HA Y-BIRD—HEART 413 
HAY-BIRD and HAY-JACK, common local names in many 
parts of England for species of the restricted genus Sylvia (or 
Curruca as some would have it)—especially the BLACKCAP and 
Garden-WARBLER, on account of the beautiful nests like open 
basket-work they build, chiefly of bents, but having a portion of 
other plant-stems, and hairs interwoven. The first name is also 
often given to the WILLOW-WREN ; but apparently by confounding 
it with the birds to which it properly applies. 
HEART, a muscular tube interposed between the central ends 
of the veins and arteries, forming the pump that forces all the 
blood through every part of the body, by its contractions, which are 
much quicker in Birds than in any other animals, numbering about 
120 in the minute when the bird is at rest, and, when it is flying 
or has just alighted, beyond count by the ear—even the first stroke 
of the wings nearly doubles the rate of pulsation, and in accordance 
with this rapid metabolism of the avine organism the Heart is com- 
paratively larger than in other Vertebrates. In shape it is conical, 
with the apex directed towards the tail, its long axis being parallel 
to that of the trunk, and it lies in the middle line of the body in 
the thoracic cavity, partly surrounded by the lobes of the Liver. 
The walls of the Heart consist of three layers, of which the 
principal (1), forming the greater part of the whole, is composed of 
striped muscular fibres, differing from voluntary muscular fibres, and 
peculiar in so far that they are individually ramified and con- 
nected with each other like network—most of them describing a 
figure of 8, starting at the base of the Heart and passing the apex 
with a spiral twist. The next layer (2) is the endocardium, lining 
the cavities of the organ, and composed of endothelial cells, elastic 
tissue and unstriped muscular fibre. Lastly (3) is the pericardium 
viscerale, a continuation of the peritonwum, and covering the Heart 
like any other viscus (cf. DIGESTIVE SYSTEM, p. 139, fig. 2). This 
visceral layer is prolonged from the base of the organ to the 
pericardium parietale or cxternum, forming a closed bag filled with a 
little serous fluid in which the Heart lies. This is the pericardium 
proper and is part of the cardio-abdominal chamber, severally con- 
nected with the membranes of the DIAPHRAGM. Owing to this 
arrangement the whole ventral surface of the pericardium is exposed 
when the sternum is removed. 
The Heart of Birds like that of Mammals consists of two com- 
pletely divided halves, each of which again is composed of an atrium 
and a ventricle. The right half receives and discharges only venous, 
the left only arterial blood (cf. CIRCULATION, p. 88). The two atria 
form the basal division of the Heart—thin-walled and darkly- 
coloured. The two ventricles, lighter in colour and with thicker 
walls, form the greater part of the cone. These two divisions are 
