90 ZOOLOGY SECT. 



(d. ao.), situated immediately beneath the notochord or vertebral 

 column. From this trunk, or from the efferent branchial arteries, 

 numerous vessels, the systemic arteries, are given off to all parts of 

 the body, the most important being the carotid arteries (Fig. 781, 

 c. a.) to the head, the subclavian (scl. a.) to the pectoral fins, the 

 cceliac (cl. a.) and mesenteric (ms. a.) to the stomach, intestine, liver, 

 spleen, and pancreas, the renal (r. a.) to the kidneys, the spermatic 

 (sp. a.) or ovarian to the gonads, and the iliac (il. a.) to the pelvic 

 fins. After giving off the last the aorta is continued as the caudal 

 artery (cd. a.) to the end of the tail. 



With the exception of the capillaries, all the vessels described 

 in the preceding paragraph, including the dorsal and ventral 

 aorta3, are arteries. They are firm, elastic tubes, do not collapse 

 when empty, usually contain but little blood in the dead animal, 

 and serve to carry the blood from the heart to the body generally. 



The systemic arteries branch and branch again into smaller and 

 smaller trunks, and finally pour their blood into a capillary network 

 (Fig. 782, B, K, and T) with which all the tissues of the body, 

 except epithelium and cartilage, are permeated. In these systemic 

 capillaries the blood parts with its oxygen and nutrient constituents 

 to the tissues, and receives from them the various products of 

 destructive metabolism carbonic acid, water, and nitrogenous 

 waste. The systemic, like the respiratory, capillaries are micro- 

 scopic, and their walls are formed of a single layer of epithelial 

 cells. 



We saw that the respiratory capillaries are in connection with 

 two sets of vessels, afferent and efferent. The same applies to the 

 systemic capillaries, with the important difference that their 

 efferent vessels are not arteries, but thin- walled, non-elastic, 

 collapsible tubes called veins. They receive the impure blood 

 from the capillaries, and unite into larger and larger trunks, 

 finally opening into one or other of the great veins, presently to be 

 described, by which the blood is returned to the heart. As a 

 general rule the vein of any part of the body runs parallel to its 

 artery, from which it is at once distinguished by its wider calibre, 

 by its dark colour due to the contained bluish-purple blood seen 

 through its thin walls, by being gorged with blood after death, by 

 the complete collapse of its walls when empty, and by its usually 

 containing valves. In some cases the veins become dilated into 

 spacious cavities called sinuses ; but sinuses without proper walls, 

 such as occur in many Invertebrates, are never found in the 

 Craniata. 



The veins from the head join to form large, paired jugular veins 

 (Fig. 781, j. v.) which pass backwards, one on each side of the head, 

 and are joined by the cardinal veins (crd. v.) coming from the trunk, 

 each jugular uniting with the corresponding cardinal to form a large 

 precavalvein (pr. cv. v.) which passes directly downwards and enters 



