220 PHYSIOLOGY. 



And where there is a circulatory system there is present some 

 means, composed in the great majority of cases of muscle, for 

 the impulsion of the circulatory fluid to every part of the organism. 

 Whenever, in animal organisms, there is transformation of energy 

 into motion or mechanical work, it may nearly always be attributed 

 to muscle. So that in the higher forms of animals there exist one 

 or more rhythmically contractile organs for the most part mus- 

 cular in nature to which is attributed the task of maintaining a 

 definite circulation. 



Comparative. Among insects and the lower orders of Crustacea 

 the heart, if such it may be called, is simply the contractile dorsal 

 blood-vessel; among the higher Crustacea, as the lobster, there 

 exists dorsally a well-defined muscular sac. Among the inverte- 

 brates in general the blood passes from the arteries into irregular 

 spaces, known as lacunas, which are situated in the tissues and from 

 which it finds its way back into the veins to terminate in the heart 

 for the completion of its cycle. That interesting creature, the 

 amphioxus, the lowest of the vertebrates, possesses a primitive, 

 lacunar vascular system. Its contractile dorsal vessel serves as its 

 systemic heart; a ventral vessel serves as a respiratory heart, ves- 

 sels proceeding from it to the gills. Fishes contain only a respira- 

 tory heart, which sends blood to the gills for aeration. It consists 

 of a venous sinus, an auricle, and a ventricle. From the gills blood 

 finds its way to the aorta, to be distributed throughout the tissues 

 without any further impulsion. Among the amphibians, as the 

 frog, there are found two auricles and a single ventricle. Reptiles 

 possess two auricles with two ventricles, though the latter are but 

 incompletely separated. Among birds and mammals there is a 

 heart which serves a double purpose it sends blood to the lungs 

 for aeration, to the body in general to serve the needs of its various 

 tissues. The passage of the blood to the lungs is accomplished by 

 the right auricle and ventricle and is known as the pulmonary sys- 

 tem. That going to the tissues of the body is propelled by the left 

 auricle and ventricle to constitute the systemic system. 



THE CIRCULATORY SYSTEM. 



This system has for its distinctive function the propulsion of 

 the blood to every part of the economy. It is a closed, vascular 

 apparatus consisting of an impelling agency, or pump, with an out- 

 going and incoming system of vessels. The central pumping organ 

 is the heart, from which proceed the vessels that carry the blood 



