AIR-TUBES AND LARYNX 



365 



(dilator, constrictor, protractor, and retractor) around the aper- 

 ture of the swim-bladder, though no cartilaginous elements are 

 formed (Fig. 271): these muscles are innerved by the vagus, 

 whether the aperture is dorsal (Lepidosteus, Amia) or ventral 

 (Polypterus). The same is true of the Dipnoi, in which there is 

 a tongue-shaped supporting plate composed of dense connective 

 tissue anteriorly to the glottis, which leads into a muscular 

 vestibule or " laryngo-tracheal " chamber communicating with the 



Ao. 



av. or. 



FIG. '271. MEDIAN LONGITUDINAL .SECTION* THROUGH PART OF THE HEAD 

 AND TRUNK OF Lepidoxleus osseus. 



Ao, aorta; Car, cavity of the swim-bladder; Co, Co 2 , and **, constrictors of 

 the pharynx, " larj*nx," and swim-bladder ; Da, dilator ; G, cranial cavity ; 

 K, cushion-like elevation of the supporting elements of the larynx ; L.B'j, 

 loose connective tissue between the swim-bladder and pharynx ; Peric, 

 pericardium ; PA 1 , 3, 4, intrapharyngeal branchial muscle ; Phar, pharynx ; 

 I\.M, neural canal ; SS, fibrous laryngeal supporting elements ; WS, vertebral 

 column. 



lung; but it is doubtful whether this plate can be regarded as 

 the first phylogenetic indication of the laryngeal skeleton of 

 higher forms, and whether the laryngeal muscles of the Amphibia 

 have been derived from those represented in the Dipnoi. 



A comparison of these parts in Ganoids and Dipnoans indi- 

 cates the possibility of the former existence in the vertebrate 

 series of two larynges, a " dorsal ". and a " ventral " (Fig. 272), 

 traces of the former of which can still be recognised in Lepidosiren 

 in addition to the ventral larynx. 



