570 ZOOLOGY SECT. 



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 corresponding, 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. 466. 



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 

 produced upwards into the respiratory division of the pharynx 

 behind the soft palate. In foetal 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, &c., are exceptional in having a third Ironchus, which passes 

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

 and to the pulmonary artery. In connection with various parts of 

 the respiratory system, there are cavities containing air. The 

 connection of the tympanic cavity with the pharynx by means of 

 the Eustachian tubes has been already mentioned. Air-sinuses, 

 connected with the nasal chambers, extend into the bones of the 

 skull, especially into the maxillai and . frontals, where they may 

 reach large dimensions . and are known as the maxillary antra 

 and frontal sinuses. Air-sacs are also developed in connection 

 with the larynx in many of the Apes. 



Nervous Sjrstem. — The brain of Mammals (Fig. 1202) is 

 distinguished by. its relatively large size, and by the large size 

 and complex structure of the cerebral hemispheres of the fore- 

 brain. 



The cerebral hemispheres of opposite sides are connected 

 together across the middle line in all Mammals, except the Mono- 

 tremes and Marsupials, by a band of nerve tissue termed t\ie corpus 

 callosum — a structure not present in the Sauropsida. The hemi- 

 spheres, in all but certain of the lower and smaller Mammals, are 

 not smooth, but marked by a number of grooves or sulci separating 

 winding ridges or convolutions. The lateral ventricles in the 



