CHAP. IV] THE GENERAL ANATOMY OF THE CERCOPITHECIDAE 137 



retractor bulbi oculi) is found constantly in the Cercopithecidae. 

 It is unknown in the New- World monkeys 1 , in the Simiidae, in 

 Man 2 , and Burmeister neither mentions it, nor does he figure it in 

 Tarsius. So irregular a distribution must own some definite signifi- 

 cance, but as yet this is obscure. 



G. Vascular System. The heart is placed mesially 3 , but 

 the apex has just reached the left side of the sternum (in Cyno- 

 cephalous monkeys it may retain the mesial position) where it 

 lies close to the sixth costal cartilage. The form of the heart also 

 shews an advance on that of the Lemurs, for the transverse 

 diameter begins to assert a superiority over the dorsi-ventral one. 

 Again the long axis of the heart is more strongly inclined to that 

 of the post-caval vein, and therefore is pointed less directly towards 

 the diaphragm. The intra-thoracic course of the post-caval vein 

 is correspondingly shortened but is still distinct ; the subpericardial 

 sinus (with the lobus impar) still persists, while a " bursa infra- 

 cardiaca " of : ' peritoneal " origin has been seen in Cynocephalus 

 by Professor Peter Thompson. The diaphragmatic contact-area 

 of the pericardium is however enlarged. The great vessels some- 

 times arise from the aortic arch as in Man. More commonly, how- 

 ever, only two trunks arise thus. Of these the first or innominate 

 gives origin to the left common carotid artery and then bifurcates 

 as in Man. These characters may be taken to indicate the early 

 stages of a thoracic enlargement, and a change in form related to 

 the shortening of the torso, and presumably adapted to the as- 

 sumption of the erect attitude. 



The axillary artery still retains a primitive arrangement 

 by providing a large subscapular trunk for the circumflex 

 arteries of the humerus and scapula. But the A. circumflexa 

 humeri anterior tends to be detached from this and to arise 

 independently from the axillary vessel. Herein the Cerco- 

 pithecidae (especially Cynocephalus) approach Man more closely 

 than do the Simiidae. The brachial vessel is almost always 

 of the "superficial" type, crossing the median nerve superficially, 



1 Ottley, P. Z. S. 1879, p. 127. 



- Except as an anomaly: cf. Mr Whitnall's specimen, J. A. P., xlvi. Oct. 1911, 

 p. 36. 



3 Ruge, Morph. Jahrb. Bel. xrx. 



