XIII 



PHYLUM CHORDATA 455 



The liyoid consists, in addition to separate vestigial sljihj-liyah, 

 of a stout, thick body or hiui-hyal, a pair of small anterior cornua 

 or cerato-liyals, and a pair of longer, backwardly-directed cornua or 

 thyro-hyals. 



The (luditory ossicles, contained in the cavity of the middle ear, 

 and cut off from the exterior, in the uninacerated skull, by the 

 tympanic membrane, are extremely small bones, which form a 

 chain extending, like the columella auris of the Pigeon, from the 

 tympanic membrane externally to the fenestra ovalis internally. 

 There are three of these auditory ossicles — the stapes, which corre- 

 sponds to the columella of the Pigeon ; the incus, and the malleus, 

 the latter with a slender process — the processtis gracilis. In addition 

 there is a small disc-like bone, the orbicular, which is attached to 

 the incus. 



The elements of the 'pectoral arch are fewer than in the Lizard. 

 There is a broad, thin, triangular scapula, the base or vertebral edge 

 of which has a thin strip of cartilage (the supra-scapular cartilage) 

 continuous with it. Along the outer surface runs a ridge — the 

 spine ; the spine ends below in a long process — the acromion-process 

 — from which a branch process or metacromion is given off behind. 

 The part of the outer surface of the scapula in front of the spine 

 is the 'pre-sp)inous or pre-scapular fossa, the part behind is the 

 p)Ost-splnous or post-scapular fossa. At the narrow lower end of 

 the scapula is a concave surface — the glenoid cavity— mio which 

 the head of the humerus fits, and immediately in front of this is 

 a small inwardly curved process — the coracoid process — which 

 is represented by two separate ossifications in the young Rabbit. 

 A slender rod — the clavicle — lies obliquely in the region between the 

 pre-sternum and the scapula, but only extends a part of the 

 distance between the two bones, and in the adult is only connected 

 with them through the intermediation of fibrous tissue. 



The skeleton of the fore-limb is more readily comparable with 

 that of the Lizard than with that of the Bird; but there is a 

 difference in the position of the parts owing to the rotation back- 

 wards of the distal end of the humerus, all the segments being 

 thus brought into a plane nearly parallel with the median vertical 

 plane of the body, with the pre-axial border directed outwards, and 

 the original dorsal surface backwards. The radius and ulna are 

 fixed in the position of pronation, i.e., the distal end of the radius 

 is rotated inwards, so that, while the proximal end is external to 

 the ulna, the distal end becomes internal, and the digits of the 

 manus are directed forwards. 



At the proximal end of the humerus are to be recognised : 

 (1) A rounded head for articulation with the glenoid cavity of 

 the scapula ; (2) externally a greater and (3) internally a lesser 

 tuberosity for the insertion of muscles ; (4) a groove, the bicipital 

 groove, between the two tuberosities. On the anterior surface 



