NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 5 



the sternum loses an essential stimulus and becomes reduced and 

 atrophied. 



The head, during swimming, is held directed as firmly as possible 

 forward. The neck is not moved, and for this reason it becomes short 

 and stiff. During motion through the water the head is pressed from 

 the front ; it is forced backward against the cervical vertebrae, which 

 thereby are squeezed excessively together and pressed back against 

 the anterior dorsal vertebrae, with the ribs of which they may even 

 come in connection. Most of the cervical vertebrae may become 

 almost as thin as paper. The odontoid process of the axis becomes 

 short and blunt ; the articular surface between the bodies of the atlas 

 and axis becomes almost flat. And there arises a strong tendency to 

 coalescence of the cervicals.° The occipital condyles lose their pro- 

 jecting form and become almost flat, only quite weakly convex, pressed 

 in against the wall of the braincase ; and the concave surface of the 

 atlas likewise becomes flattened out. The occipital crest in its 

 capacity as an attachment for the upper neck muscles is restricted ; 

 the points of attachment for the lower neck muscles on the basal part 

 of the occipital bone are effaced, and the under side of the occipital 

 bone is formed more as a sheath around the gullet and windpipe. 



The pressure of the water on the head when the cetacean swims 

 has a highly modifying effect on the skull. 



From above the water presses especially during the animal's con- 

 stant rising to the surface to breathe. This gives the skull a tendency 

 to acquire a flat and broad upper surface, with thick bones. The size 

 of the horizontally outspread supraorbital process of the frontal, 

 which pushes itself far out over the orbit, becomes particularly notice- 

 able. The facial part of the cheek bone may likewise become pecu- 

 liarly flattened out. 



From in front the water presses during forward motion, the more 

 strongly as the motion is faster. Its effect is to develop an unusual 

 strength in those bones of the face which project furthest forward, 

 the intermaxillary, maxillary, and vomer, as well as in the cartilaginous 

 nasal partition which the vomer embraces. This strengthening may 

 show itself in different ways : in the noticeable lengthening forward 

 of the bones in question, in their solid ossification, in their tendency 

 to coalesce. It also appears in the backward spreading of the inter- 

 maxillary and especially of the maxillary. The latter may extend 

 itself out over the facial part of the zygoma and over the frontal, 

 which it almost entirely covers to the hinder margin, so that the supra- 

 orbital foramen may pierce not only the frontal as in other mammals. 



