MAMMALIA. 365 



by a fossa ovalis, indicating the position of the foramen ovale in the foetus 

 by which blood passed from the right to the left auricle. The right, like 

 the left, auriculo-ventricular valve is membranous, and consists of three flaps 

 (hence tricuspid), one of which, the septal flap, is attached to the septum 

 of the ventricles. The aorta crosses the left bronchus, and the right sub- 

 clavian artery represents the corresponding, i. e. the fourth aortic arch of 

 the right side. The sub-clavians and carotids arise from the aortic arch 

 in various ways. The blood of the left vena cava superior is frequently 

 carried across to the right cava superior by a transverse vein, and the 

 cardiac end of the vessel (= coronary sinus) alone persists receiving the 

 coronary veins of the heart, and sometimes, as in many Ungulata, a left 

 azygos vein. The veins of the extremities at least possess valves, structures 

 wanting, however, in Cetacea. The lymphatic system is well developed, and 

 there are numerous lymphatic glands. The thoracic duct opens into the 

 left subclavian vein. The lymphatic vessels have valves. The tonsils, 

 masses of adenoid tissue at the entrance to the pharynx, are peculiar to 

 Mammals. For the hibernating gland, see pp. 2 and 4. 



The nostrils which lead to the air-passages are double, except in some 

 toothed Whales. The entrance to the trachea is protected by an epiglottis. 

 There is a larynx composed of two arytenoid cartilages, a single thyroid 

 and cricoid cartilage, and provided with well-developed muscles. The 

 cartilage rings of the trachea are incomplete, posteriorly or dorsally. The 

 trachea divides into two principal bronchi. There is but a single bronchia 

 rising above the entrance of the pulmonary artery. This eparterial bronchia 

 is present either on both sides or on the right only, e. g. Primates, and 

 in some instances it rises on the right side from the trachea, e. g. in 

 Artiodactyle Ungulate. It is wanting in the Porcupine (Hystrix). There 

 are 9 hyparterial bronchia. The lungs are suspended freely in pleural 

 sacs, and they are frequently lobed, especially on the right side. The 

 respiratory capillaries are distributed on the walls of the lobed air-cells, or 

 alveoli, in which the ultimate branches of the bronchiae terminate. 



The kidneys sometimes retain the embryonic lobed character (Cetacea, 

 the Seals, "Bear, &c., among Carnivora), but, as a rule, the lobes fuse com- 

 pletely at their outer extremity, while at their inner they unite to form 

 the Malpighian pyramids, upon which the tubuli uriniferi open. These 

 pyramids project into the dilated upper ends or pelves of the ureters. The 

 blood-vessels of the gland and of the surrounding adipose tissue anastomose, 

 but there is no functional renal-portal system. The ureters enter the dorsal 

 aspect of the urinary bladder, either near its apex or more usually its 

 fundus (=base). This bladder is a remnant of the urachus, or internal 

 portion of the allantois. It has an outlet or urethra which, in the male, 

 unites with the genital ducts to form a well developed uro-genital canal, 

 while in the female the urethra and genital outlet alike fall into a shallow 



