436 



TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



one median on the basioccipital and two lateral on the exoccipitals. The articular surfaces 

 of these three condyles are inclined toward each other in such a way that forces coming, for 

 example, from a horizontal thrust of the backbone are divided into three streams which 

 spread out as they go forward and upward. Conversely, thrusts coming from the front 

 are collected and concentrated into three streams, which find their exits in lines normal to 

 the three condyles. Thus owing to the concentration of the bony tissue these three sets of 

 forces pass below and on either side of the medulla without in the least disturbing it. In 

 the side view (Fig. 299) the half downwardly-facing exoccipital condyles and the half 



B 



Hemitrlpterus 



Myoxocephalus 



Fig. 299. Neurocranium of scorpaenoids. 



upwardly-facing basioccipital condyle are inclined toward each other at an angle of about 

 ninety degrees; this arrangement makes it easy for the collateral ligaments to prevent dis- 

 location of the neck and consequent strangulation of the spinal cord. Moreover, in typical 

 fish the basioccipital condyle has a deep cotylus, which is filled in life with more or less 

 elastic intercentral tissue, and a raised rim, to which are attached the ligaments that tie it 

 to the first vertebra. On the exoccipital condyles the articular surface is shallowly convex, 

 with raised rims. Doubtless the ligaments and interarticular discs take up some of the 

 shocks, while permitting some movement of the head on the column in connection with the 

 alternate left and right movement of the head in swimming. 



The fact that the forces coming from the rear and impinging on the lateral condyles 

 are spread outward and upward is reflected in the construction of the occipital segment of 

 the skull, for from the arrangement of the main braces above the condyles it is seen that 



