THE SENSE-ORGANS 495 



In the Sauropsida there is but little change in the tympanic 

 cavity from that of the Anura. The two Eustachian tubes 

 often form by their union a median duct, which opens into the 

 pharynx in the mid-dorsal line. Such is the case in birds and 

 in crocodiles, and in the latter the tubes under consideration 

 form a complicated system of cavities, many of which are 

 lodged in the bones of the cranium. In other cases similar sys- 

 tems extend out from the main tympanic cavity, and in certain 

 instances the two latter communicate with one another across 

 the median line. 



A characteristic and important addition is gained in mam- 

 mals by the appearance within the tympanic cavity of the artic- 

 ular and quadrate bones, hitherto employed in forming the 

 mandibular articulation. These form respectively the malleus 

 and incus, and become added to the columella to form a chain 

 of ossicles which reaches across the cavity from the other drum 

 head to the fenestra ovalis, thus assuming the function formerly 

 sustained by the columella alone. This latter apparently be- 

 comes reduced in size and forms the stapes. (Cf. Chap. V.) 

 The singular and characteristic foramen in this bone, to which 

 it owes its similarity to a stirrup, is caused by the development 

 of a small artery, which perforates the columella. This re- 

 lation appears only in the embryo in most mammals, includ- 

 ing Man, but in some (certain rodents and insectivores) it per- 

 sists throughout life. (Cf. Chap. IX.) In others still, mainly 

 monotremes and marsupials, the perforation does not take 

 place, but the bone remains in the primitive cylindrical form. 



The stapes is supplied by a tiny muscle, the stapedius, which 

 is shown by its embryology to be a slip separated from the 

 digastric muscle, an element primarily associated with the 

 hyoid arch. To the malleus is attached a second small muscle, 

 the tensor tympani. This was originally a portion of the com- 

 mon mass from which the masticatory muscles of the jaw have 

 differentiated, the abductor mandibuli of the selachians. This 

 little slip arises from that portion which forms the pterygoid 

 muscles, and is innerved by a branch from Trigeminus, the 

 nerve associated with the first or mandibular arch. These two 



