292 COMPARATIVE ANATOMY 



which fits against the capitulum of the malleus. The 

 shorter of the two processes is directed backwards, and 

 is attached to the wall of the tympanic cavity ; the longer 

 process is directed downwards and inwards, and bears at 

 its extremity a small nodule of bone, the orbiculare. The 

 stapes is a stirrup-shaped bone, the base of which is 

 attached to the membrane covering the fenestra ovalis, or 

 aperture leading into the inner ear, and its arch is articulated 

 with the orbiculare. The study of the development of the 

 auditory ossicles shows that the malleus and incus are formed 

 from the upper end of the mandibular arch, and that the 

 incus is the equivalent of the quadrate bone or cartilage of 

 lower vertebrates. The malleus is derived from the upper 

 end of Meckel's cartilage, which in birds and reptiles is 

 ossified, and forms the os articulare of the lower jaw. The 

 malleus of mammals, then, represents a part of the lower 

 jaw of lower vertebrates. The homology of the stapes is 

 more doubtful, but it appears to be formed in connection 

 with the upper end of the hyoid arch, and therefore may be 

 in some degree representative of the hyomandibular of fishes. 



With the exception of the lowest forms, the sternum of 

 mammals is characteristically formed of a number of seg- 

 ments or sternebrse. The first pair of ribs articulate with 

 the body of the first sternebra, but the remaining ribs that 

 reach the sternum articulate with the interspaces between the 

 sternebrae. The last segment of the sternum has no ribs 

 attached to it, and is called the xiphisternum. The hind- 

 most ribs do not reach the sternum, and are described as 

 free or floating ribs. 



The shoulder-girdle of the higher mammals is characteristic. 

 The scapula is a flattened, more or less triangular plate of 

 bone, the outer surface of which is divided into two by a 

 stout ridge running from the upper border to the glenoid 

 cavity. This ridge is called the spine of the scapula, and 

 its lower extremity is produced into a process called the 

 acromion, with which the outer end of the clavicle, when 

 present, articulates. The coracoid is reduced to a blunt, 

 hook-like process, which overhangs the anterior border of 

 the glenoid fossa, but does not extend to the sternum. In 

 the lowest group of Mammalia, the Prototheria, comprising 

 the genera Ornithorhyncus and Echidna^ the shoulder-girdle 



