THE HEART AND ARTERIES 461 



trunks lie in the space on each side between the two ventricles. The 

 movements of the heart may be carried on independently of the brain and 

 spinal cord, if these parts are gradually removed ; but if they are suddenly 

 destroyed or partially injured, it ceases to beat. Its nerves are derived 

 from the pneumogastric and sympathetic. 



The pericardium is made up externally of a thin layer of white fibrous 

 matter, attached to the roots of the great vessels above, and by a few pi-o- 

 longations to the sternum below, and the central tendon of the diaphragm 

 behind. Within this the heart lies, loosely covered with a serous bag, 

 which also lines the fibrous coat above mentioned, and forms with it the 

 pericardium as a whole. The use of the external layer is to restrain the 

 movements of the heart within due bounds, and of the serous layer to allow 

 it to play freely without being restrained by the friction of its exterior 

 against the surrounding parts, which would be the case in the absence of 

 the double sac of serous membrane which it is endowed with. Like the 

 pleura, this sac, during health, contains only sufficient serum to lubricate 

 it ; but after inflammation or congestion, serum, lymph, or pus are thrown 

 out, so as to interfere with its proper functions. 



Each artery has three distinct coats : an outer cellular coat, capable 

 of great distension ; a middle coat, consisting in part of yellow fibrous 

 tissue and in part of non-striated musculai fibres, which is highly elastic ; 

 and an inner serous coat, intended to diminish the friction of the blood 

 as it rushes on. It is in the elastic middle coat that the power resides of 

 equalizing the flow of blood, retarding its velocity when the vessel con- 

 taining it is near the heart, and accelerating it at a distance from it. In 

 this way the intermittent jets which are produced by the ventricular con- 

 traction become at length converted into a continuous stream, having 

 midway between the two extremities developed the arterial pulse, which 

 can be felt in all the arteries of any size throughout the body, and most 

 conveniently within the lower jaw, at the submaxillary artery. 



The capillaries are generally spoken of as a distinct system of small 

 blood-vessels, but no line of demarcation can be demonstrated either at 

 their junction with the larger branches of the arteries, or with the veins ; 

 and they should be regarded simply as the minute terminations of the one 

 set and commencement of the others, together making a fine network of 

 vessels which vary greatly in the mode of their ramifications, according as 

 they minister to muscular fibre, gland, or membrane. Like the arteries 

 themselves, they possess the power of contraction and dilatation, which is, 

 apparently, under the influence of the nervous system. Thus, on the 

 application of a local stimulus, the capillaries of the part admit more blood 

 without any increase of the heart's action, and this may go on to the states 

 known as congestion and inflammation according to the presence or absence 

 of other circumstances bearing upon their action. 



The arteries are arranged in two great groups, one of which has been 

 sufiiciently alluded to at page 460, as conveying black blood to the lungs; 

 the other commences at the left ventricle as the aorta, and dividing at once 

 into the anterior aorta and posterior aorta, supplies the corresponding 

 parts of the body with arterial blood, after branching off into innumerable 

 subdivisions. This is clearly marked in the accompanying plan, which 

 indicates the position of the lieart in the thorax, and most of the principal 



