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 eac — 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 peritoneum, 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 externum, 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 membi'anes 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 



