Chap, ix.] RIBS. 345 



forms where a powerful inspiration might lead to an 

 insect being carried from the tongue into the trachea, 



In various Mammals the roof of the skull is in one 

 way or another enlarged ; in the sperm-whale, the 

 nasals, rnaxillaries, and supraoccipital unite to form a 

 large basin ; as the contents of this basin are, how- 

 ever, of an oily nature, we may believe that the 

 specific gravity of the bone is neutralised (even if no 

 advantage is gained) by that of the spermaceti, the 

 specific gravity of which is '843 at 50 C. In the 

 Khinoceros and in the Ungulata the roof of the skull 

 carries horns. (See page 369.) In the porcupines 

 many of the upper cranial bones are expanded, and 

 contain large air sinuses, as in the frontal region of 

 the sloths. 



The morphological characters and homologies of 

 the ribs present a problem that is yet unsolved ; the 

 evidence as to their origin in the embryo being not a 

 little conflicting. In a number of forms they un- 

 doubtedly arise in the tissue of the proto vertebrae 

 (see page 529) quite independently of the vertebrae 

 themselves, their connection with which is only 

 secondary. In the Cyclostomata, Holocephala, and 

 some rays, there are no ribs, and the supporting 

 function of these absent rods is undertaken, as in 

 Amphioxus, by the fibrous tissue which lies between 

 the dorsal and ventral muscular layers. 



Among the Amphibia and the Amniota, the ribs, 

 when present, and typically developed, are connected 

 with the vertebrse by an upper and lower process j in 

 the Csecilise, where, among amphibians, ribs are best 

 developed, these two processes are widely separated ; 

 in the Salamanders the proximal ends of the anterior 

 ribs are forked, and of the hinder single ; in the 

 Anura the ribs are reduced or quite rudimentary. 



In the Amniota the ribs have a greater functional 

 importance, as is shown by the length of the more 



