The lungs are remarkable for their unlobulated character; in this they agree with the lungs of the Sirenia. The thoracic cavity in which they lie is barrel-shaped, and not, as is usual in terrestrial mammals, boat-shaped, _i.e._ narrower sternally than above. The alteration of the shape of the thoracic cavity is associated with the aquatic life; so at any rate the fact that it is also marked in Seals and even in the Otter seems to show. The Whales are also characterised by the great obliquity of the diaphragm, which is extremely muscular. In this character again we find an agreement with the Sirenia, and also with other aquatic mammals; it is not therefore a character of Whales so much as evidence of an adaptation to the aquatic life. The advantage is, it appears, in the increased capacity of the thoracic cavity, and the consequent greater possibilities of expansion of the lungs, which it must be remembered serve as hydrostatic as well as breathing organs. Some of the internal arteries of Whales break up into retia mirabilia. Their kidneys are lobulated; whether this has {349} anything to do with the aquatic life is not so clear. It also characterises the Sirenia, more or less, and the Otters; but, on the other hand, the terrestrial Bears show the same structure as do also some Ungulates. It must be borne in mind, too, that the kidneys of foetal Man are lobulated. The liver is a compact organ not showing such lobulation as is common, but not universal, among mammals. The bones of Whales have a somewhat loose structure, and are much impregnated with oil. In many features the skeleton of Whales is highly distinctive of the order. [Illustration] FIG. 183.--Skeleton of Porpoise (_Phocoena communis_), _acr_, Acromion process of scapula; _cerv_, united cervical vertebrae; _chev_, chevron bones; _cor_, coracoid process; _hu_, humerus; _hy_, hyoid; _jug_, jugal; _lumb.trans_, lumbar transverse processes; _max_, maxilla; _nas_, nasal; _orb_, orbit; _par_, parietal; _pelv_, vestige of pelvis; _per.ot_, periotic; _pr.max_, premaxilla; _rad_, radius; _rb_^1, first rib; _rb^{12}_, twelfth rib; _sc_, scapula; _s.occ_, supra-occipital; _st_, sternum; _uln_, ulna; _zyg_, prezygapophysis. (From Parker and Haswell's _Zoology_.) [Illustration] FIG. 184.--Under surface of the cranium of a young Caa'ing Whale (_Globicephalus melas_). x 1/5. _AS_, Alisphenoid; _BO_, basioccipital; _cf_, condylar foramen; _ExO_, exoccipital; _Fr_, supra-orbital process of frontal; _gf_, glenoid fossa of squamosal; _Ma_, body of malar; _Mx_, maxilla; _OS_, orbitosphenoid; _Per_, posterior (mastoid) process of periotic; _Pl_, palatine; _PMx_, premaxilla; _Pt_, pterygoid; _Sq_, squamosal; _tg_, deep groove on squamosal for meatus auditorius externus, leading to tympanic cavity; _Ty_, tympanic; _Vo_, vomer; _ZM_, zygomatic process of malar. (From Flower's _Osteology_.)