664 
ANATOMY OF THE HORSE. 
the upper and back part of this cavity; and its mouth is guarded 
by three semilunar valves, which present little pouches within the 
cavity of the vessel: these valves consist of doublings of the lining 
membrane of these parts, infolding, about the middle of their loose 
edges, three small granular substances, described as the corpus- 
cula Arantii . Opposite to the valves, three depressions are ap¬ 
parent in the coats of the vessel: these are named the sinus Val¬ 
salva. 
The left auricle is smaller than the right, and has thicker 
parietes. It contains, in general, but little blood, and in some 
subjects none. It presents nearly the same aspect internally as 
the right. It has not so much of the reticulated structure how¬ 
ever—fewer rnusculi pectinati; what there are, are more strongly 
marked, and are principally confined to the appendix. The pul¬ 
monary veins terminate by four openings in the superior and pos¬ 
terior part of this cavity. The auriculo-ventricular opening is 
somewhat larger than that of the right side, and is rather square 
than round. Now, that the auricles are both laid open, the septum 
auricularum , fossa ovalis , and foramen ovale , may be distinctly 
viewed. 
The left ventricle , though smaller within, is longer, and more 
prominent and extensive without than the right: it forms, of it¬ 
self, the apex cordis . Its outer wall far exceeds in thickness that 
of any other cavity of the heart; it is thrice that of the right ven¬ 
tricle. Its rnusculi pectinati appear mostly upon the septum, 
within the apex, and under the valves. It has but two carnea 
columna , but they are very bulky, and project much into the 
cavity. Its auriculo-ventricular opening is only furnished with 
two valvular productions; in other respects the cortina tendinea 
and corda tendinea resemble those on the right side: this valve 
is called the valvula bicuspis vel mitralis . The aorta takes its 
rise from the upper and fore part of this ventricle, and, concealed 
at its origin by the pulmonary artery on one side and venrn cavae 
on the other, makes it exit close to the spine. The mouth of the 
aorta is shut by three semilunar valves , similar in formation and 
disposition to those at the origin of the pulmonary artery: but the 
sinus Valsalva are much larger and deeper. Just above two of 
them are seen the mouths of the coronary arteries . The ventri¬ 
cles are divided by a thick fleshy partition, called the septum ven- 
triculorum . 
Organization. ™-Though the heart is composed mostly of fleshy 
fibres, a tendinous structure is demonstrable in its middle, which 
tendinous intertexture appears to be the common medium of at¬ 
tachment of its auricles, ventricles, vessels, and valves, to one 
another. The fleshy fibres composing the parietes of the auricles, 
