ANATOMY OF THE PIGHON. 529 
opening into the upper part, while the longer duct, after 
uniting with that of the pancreas, opens into the lower part 
of the duodenum. 
The length of the neck in birds is never less than the 
height at which the body is carried from the ground ; the 
number of vertebre entering into its formation varies from 
9 to 24 (swan); in the pigeon there are twelve, accompanied 
by a corresponding number of spinal nerves, the branches of 
which may be observed immediately underneath the skin. 
The main mass of the neck is composed of the vertebral col- 
umn and muscles, the trachea and esophagus. On either 
side of the base of the neck, in close proximity to the trachea 
and carotid artery, is a small oval white body, the thyroid 
gland (77), at first developed as an evagination of the fore- 
gut, but afterward becoming a closed and ductless sac, 
which is found in the majority of vertebrates, but the use of 
which to the organism is entirely unknown. Above the thy- 
roid lie the carotid artery and jugular vein, the main vas- 
cular trunks of the head and neck. The right jugular vein 
is usually the largest. Along the side of the neck, above 
the trachea on the left and the esophagus on the right, lies. 
the elongated thymus gland (7m), drawn somewhat dia- 
grammatically ; ; this gland forms part of the lymphatic sys- 
tem, and in minute structure resembles the spleen. 
The heart (H#) lies immediately below the lungs and: 
against the sternum, with its apex between the two lobes of 
the liver pointing obliquely downward and backward ; it 
is enclosed in a thin membranous bag, the pericardium, 
which is filled with serous fluid and attached to the roots of 
the main vascular trunks. To study the heart, it must be 
excised, taking the greatest care to leave as much as possible: 
of the vessels, especially the large veins behind, in connec- 
tion with it. Viewed from behind (Fig. 459, C), the heart 
is seen to be composed of four chambers, the two anterior 
ones, the auricles, being the smaller. The left auricle receives 
upon its dorsal side the opening of the united pulmonary 
veins (Pv), one from each lung ; the right auricle is larger 
than the left, and receives in its upper portion the right vena 
cava superior (Vsd); in its lower portion the left vena 
