86 ZOOLOGY 



SECT. 



them and the surrounding water but. the epithelium of the 

 filaments. The blood, driven by the contractions of the heart into 

 the ventral aorta, is pumped into these respiratory capillaries, and 

 there exchanges its superfluous carbonic acid for oxygen. It then 

 passes from the capillaries into another set of vessels which join 

 with one another, like the tributaries of a river, into larger and 

 larger trunks, finally uniting, in each gill, into an efferent branchial 

 artery (e. br. a.). The efferent arteries of both sides pass upwards 

 and discharge into a median longitudinal vessel, the dorsal aorta 

 (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 (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 



aortse, 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. 728, 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 



