276 THE MECHANICAL FUNCTIONS. 



one another, and a third posterior to these. In the Orni- 

 thorhyncus, while the latter retains its situation in the mid- 

 dle, the other surfaces have separated from each other, and 

 have travelled outwards, taking their stations upon the 

 leaves. In the Mammalia, the middle surface has Avholly 

 disappeared, and the outer surfaces have risen into what are 

 termed the oblique processes. 



In addition to these, accessory bones are often developed 

 to suit particular occasions. Thus, in fishes, we see that one 

 or two additional pieces (i) are affixed to the ends of each 

 spinous process. In many cases, instead of being thus placed 

 in a line with these processes, they appear at a little distance, 

 as if they had slipped from their proper situations; they are 

 then found between the spinous processes, and receive the 

 name of interspinous hoiies. 



The spinous processes have a tendency, when their de- 

 velopment proceeds, to divide into two branches, and this bi- 

 furcation frequently takes place also in the interspinous bones. 

 The transverse processes, likewise, occasionally develope ac- 

 cessory pieces, as is found to be the case in some reptiles ; 

 but, in other instances, they undergo a gradual change of po- 

 sition, as we follow them backwards along the spinal column, 

 where they descend tow^ards the abdominal region. 



The flexibility of particular portions of the spinal column 

 is regulated by the size and form of its processes. When 

 these are much developed, they necessarily obstruct the flex- 

 ion of the vertebrae in the directions in which they are situ- 

 ated : when they are small, no such hinderance arises, and the 

 spine is free to move in all directions. Thus, when we see 

 the spinous processes much enlarged, while the transverse 

 processes are small, we may infer that the spine is incapable 

 of any bending in that direction ; but that it has the power of 

 free lateral flexion. This is the condition of the spine of 

 fishes, where this latter kind of motion is the one principally 

 wanted. In dolphins, and other cetacea, on the contrary, 

 where the actions are required to be vertically upwards and 

 downwards, the spinous processes are small, and the trans- 

 verse processes very long and broad. 



