92 ZOOLOGY SECT. 



ventral aorta (Figs. 760, B, and 772, t. ao.). At its origin, 

 which may be dilated to form a hidhus aortce, are valves so 

 disposed as to allow of the flow of blood in one direction only, 

 viz. from the ventricle into the aorta. It gives off on each side 

 a series of halt-hoop- like vessels, the afferent Iranchial arteries 

 (a. hr. a.), one to each gill. These vessels ramify extensively, 

 and their ultimate branches open into a network of microscopic 

 tubes or capillaries (Fig. 773, G.), having walls formed of a single 

 layer of epithelial cells, which permeate the connective- tissue layer 

 of the branchial filaments, and have therefore nothing between 

 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 hranchial 

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

 and discharge into a median longitudinal vessel, the dorsal aorta 

 {d. eio.), 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. 772, 

 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 

 (cSjj. 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 ecmdal 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 svstemic arteries branch and branch asain into smaller and 

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

 (Fig. 773, B, K, and T)\vith which all the tissues of the body, 

 except epithelium and cartilage, are permeated. In these systemic 

 cwpillaries the blood parts with its oxygen and nutrient constituents 

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

 destructive metabolism — carbonic acid. Mater, and nitrogenous 

 waste. The s^-stemic, 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 



