CIRCULATING SYSTEM. 339 
retrograde motion of the blood. This inner coat is found in 
all the ves; 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 venze cavee 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 Ariery, 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 
yg 
