544 ZOOLOGY SECT. 



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. In foetal Marsu- 

 pials, 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 Artiodactyla are exceptional in 

 having a third bronchus, 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 cham- 

 bers, extend into the bones of the skull, especially into the 

 maxillae 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 System. The brain of Mammals (Fig. 1133) 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 the corpus 

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

 spheres, in all but certain of the lower groups of Mammals, are 

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

 winding ridges or convolutions. The lateral ventricles in the 

 interior of the hemispheres are of large size and somewhat complex 

 form. 



The optic lobes, which are relatively small, are divided into four 

 parts, and are hence called the corpora quadrigemina. The 

 pineal body is always a small gland-like structure. Connecting 

 together the lateral parts of the cerebellum, which, in the higher 

 Mammals, attains a high degree of development, is a transverse 

 flattened band the pons Varolii (Po.) crossing the hind brain 

 on its ventral aspect. 



In the Monotremes and Marsupials (Figs. 1134, 1135) there is 

 no corpus callosum, while the anterior commissure (ant. com.) is of 

 relatively large size. The hippocampi extend along the whole 

 length of the lateral ventricles. The layer of nerve-cells in each 

 hippocampus gives origin, as in Eutheria, to numerous fibres, which 

 form a layer on the surface, the alveus, and become arranged in a 

 band the tsenia hippocampi. In the Eutheria, as we have seen 

 in the case of the Rabbit, the tsenise unite mesially to form the body 

 of the fornix. In the Monotremes and Marsupials, on the other 



