i6o 



THE LOWER PRIMATES 



none of the bold markings in the way of fissures and convohitions which 

 ultimately become the identifying (Fig. 75) features of this part of the 

 nervous system in primates. The marmoset brain may be considered as 



FIG. 75. LEFT LATERAL SURFACE OF BRAIN, CALLITHRIX JACCHUS (MARMOSET). 



[Actual Length, 31 mm.] 

 Kev to Diagram, ob., Oblongata. 



something more than a mere transitional form. It must be taken to represent 

 a transition already achieved from the lower mammals into the primate 

 order but also manifesting certain retrograde changes. 



THE ORBITAL SURFACE 



On the orbital surface the brain has all of the typical markings of the 

 lower primates. The two orbital concavities are well defined. The inception 

 of these concavities seen in the lemur's brain is here carried to its ultimate 

 development. The orbital surface now rests upon an expanded orbital plate 

 of the frontal bone. The interorbital keels are also well defined and more 

 marked than in the lemur brain. The olfactory bulbs, though fairly large, are 

 less pronounced than in the lemur, and the olfactory tract is detachable as far 

 back as the trigonum which is fairly prominent (Fig. 74). 



THE OCCIPITAL REGION OF THE BASAL SURFACE 



In the occipital region of the basal surface a well-marked occipital con- 

 cavity exists which is most emphasized in and about the midline, particu- 



