VERTEBRATA : GENERAL CHARACTERS. 449 



which are known as the " transverse processes." (In the typi- 

 cal vertebra the transverse processes consist each of two pieces, 

 an anterior piece or " parapophysis," and a posterior piece or 

 " diapophysis.") These elements form the vertebra of the 

 human anatomist, but the " vertebra " of the transcendental 

 anatomist is completed by a second arch which is placed be- 

 neath the body of the vertebra, and which is called the " hae- 

 mal " arch, as it includes and protects the main organs of the 

 circulation. This second arch is often only recognisable with 

 great difficulty, as its parts are generally much modified, but a 

 good example may be obtained in the human chest, or in the 

 caudal vertebra of a bony fish. 



The haemal arch in the case of the human thorax (fig. 240, 

 B) is formed by the ribs (rr) and the costal cartilages (//), 

 and is completed in front by the breast-bone or sternum (^), 

 which in some cases but not in man develops a spine (the 

 haemal spine) which corresponds to the neural spine on the 

 opposite aspect of the vertebra. 



It follows from the above, that the typical vertebra consists 

 of a central piece or body from which two arches are given off, 

 one of which protects the great masses of the nervous system, 

 and is therefore said to be "neural;" whilst the other protects 

 the main organs of the circulation, and is therefore said to be 

 " haemal." The correspondence of the typical bony segment 

 or vertebra with the doubly tubular structure of the body in all 

 Vertebrates is thus too obvious to require to be specially 

 pointed out. 



As a general rule, the vertebral column is divisible into a 

 number of distinct regions, of which the following are recog- 

 nisable in man and in the higher Vertebrata : i. A series of 

 vertebrae which compose the neck, and constitute the " cervical 

 region" of the spine (fig. 241, n). 2. A number of vertebrae 

 which usually carry well-developed ribs, and form the " dorsal 

 region " (d). 3. A series of vertebrae which form the region 

 of the loins, or "lumbar region" (/). 4. A greater or less 

 number of vertebrae which constitute the " sacral region," and 

 are usually amalgamated or " anchylosed " together to form 

 a single bone, the " sacrum." 5. The spinal column is com- 

 pleted by a variable number of vertebrae which constitute the 

 " caudal " region, or tail (c). 



As regards the skull of the Vertebrata, it has been thought 

 advisable not to enter into any general details here, partly 

 because the subject is one which can only be properly dis- 

 cussed in a work specially devoted to Human or Comparative 

 Anatomy, and partly because there is still much diversity of 



2 F 



