The _heart_ lies in the middle line of the body, its long axis being
parallel with that of the trunk. The whole ventral surface of the
pericardium is exposed when the sternum is removed. The right and left
halves are completely divided by septa, no mixture of the venous and
arterial blood being possible, an advance upon reptilian conditions,
even the highest.
The atria are comparatively small, the walls being thin, especially
those of the right, which possesses numerous muscular ridges
projecting into the cavity presenting a honeycombed appearance. The
interauricular septum is mostly entirely membranous; in the middle it
is thinner, rather transparent, but there is no depression or _fossa
ovalis_. The whole sinus venosus has become part of the right atrium.
It receives the three great venous trunks of the body, namely the
_vena cava superior dextra_, the _vena cava superior sinistra_ more
dorsally, and the _vena cava inferior_ more to the right and below;
the opening of the last is guarded by two prominent valves in place of
the mammalian _valvula Eustachii_. The right ventricle occupies the
ventral portion of the heart. The communication with the atrium is
guarded by a _valvula cardiaca dextra_, which only in function
represents the mammalian tricuspid; it consists of an oblique
reduplication of the muscular fibres together with the endocardiac
lining of the right ventricle, while the opposite wall is convex and
forms neither a velum nor papillary muscles, nor _chordae tendineae_.
The right anterior corner of the right ventricle passes into the short
stem, guarded by three semi-lunar valves, which divides into the two
pulmonary arteries. There are likewise two pulmonary veins, entering
the left atrium by one orifice. Two or three membranous flaps, held by
numerous _chordae tendineae_, form a true mitral valve, and allow the
blood to pass through the left _ostium atrioventriculare_. The blood
leaves the heart past three semi-lunar valves, by the _right_ aorta,
this being alone functional, a feature characteristic of, and peculiar
to, birds. Remnants of the left aortic arch persist sometimes in the
shape of a ligamentous strand. The aortic trunk is very short, sends
off the coronary arteries and then the left _aorta brachiocephalica_,
while the rest divides into the right brachiocephalic and the _aorta
descendens_. Each brachiocephalic soon sends off its subclavian, while
in the normal or more usual cases the rest proceeds as the carotid
trunk, inclusive of the vertebral artery. But the carotids show
several interesting modifications which have been examined chiefly by
C.L. Nitzsch and by A.H. Garrod. (1) The right and left carotids
converge towards the middle and extend up the neck, imbedded in a
furrow along the ventral surface of the cervical vertebrae. This is
the usual arrangement. (2) The two carotids are fused into one
_carotis conjuncta_, imbedded in a special median osseous semicanal of
the vertebrae; e.g. herons, flamingos, and some parrots. (3) There is
one _carotis conjuncta_, but the basal portion of its original right
component is obliterated, leaving a so-called _c. primaria sinistra_,
an unfortunate name. Such _Aves laevocarotidinae_ of Garrod are
common, e.g. all the Passeriformes. (4) The reverse of the third
modification, producing a _c. primaria dextra_ in the bustard
_Eupodotis_. In other likewise very rare cases a left, or a left and
right, superficial carotids are developed and take the place of the
then vanished deep or primary carotids.
_Venous System._--The bird's liver receives nearly all the blood from
the stomach, gut, pancreas and spleen, as well as from the left liver
itself, into the right hepatic lobe, by a right and left portal vein.
The _venae hepaticae magnae_ join the _vena cava posterior_ and
thereby form with it the _vena cava inferior_. The left _hepatica
magna_ receives also the umbilical vein, which persists on the
visceral surface of the abdominal wall, often anastomosing with the
epigastric veins. A likewise unpaired vena _coccygeo-mesenterica_ is
usually present. There is no renal portal system, excepting
unimportant vestiges of such a system in the head kidneys.
_Lymphatic System._--The white blood-corpuscles are produced in the
follicles at the base of the intestinal villi. The lymph vessels of
the tail and hinder parts of the body enter the hypogastric veins; and
at the point of junction, on either side, lies a small lymph heart,
which often persists until maturity. The red blood-corpuscles are
invariably oval disks, with a central nucleus which causes a slight
swelling; hence they are oval and biconvex.
See A.H. Garrod, "On the Carotid Arteries of Birds," _Proc. Zool.
Soc._, 1873, pp. 457-472; E.A. Lauth, "Memoire sur les vaisseaux
lymphatiques des oiseaux," _Ann, Sci. nat._ (iii. 1824), p. 381; J.J.
Mackay, "The Development of the Branchial Arterial Arches in Birds,
with special reference to the Origin of the Subclavians and Carotids,"
_Phil. Trans._ 179 B (1888), pp. 111-141; L.A. Neugebauer, "Systema
venosum avium," _Nov. Act. Leopold. Carol._ xxi., 1844, pp. 517-698,
15 pls.; R. Gasch, "Beitraege zur vergl. Anatomic des Herzens der Voegel
und Reptilien," _Arch. f. Naturgesch._, 1888.