558 ZOOLOGY SECT. 



as in the Rabbit, an innominate may give origin to the right sub- 

 clavian and both carotids, the left subclavian coming off separately. 



In Monotremes and Marsupials, in most Ungulates, and in the 

 Rodentia, Insectivora, and Chiroptera, both right and left pre- 

 cavals persist ; in the others the left aborts, its vestige giving 

 rise to the coronary sinus. In the Monotremes the openings of 

 all three cavals are provided with valves, only vestiges of which 

 exist in the other groups. In the Monotremes all the pulmonary 

 veins open by a common trunk. In the Metatheria and Eutheria 

 the four veins sometimes open separately, sometimes the two 

 veins of each side unite to form a single lateral trunk. In Echidna 

 there is an abdominal vein corresponding to that of the Frog and 

 Lizard (pp. 272 and 316). 



The following are some of the principal variations, in the struc- 

 ture of the heart which occur in the different groups of Mammals. 

 In the Monotremes there is a deep fossa representing the fossa 

 ovalis in the auricular septum. The auriculo-ventricular valves 

 depart from the structure typical of Mammals and approach the 

 corresponding valves in the heart of Birds. In the Marsupials the 

 fossa ovalis and annulus ovalis are absent ; in the uterine foetus of 

 the Kangaroo the auricles communicate by a fissure, but all trace 

 of this becomes lost before the adult stage is reached. 



In the Cetacea, Eustachian and Thebesian valves are both 

 absent. In some of the Cetacea the apices of the ventricles are 

 separated by a slight depression. In the Sirenia there is a corre- 

 sponding, but much deeper and wider, cleft, so that the apex of the 

 heart is distinctly bifid. 



In the Ungulata, Eustachian and coronary valves are both 

 absent ; in some there is a cartilage or a bone the os cordis 

 often double, at the base of the heart. The Eustachian valve is 

 absent in most of the Carnivora. In the Pinnipedia an aperture of 

 communication between the auricles often persists in the adult. 



The organs of respiration resemble those of the Rabbit in 

 the general features mentioned on p. 453. 



In the Cetacea, the epiglottis and arytenoids are prolonged to 

 form a tube, which extends into the nasal chambers and is em- 

 braced by the soft palate, so that a continuous passage is formed 

 leading from the nasal chambers to the larynx, and giving rise to 

 the condition of intra-narial epiglottis. In all the remaining orders 

 a similar condition occasionally occurs the epiglottis being pro- 

 duced upwards into the respiratory division of the pharynx behind 

 the soft palate. In fcetal Marsupials, in which the intra-narial 

 condition is very complete, it is obviously associated with the 

 passive absorption of the milk, while breathing is being carried on 

 continuously through the nostrils. Some Cetacea and Artiodac- 

 tyla, etc., are exceptional in having a third bronchus, which passes 

 to the right lung anteriorly to the ordinary bronchus of that side 



