182 



CIEC0LATION 



This is conical like the heart, but the broad end is down- 

 ward and attached to the diaphragm. Both the inside of 

 the pericardium and the outside of the heart are covered 

 with smooth sl'ippery membranes, while between these two 

 layers is a fluid which moistens the surfaces and lessens 

 the friction between them as the 

 heart beats. The heart is also lined 

 ■>;/ internally with fibrous membrane 

 while the walls between these outer 

 and inner membranes are composed 

 of a peculiar kind of striped muscle 

 tissue whose cells differ markedly 

 from the striped cells of ordinary 

 voluntary muscle. These muscular 

 walls contain nerves, blood vessels, 

 and connective tissue. Their thick- 

 ness varies markedly in different 

 parts of the organ. 



Internal structure of the heart. — 

 (See Fig. 66.) On opening the 

 heart, it is found to be completely 

 separated into two distinct halves 

 by a longitudinal partition or 

 septum. Each of these halves, 

 which are right and left in posi- 

 tion, is divided by transverse partitions into an upper 

 and lower cavity. The cavities which are at the broad 

 end of the heart are thin walled and flaplike in appear- 

 ance. From a fancied resemblatice to the ears of a dog 

 they are called auricles. The other two cavities and 

 the thick walls of muscle which surround them, make up 

 the bulk of the heart. These latter cavities are called 



Fig. GG — Kelation of cavities 

 in one side of the lieart. 



