THE EVOLUTION OF THE EYEBROW REGION OF THE FOREHEAD. 307 



turns round a wide and conspicuous notch on the inner part of the supraorbital margin 

 and immediately to the outer side of the glabella (fig. 19, PL I.). It follows from this 

 that the supratrochlear nerve in man is the representative of the whole nerve in the 

 baboon and macaque, and that the occasional groove or foramen on the orbital margin 

 associated with it is the morphological equivalent of the large single notch in the lower 

 ape. The new position of the frontal nerve in man (on the upper surface of the levator 

 palpebrse superioris and midway between the outer and inner walls of the orbit) and 

 the passage of the greater number of its fibres through a new nerve (the supraorbital) 

 is a condition which has probably been brought about by the increase in the breadth of 

 the human forehead, which renders it necessary for the greater number of the group of 

 nerve-fibres which go to supply the skin of this area to be shifted in an outward 

 direction. Like most acquisitions of recent phylogenetic origin, the condition is one 

 which is liable to considerable variation. Not only is the relative size of the two 

 branches of the frontal nerve in man subject to variation, but also the position of the 

 supraorbital notch on the orbital margin is very far from being constant. 



The arrangement of the frontal nerve in the orbit of the chimpanzee and orang is 

 slightly different from that which was seen in the baboon and macaque. The nerve still 

 clings to the inner wall of the orbit, but when it comes to the region above the trochlea 

 it divides into two branches, which no doubt correspond to the supratrochlear and 

 supraorbital branches in man. This division takes place below the front part of the 

 torus, and the supratrochlear is carried onwards in an upward and inward direction 

 around the inner part of the supraorbital arch and immediately to the outer side of the 

 glabella. The outer branch or the supraorbital nerve turns sharply outwards on the 

 under surface of the torus, and then winds on to the forehead in an oblique and often 

 very obscure groove, which is the representative of the supraorbital notch in man. 



This arrangement of the nerve can usually be made out in a very distinct manner 

 in the skull of the adult gorilla. From the markings on the bone it becomes evident 

 that the main portion of the nerve turns over the torus close to the inner wall of the 

 orbit in a very shallow groove which partakes more of the nature of a smooth pathway. 

 This lies above and in front of the trochlear pit. 



In nine out of eleven gorilla skulls there was evidence that the frontal nerve had 

 divided close to the margin of the orbital opening, and further, that the outer branch 

 (i.e. supraorbital) had diverged from the inner branch almost at a right angle before 

 turning round the supraorbital margin. The two shallow pathways for these nerves 

 are distinctly marked on the bone, and in a large male skull they were separated at the 

 points at which they turned upwards by an interval of 10 mm. As a rule they are 

 separated from each other by a low spine-like projection upon the under surface of 

 the torus. 



In no sense either in the chimpanzee or the gorilla can either of these grooves be 

 taken as giving an indication of the line of fusion between the superciliary and supra- 

 orbital elements of the torus supraorbitalis. This can be seen in those young specimens 



TRANS. ROY. SOC. EDIN., VOL. XLVI. PART II. (NO. 12). 46 



