 with observations on the Homologies of the Pelvic Arteries," in " Studies in Anatomy," the 

 Owens College, 1891.) 



SYSTEMIC VEINS. 



The systemic veins commence by small branches which receive the blood from 

 the capillaries throughout the body, and unite to form larger vessels, which end 

 at last by pouring their contents into the right auricle of the heart through 

 two large venous trunks, the superior and inferior venae cavae. The blood from the 

 walls of the heart itself is returned by the cardiac veins also to the right auricle. 



The veins, however, which bring back the blood from the stomach, intestines, 

 spleen, and pancreas, have an exceptional destination, not conveying the blood 

 directly to the heart, but joining to form a single trunk the portal vein, which 

 again becomes ramified in the substance of the liver, and carries its blood to the 

 capillaries of that organ. Thence the blood passes into the ultimate twigs of the 

 hepatic veins, and is conveyed by these veins into the inferior vena cava. The 

 veins thus passing to the liver constitute the portal system. 



The anastomoses of veins are much larger and more numerous than those of 

 arteries. The veins of the body generally consist of a subcutaneous and a deep 

 set, which have very frequent communications with each other. In some parts of 

 the body, chiefly in the limbs and at the surface, the veins are provided with valves, 

 while in others no valves exist (see Vol. I, p. 368). 



The systemic veins are naturally divisible, into two groups : firstly, those from 

 which the blood is carried to the heart by the superior vena cava, viz., the veins of 

 the head and neck and upper limbs, together with those of the spine and a part of 

 the walls of the thorax and abdomen, with which may be associated also the veins 



