GENERAL CHARACTERS OF THE VERTEBRATA. 373 



spoken of as the "lesser" or "pulmonary" circulation, and 

 the "greater " or " systemic" circulation. 



In all Vertebrates there is that peculiar modification of the 

 venous system which is known as the " hepatic portal system." 

 That is to say, a portion of the blood which is sent to the ali- 

 mentary canal, instead of returning to the heart by the ordinary 

 veins, is carried to the liver by a special vessel the vena 

 porttz which ramifies through this organ after the manner of 

 an artery. 



In all Vertebrates, also, is found the peculiar system of ves- 

 sels known as the " lacteal system." This is to be regarded 

 as an appendage of the venous system of blood-vessels, and 

 consists of a series of vessels which take up the products of 

 digestion from the alimentary canal, elaborate them, and finally 

 empty their contents into the veins. 



Lastly, the masticatory organs of Vertebrates are modified 

 portions of the walls of the head, and never " hard productions 

 of the alimentary mucous membrane, or modified limbs " (Hux- 

 ley), as they are amongst the Invertebrata. 



The above are the leading characters of the Vertebrata as a 

 whole ; but before going on to consider the primary divisions 

 of the sub-kingdom, it may be as well to give a very brief and 

 general description of the anatomy of the higher and more 

 typical Vertebrates, commencing with their bony framework, 

 or skeleton. 



The skeleton of the Vertebrata may be regarded as consisting 

 essentially of the bones which go to form the head and trunk 

 on the one hand (sometimes called the " axial " skeleton), and 

 of those which form the supports for the limbs ("appendicular" 

 skeleton) on the other hand. The bones of the head and 

 trunk may be looked upon as essentially composed of a series 

 of bony rings or segments, arranged longitudinally, one behind 

 the other. Anteriorly these segments are much expanded, and 

 likewise much modified, to form the bony case which encloses 

 the brain, and which is termed the cranium or skull. Behind 

 the head the segments enclose a much smaller cavity, which is 

 called the " neural " or spinal canal, as it encloses the spinal 

 cord ; and they are arranged one behind the other, forming 

 the vertebral column. The segments which form the vertebral 

 column are called " vertebrae," and they have the following 

 general structure : Each vertebra (fig. 154, A) consists of a 

 central piece, which is the fundamental and essential element 

 of the vertebra, and is known as the "body" or "centrum" (c}. 

 From the upper or posterior surface of the centrum spring two 

 bony arches (n n), which are called the " neural arches " or 



