378 THE ANIMAL KINGDOM. 



of the femoral veins with the two efferent renal veins (see p. 56). Lym- 

 phatic glands are present, few however in number. A pair of lymph-hearts 

 with striated muscle fibres in their walls are found in the pelvic region of 

 the Ostrich, Goose, Swan, and Stork, and also in the embryo chick in 

 which they subsequently disappear. The valves of the lymphatics are not 

 so numerous as in Mammalia. 



The nostrils lie at the tip of the beak in Apteryx : in other birds at its 

 base in front of the orbits. In the Gannets (Sula] they are closed. The 

 larynx consists of two arytenoid cartilages and a cricoid ring. The 

 trachea is always of considerable length : its cartilaginous rings are usually 

 perfect and at least partially ossified : it is often tortuous, forming convolu- 

 tions beneath the skin and sometimes lodged in the keel of the sternum, 

 e.g. in the Swan : it may be dilated at intervals and its cavity is divided 

 by a longitudinal septum in Aptenodytes and Procellaria (a Penguin and 

 a Petrel). In most birds including the Ratitae (see p. 55) a lower 

 larynx or syrinx is developed at the junction of the trachea with the 

 bronchi. It is produced by modifications of some of the lower tracheal 

 rings only (some American Passeres=Coracomorphae\ of the upper bron- 

 chial rings only (Steatornis among Caprimulgidae or Goatsuckers, and 

 Crotophaga, a Cuckoo, both from S. America), or, as most usual, of both 

 tracheal and bronchial rings. It has often a complicated structure, and in 

 many singing birds six intrinsic pairs of muscles. The voice is caused by 

 the varying tensions of an internal and external tympaniform membrane 

 developed, the former on the inner aspect of the incomplete upper bron- 

 chial rings, the latter between two of the same rings. In the males of 

 many Ducks the lower tracheal rings are dilated asymmetrically into a 

 resonant cavity with walls partly membranous, partly ossified. The 

 bronchi lose their cartilaginous rings to a great extent when they enter the 

 lung. There are four eparterial bronchia, and usually nine hyparterial. Of 

 the former, the first winds round the trachea and branches in a dorsal and 

 ventral direction : the fourth is rudimentary : but the second and third 

 run towards the inner border of the lungs parallel to one another. The 

 hyparterial bronchia branch on the outer aspect of the lungs dorsally and 

 ventrally. The lungs are fixed to the back of the thorax and are deeply 

 indented by the ribs but not otherwise lobed. The pleura cover only their 

 ventral surfaces on to which pass from the ribs small muscular bundles. 

 The ultimate branches of the bronchia are tubes more or less hexagonal, 

 and containing transverse and longitudinal ridges upon which the respira- 

 tory capillaries are distributed. Certain of the chief bronchia traverse the 

 lungs and open at their surface into air-sacs. Of these there are nine (see 

 PP- 55-6). Processes are prolonged from the anterior and posterior air-sacs 

 into the bones, with the exception of those of the head, which are supplied 

 with air from the nasal fossae and the tympanic cavity. The bones of the 



