CHAP. IV] THE GENERAL ANATOMY OF THE CERCOPITHECIDAE 145 



I. Respiratory System : the Larynx. The hyoid bone 

 is characterized by the enlargement and downward extension of 

 the basi-hyal. The latter however is not excavated in the 

 Cercopithecidae as in the Cebian Mycetes (Howling Monkey). 

 The laryngeal apparatus generally resembles that of Man, all the 

 muscles and cartilages of the human larynx being recognizable. 

 Coronal sections of the larynx reveal one or two distinctive 

 features (cf. Fig. 90). 



Most obvious perhaps is the relatively long and thin plica 

 vocalis. The unusual extent of the M. crico-thyroideus on the 

 deep aspect of the thyroid cartilage is also evident. 



The Lungs. The lungs are more subdivided than in Man ; 

 in the left lung (Fig. 92) there are three lobes corresponding in a 

 general way to the three lobes normally found in the right human 

 lung. In the right lung (Fig. 91) the three lobes (of human 

 anatomy) will be found supplemented by a fourth lobe ; this lobe, 

 which is small and pendulous, is known as the lobus impar : it is 

 situated beneath the right bronchus and sometimes it sends a 

 process leftwards crossing the oesophagus anteriorly and thus 

 encroaching on the posterior mediastinum and left side of the 

 thorax. This involves a modification of the right pleural 

 membrane, and the production of a diverticulum to contain the 

 lobus impar. The diverticulum is called the sinus subperi- 

 cardiacus 1 , and its passage between the oesophagus and the post- 

 caval vein beneath the pericardial sac is doubtless related to the 

 limited attachment of the latter to the diaphragm. 



The pleura may form a cervical dome in rare instances among 

 the Cercopithecidae, but such cases are exceptional 2 in these 

 animals and absent from other Primates. Behind the sternum 

 the first contact of the two sacs is nearer the head than in Man, 

 and the sacs accompany each other further before they diverge. 

 The left pleura first breaks away over the pericardium, but the 

 uncovered area of the latter is relatively small in extent. The 

 pleural sacs extend far down the vertebral column behind, in 

 Cercopithecidae reaching to the vertebra below the last thoracic. 



1 Ruge, Morphologischea Jahrbuch. The Lemur possesses a lobus impar and 

 the corresponding pleural pouch. 



2 Patten, op. cit., 1899, p. 10. 



D. M. 10 



