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130 Veterinary Histology of Domestic Mammals and Birds

The endocardium is similar to the tunica intima of blood ventricularis, bundle of His) to the bundle branches and
VetBooks.ir vessels. Its innermost layer consists of a thin endothelium the walls of the ventricles. The sinoatrial and atrioventricu-
and loose subendothelial connective tissue. Adjoining lar nodes are interconnected by the atrial myocardium and
this is a meshwork of collagen and elastic fibres in which are particularly well vascularised. The conducting system
smooth muscle cells are embedded (stratum myoelasti- is composed of three cell types:
cum). The endocardium is joined to the myocardium by
a vascularised tela subendocardialis. This subendocardial · nodal cells (pacemaker cells),
layer contains the fibres of the autonomic conduction · transitional cells and
system (myofibra conducens cardiaca), known as Purkinje · Purkinje cells (myofibra conducens cardiaca).
fibres (see Figure 4.17).
The heart valves are endocardial outfoldings with a Nodal (pacemaker) cells are found, together with tran-
tough ‘skeleton’ of collagen fibres. They are avascular, yet sitional cells, in the sinoatrial and atrioventricular nodes,
richly supplied with autonomic nerve fibres. The chordae and in the conduction pathways. These pacemaker cells
tendinae attach the ventricular side of the atrioventricular are characterised by lightly staining cytoplasm rich in
valves to the papillary muscles. mitochondria, a high density of glycogen granules and
The myocardium is composed of cardiac muscle cells numerous pinocytotic vesicles. Myofibrils are sparse and
(see Chapter 4, ‘Muscle tissue’) and a modestly devel- are distributed diffusely within the cytoplasm.
oped connective tissue meshwork incorporating a dense Transitional cells appear to be localised at the final
network of capillaries. Arterioles supplying the myocar- branches of the conduction pathway, between the Purkinje
dium exhibit several physiological specialisations that aid cells and the myocardium. They contain few mitochondria
in meeting the oxygen requirements of this tissue. The and little glycogen. The cells are arranged in longitudinal
myocardium also contains autonomic nerve fibres and spirals.
numerous lymph vessels. The conducting system terminates in the ventricles as
The heart muscle is attached to the fibrous cardiac bundles of Purkinje cells. Transmission of impulses by the
skeleton, consisting of the annular rings (annuli fibrosis) Purkinje cells results in myocardial contraction. Measuring
surrounding the valves, the fibrous trigones that connect up to 100 μm, Purkinje cells have pale cytoplasm with few,
the rings, and the membranous part of the interventricular longitudinally aligned myofibrils, and are high in glycogen.
septum. Variation in the orientation of fibres in the ventri- Purkinje fibres travel beneath the endocardium, eventually
cles gives rise to an outer layer of longitudinally oriented ramifying in the myocardium. They are connected by gap
muscle fibres, a circular middle layer and an inner layer in junctions with ordinary myocardial cells, either directly or
which the muscle cells are arranged in spirals. The inner via transitional cells.
layer is continuous with the papillary muscles. In addition to the conducting system, the heart contains
The epicardium is the visceral leaf of the pericardium, a dense network of sympathetic nerve fibres that secrete
also constituting the pericardial subserosa. It consists of adrenergic agonists (acting primarily on β-receptors).
a simple epithelium overlying a thin connective tissue Vagal (parasympathetic) fibre bundles primarily inner-
layer. vate the atria. Atrial cells synthesise the peptide hormone
In contrast to the muscle of the blood vessels, the ANP (atrial natriuretic peptide), which promotes diuresis
myocardium is composed of a specialised form of stri- (see Chapter 9, ‘Endocrine system’).
ated muscle (see Chapter 4, ‘Muscle tissue’). Moreover,
the heart is capable of generating and conducting electri- Lymph vessels (systema lymphovasculare)
cal impulses, giving rise to rhythmic cardiac contractions Lymph vessels are the drainage system for the extracel-
without input from the central nervous system. lular fluid of the connective tissue, serving to convey
lymph to the blood vascular system (Figures 6.20 to 6.24).
The conducting system of the heart The system of lymph vessels begins as a network of anas-
A feature of the cardiac muscle is its capacity for autono- tomosing lymph capillaries that merge to form larger
mous generation and propagation of rhythmic electrical vessels. Lymph vessels typically pass via lymph nodes as
activity (bioelectric impulses). This so-called conducting they conduct lymph to the venous system, which they join
system of the heart is composed not of nerve fibres, but near the thoracic inlet.
of modified cardiac muscle cells. Lymph is composed of cells (particularly lymphocytes)
The impulse is initiated at the sinoatrial node (nodus and lymph plasma. The lymph plasma, a colourless to
sinuatrialis), referred to as the pacemaker of the heart. The pale yellow fluid, contains proteins including albumin,
impulse spreads smoothly and rapidly via the atrioventric- prothrombin, fibrinogen and globulins. The fat content
ular node (nodus atrioventricularis, Aschoff–Tawara node) is determined largely by short-chain fatty acids absorbed
and the atrioventricular bundle (truncus fasciculi atrio- from the intestine. These join with phospholipids,

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