102 ARBOREAL MAN 



characters of those processes which project, one from the 

 dorsal aspect of each vertebra, and which are named 

 spinous processes or neural spines. These spinous pro- 

 cesses stand up in line all down the middle of the back, 

 and to them are attached portions of the great muscle 

 (M. erector spinse) which acts upon the vertebral column. 

 In actual disposition these spinous processes differ greatly 

 in different animals ; and the most conspicuous differences 

 are to be noted in the direction in which they slope. 

 Some, or all of them, may stand up quite straight, or 

 they may lean towards the head end, or the tail end, of 

 the animal. In the Reptiles, the arrangement of the 

 spines is comparatively simple, for in most existing types 

 all the processes stand directly upwards, or they are 

 directed slightly backwards at their free tips. There is 

 a very primitive plan, seen in many Reptiles, both living 

 and extinct, as well as in some existing and many extinct 

 Mammals, in which the vertebra to which the pelvic 

 girdle is united forms a definite landmark by possessing 

 an upright spinous process. All the vertebrae in front 

 of this one may have their spines directed slightly back- 

 wards towards it, or with some variation displayed in the 

 forward spines, uprightness is again found in the spine 

 of this pelvic or sacral vertebra. 



In many extinct reptilian forms the only truly upright 

 " spine is at the pelvis. In the Gavial some ten anterior 

 spines point backwards, about nine are upright, and five 

 are directed slightly forwards. In the modern Nilotic 

 Crocodile some seventeen anterior spines are directed 

 backwards, the last rib-bearing vertebra having an erect 

 spine, and being followed by some five elements in which 

 the spines are directed slightly forwards; and after that 

 comes the pelvic region with an upright spine again. 



The upright spine of the pelvic vertebra of the Reptiles 

 entitles this vertebra especially to the distinctive name 

 of anticlinal vertebra. This may be the only spine in the 

 whole vertebral column which does not slope, but we 



