ANATOMY OF THE PIGEON. 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 vertebras 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 oesophagus. 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 (Tr), 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 oesophagus on the right, lies 

 the elongated thymus gland (Tm), drawn somewhat dia- 

 gram matically ; this gland forms part of the lymphatic sys- 

 tem, and in minute structure resembles the spleen. 



The heart (Hi] 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. 



