INTRODUCTORY. RIBS AND STERNUM. 23 



The quadrate in all animals with ossified skeletons, except 

 the Mammalia, forms the suspensorium of the mandible or the 

 skeletal link between the jaw and the cranium ; in the Mam- 

 malia, however, the mandible articulates with the squamosal, 

 while the quadrate is greatly reduced, and is now generally 

 considered to be represented by the tympanic ring of the ear. 



The second visceral or hyoid arch in fishes consists of two 

 pieces of cartilage, a proximal 1 piece the hyomandibular, 

 and a distal 1 piece the cerato-hyal. The cerato-hyals of the 

 two sides are commonly united by a median ventral plate, the 

 basi-hyal. The hyoid arch bears gills on its posterior border, 

 but its most important function in most fishes is to act as the 

 suspensorium. In higher vertebrates the representative of 

 the hyomandibular is much reduced in size, and comes into 

 relation with the ear forming the auditory ossicles; the 

 cerato-hyal looses its attachment to the hyomandibular and 

 becomes directly attached to the cranium, forming a large part 

 of the hyoid apparatus of most higher vertebrates. 



Behind the hyoid arch come the branchial arches. They 

 are best developed in fishes, in which they are commonly five 

 in number and bear gills. Their ventral ends are united in 

 pairs by median pieces, the copulae. 



In higher vertebrates they become greatly reduced, and 

 all except the first and second completely disappear. In the 

 highest vertebrates, the mammals, the second has disappeared, 

 but in birds and many reptiles it is comparatively well 

 developed. 



3. THE RIBS AND STERNUM. 



The ribs are a series of segmentally arranged cartilagi- 

 nous or bony rods, attached to the vertebrae; they tend 

 to surround the body cavity, and to protect the organs 

 contained within it. Ribs are very frequently found 



1 The proximal end of anything is the one nearest the point of origin 

 or attachment, the distal end is the one furthest from the point of origin 

 or attachment. 



