PRINCIPLES OF VERTEBRATE MORPHOLOGY 3 



gans of highest dynamic activity and most pronounced sensitivity are at 

 the apical or anterior end and the organs of lowest dynamic activity and 

 least sensitivity are at the basal or posterior end of this axis. Between 

 these extremes, or opposite poles, of the axis the remaining organs 

 or functions are arranged, at least primitively, in a graded series of 

 diminishing dynamic activity and sensitivity. These geometrical 

 relations serve as an index of an inherent spatial orderUness in the 

 arrangement of the functions with reference to one another, and 

 demonstrate that the organism is based on a single plan — ^is a coher- 

 ent entity. 



From a purely physiological point of view this gradient represents 

 a linear series of functions, ranging from dominant or controlling func- 

 tions to subordinate or controlled functions, a series which, broadly 

 speaking, runs somewhat as follows: — olfactory and visual, the most 

 anterior and dominant functions, entirely sensory in character; motor 

 functions associated with movement of the eyes, and motor centers 

 for most voluntary functions; sensory and motor activities associated 

 with feeding, including the sense of taste, and the motor activities of 

 jaws and tongue; sensory and motor activities associated with hearing 

 and equilibrium; the active functions of respiration and circulation, 

 which are closely correlated; the most anterior locomotor functions, 

 associated with the pectoral appendages; the most active phases of 

 the alimentary or digestive functions, associated with the stomach 

 and the larger glands; the excretory and lower alimentary functions, 

 associated with the kidneys, the lower intestine and rectum; the re- 

 productive functions, associated with ovaries and testes, their acces- 

 sory ducts and copulatory organs; the functions of the tail or post-anal 

 body, which may be considered as a developmental afterthought and 

 as more or less beyond the limits of the original primary axis. The 

 tail has a gradient of its own and does not belong to the primary gra- 

 dient. This is only a rough outhne of the real physiological gradient, 

 but is clear enough for our purposes. The true gradient no longer 

 exists in modern vertebrates because there has been a great deal of 

 secondary concentration at various levels, especially at the anterior 

 end, where the original metameric arrangement of the functional 

 series has been profoundly disturbed and distorted. The primary 

 gradient is further obscured by the fact that various systems of or- 

 gans, such as the heart and blood vessels, the brain, the alimentary 

 tract, etc., have developed secondary axes of functional activity and 



