52 COMPARATIVE ANATOMY 



the Cetacea and Sirenia, in correspondence with the absence of 

 hind-limbs. 



The caudal vertebrae vary extremely in their development, and 

 excepting in most long-tailed Mammals such as Kangaroos, 

 Sirenians, Cetaceans and certain Apes no longer develop lower 

 arches. When present these " chevron bones " are intervertebral 

 in position. 



The greatest number of caudal vertebrae is found in Microgale longicauda 

 (forty-eight), Manis macrura (forty-six to forty-nine), Paradoxurus (about 

 thirty-six), and certain Monkeys (thirty- two to thirty-three). 



The caudal region is most reduced in the higher Primates, in 

 which it gives rise to a stump-like coccyx consisting of at most five to 

 six rudimentary vertebrae, all fused together, and these may even (in 

 the human subject, especially in the male) fuse with the sacrum. 

 Many facts as regards the development as well as the structure of 

 the whole tail-region in the adult show however that the ancestors 

 of Man must have been provided with a distinct and functional 

 taiJ. 



II. RIBS. 



The ribs do not as a general rule (with the exceptions to be 

 noted presently) arise as outgrowths from the vertebral column, but 

 become developed independently in the skeletogenous layer that 

 is, in the tissue of the somites, and their connection with the 

 vertebral column is a secondary one. They stand in the closest 

 connection with the inter-muscular septa or myocommata of the 

 great lateral muscles of the body (Fig. 40 A,) are arranged seg- 

 mentally, and onto- as well as phylogenetically, pass through a 

 membranous, a cartilaginous, and a bony stage : their ossification is 

 independent of that of the vertebral column. In their primitive 

 form, the ribs have simple, unbifurcated heads, the articulation of 

 which with the vertebral column first takes place in the region of 

 the " intercentra " (p. 47), and from this condition all the later modi- 

 fications as regards their form and connection are to be derived. 



The ribs present great .variation in the various vertebrate 

 Classes : they may be short and stump-like and almost horizontal 

 in position, or may grow ventral wards so as to encircle the body- 

 cavity. Primitively, ribs may be present all along the vertebral 

 column, but in the higher types they become reduced in certain 

 regions. 



In order to arrive at sound conclusions as to the morphological 

 value of the ribs, their relations to the soft parts must be taken into 

 consideration. It is then seen that they are not completely homo- 

 logous throughout the vertebrate series, and that those of Ganoidei, 

 Teleostei and Dipnoi are not exactly comparable to those of Elasmo- 

 bianchii, Amphibia, and Amniota (Fig. 40A). 



