412 THE HORSE. 
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 pro- 
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. 
Eacu 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 muscular 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. 
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 net-work 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 circumstanees bearing upon their action. 
THE ARTERIES are arranged in two great groups, one of which has been 
sufficiently alluded to at page 411, as conveying black blood to the lungs ; 
the other commences at the left ventricle as the aorta, and dividing at once 
into the aorta anterior and aorta posterior, 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 heart in the thorax, and most of the principal 
arteries of the body; but being on so small a scale, it can only convey a 
general idea of their numbers and the situation at which they each leave 
the parent trunk. 
T'Ae AORTA, or great artery of the body, as it emerges from the sub- 
stance of the heart and rises towards the spine, describes a curve whose 
convexity looks upwards and forwards. Immediately above the valves at 
its root are the origins of the two coronary arteries, supplying the heart as 
