CIRCULATORY AND RESPIRATORY -SYSTEMS. 63 



they are transient, and are represented in the infant as anas- 

 tomoses between the arteries of the neck. But in the latter, 

 they are persistent, and sustain the branchiae throughout life. 

 In the reptilian heart, the transmission of venous blood from 

 the right to the left side through a septal foramen recalls the 

 arrangement seen in the human foetus at term. 



SPLEEN. 



The spleen is an appendage to the blood-vessel system. 

 Commonly oval, it is seldom narrow, and elongate, as in 

 Perca (perch). Usually single, it is occasionally double, 

 as in Squatina, or multiple, as in Lamna (shark). (Van der 

 Hoeven). It is situated in the cavity of the abdomen in prox- 

 imity with the stomach, and consists of a red parenchyma, 

 supported by fibrous bands (trabeculae) extending inward 

 from a capsule. The spleen is peculiar in the arrangement of 

 its blood-vessels. The splenic artery abruptly terminates in 

 large spaces (lacunae), from the opposite sides of which the 

 splenic veins as abruptly begin. Small branches of the ar- 

 tery, prior to entrance into the lacunae, are distributed upon 

 their walls, and have appended to their smaller twigs a 

 number of minute white bodies the splenic corpuscles. 



Were no provision made for the reception of large volumes 

 of blood which, from various causes, are poured into the 

 vessels of the abdominal viscera from other parts, stagnation 

 of vascular currents supplying organs essential to nutrition 

 might at any time occur. It is thought to be the function of 

 the spleen to avert this by receiving and retaining the excess 

 of blood until an equalization of currents is again assured. 

 This view would seem to receive confirmation from the 

 organ appearing only in animals having a complete system 

 of vessels, as in the Yertebrata.* With others, as in the 

 Mollusca, the distensible lacunae (which resemble in plan of 

 construction that of the splenic lacunae) are placed at all por- 



* Other animals having closed vessels, but without spleen, as Annelida, 

 Amphioxus, and marsipobranchiate fishes, have a compensatory feature in 

 the extreme dilatability of their numerous vascular trunks. 



