THE SKELETON 



43 



and 23 in Terissodactyla. The highest and quite exceptional 

 numbers are in the Two-toed Sloth (Choke/pus) 27, and the Hyxax 

 30. The prevailing number of rib-bearing vertebra? is 12 or 13, 

 any variation being generally in excess of these numbers. 



Sacral Vertebra*. — The sacral region offers more difficulties o. 

 definition. Taking the human " os sacrum " as a guide for 

 comparison, it is generally defined as consisting of those vertebras 

 between the lumbar and caudal regions which are ankylosed 

 together to form a single bone. It happens, however, that the 

 number of such vertebras varies in different individuals of the 

 same or nearly allied species, especially as age advances, when a 

 certain number of the tail vertebras generally become incorporated 

 with the true sacrum. Other suggested tests — as those vertebras 

 which have a distinct additional (pleurapophysial) centre of ossifica- 

 tion between the body and the ilium, those to which the ilium is 

 directly articulated, or those in front of the insertion of the ischio- 

 sacral ligaments — being ecpially unsatisfactory or unpractical, the 

 old one of ankylosis, as it is found to prevail in the average 

 condition of adults in each species, is used in the enumeration of 

 the vertebras in the following pages. The Cetacea, having no iliac 

 bones, have no part of the vertebral column modified into a 

 sacrum. 



Caudal Vertebra}. — The caudal vertebras are those placed behind 

 the sacrum, and terminating the vertebral column. They vary 

 in number greatly — being reduced to 5, 4, or even 3, in a most 

 rudimentary condition, in Man 

 and in some Apes and Bats, and 

 being numerous and powerfully 

 developed, with strong and com- 

 plex processes, in many mammals, 

 especially among the Edentata, 

 Cetacea, and Marsupialia. The 

 highest known number, 46, is 

 possessed by the African Long- 

 tailed Pangolin. Connected with 

 the under surface of the caudal 

 vertebras of many mammals which 

 have the tail well developed are 

 certain bones formed more or less 

 like an inverted arch, called chev- 

 ron bones, or by the French os en 

 V. These are always situated 

 nearly opposite to an interverte- 

 bral space, and are generally artic- 

 ulated both to the vertebra in front and the vertebra behind, but 

 sometimes chiefly or entirely either to one or the other. 



Fig. 12. — Anterior surface of fourth 

 caudal vertebrae of Porpoise (Phoccena com- 

 munis), s, Spinous process ; m, metapophy- 

 sis ; t, transverse process ; h, chevron bone. 



