CIRCULATING SYSTEM. 339 



retrograde motion of the blood. This inner coat is found in 

 all the veins ; but the external ones cannot be detected, 

 where the vessels penetrate bone, or are otherwise protected. 



In many animals, the blood appears to be conveyed di- 

 rectly to the aerating organs, by means of the veins, with- 

 out the intervention of any other apparatus. In other 

 cases, especially among the higher classes of animals, it has 

 to pass through a muscular enlargement or bag, termed the 

 HEART, by which its motion is accelerated. 



The heart is inclosed in a membranaceous sac, termed 

 the Pericardium, of dimensions somewhat larger than its 

 greatest expansions. In the walls of this sac, fibres, pass- 

 ing in different directions, may be readily perceived. It is 

 kept moist internally, by a serous fluid, consisting chiefly 

 of water, with a little albumen, mucus, and muriate of 

 soda. 



In the mammalia and birds, the heart consists of four ca- 

 vities. Two of these are connected with the pulmonic, and 

 two with the systemic vessels. With the former our atten- 

 tion is at present occupied. The venae cavae empty their 

 contents into what is termed the Pulmonic Auricle ; or, in 

 reference to its position in man, the Right Auricle. The 

 blood passes directly from the auricle, by a valvular orifice, 

 into the pulmonic or right ventricle, and from thence by a 

 vessel termed the Pulmonary Artery, to the aerating organs. 

 The structure of the auricle and ventricle exhibits a dense 

 muscular substance, consisting of fibres intimately interwo- 

 ven, and liberally supplied by bloodvessels and nerves, lined 

 with a smooth membrane. Hence we may regard the heart 

 merely as an enlargement of the veins, and its walls endow- 

 ed with corresponding strength by the increase of the thick- 

 ness of the muscular coat. The walls of the auricle are 

 much thinner than those of the ventricle. The former bag 

 has merely to convey the blood to the ventricle ; while this 



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