122 THE BIOLOGY OF BIRDS 



the right and left members of a pair. In lampreys both the 

 epiphysis and the parietal organ show an eye-like structure 

 at the tip, most marked in the case of the epiphysis. In the 

 so-called New Zealand lizard and in some other reptiles 

 the eye-like structure is borne by the parietal organ. In 

 birds " the pineal body," which has no trace of optic 

 function, corresponds to the parietal organ. 



On the under surface of the optic thalami region there 

 is a downgrowth (the cerebral part of the hypophysis) which 

 meets an upgrowth from the mouth (the oral part of the 

 hypophysis). The two components unite to form the 

 enigmatical pituitary body, which loses all connection with 

 the mouth and rests on the basisphenoid bone. It is partly 

 glandular and partly nervous, and it forms an internal 

 secretion or hormone which is swept away by the blood and 

 distributed through the body — a subject to which we must 

 return later. 



The spinal cord passes downwards in the protected 

 tunnel formed by the neural arches of the vertebrae, and 

 gives off numerous nerves. Very noteworthy are the 

 strong nerves forming the *' brachial plexus " for the wing 

 and its muscles, and the corresponding " sacral plexus " 

 forming the sciatic nerve which passes down the leg. 



In the pelvic region of the spinal cord there is an inter- 

 esting dilatation, leaving a median sinus. The expansion 

 is highly differentiated and no doubt connected with the 

 bipedal habit and the consequently increased importance 

 of the innervation of the hind leg. It is manifested (Imhof, 

 1905) at a relatively late period in embryonic life — which 

 points to its being a secondary acquisition. 



§ 4. The Sustentative Functions 



The master-activities of the bird's body are those of 

 the muscular and nervous systems. Most of the other 

 functions may be called sustentative or subservient to these, 

 always excepting growth and reproduction which stand by 

 themselves. The master-activities imply wear and tear 



