xvi. 27 BRAIN 477 



parts, the upper secreting mainly albumen, the lower producing the 

 shell, and the lowest mucus, to assist the act of laying. The blue back- 

 ground colour of the egg (oocyanin) is produced during shell-forma- 

 tion in the upper part of the tube ; spots of red-brown ooporphyrin are 

 added lower down. The pigments are derived from the bile, ultimately 

 from haemoglobin. 



As much as a third of the weight of calcium in the whole skeleton 

 is needed for the shells of the two eggs laid by a pigeon. A reserve is 

 collected as the ovarian follicles mature. The oestrogen they produce 

 increases the uptake of calcium from the food and stimulates its 

 deposition in the bones. After ovulation the oestrogen level falls, the 

 calcium is mobilized from the bones, and its concentration in the 

 blood becomes very high, until used by the eggs. 



27. The brain of birds 



The brain is larger relative to the body in birds than in any other 

 vertebrates except mammals (Fig. 288), and there is no doubt that one 

 result of the high temperature has been to allow opportunity for an 

 elaborate nervous organization and complicated behaviour. Unfortu- 

 nately we have little information about the w^ay in which the large 

 masses of tissue of the brain function ; they are certainly different from 

 anything found in mammals. There are considerable differences in the 

 development of the parts in various birds, for instance, the forebrain is 

 especially large in the rooks and crows (Corvus) and in the parrots, the 

 behaviour of which also shows signs of outstanding 'intelligence'. 



In the spinal cord the most characteristic feature is the relatively 

 small size of the dorsal funiculi, and their nuclei in the medulla are 

 also small. Evidently the sense of touch is less well developed over the 

 body than it is in mammals, perhaps less than in reptiles. No doubt 

 movement of the feathers provides impulses leading to reflex actions, 

 but it is not surprising that the loose covering does not allow elaborate 

 organization of the sense of touch. The finer senses of birds are 

 restricted to the eyes, ears, and bill. On the other hand, there are large 

 spino-cerebellar tracts, presumably proprioceptive and concerned with 

 the delicate adjustments necessary for flight. The spinal cord is con- 

 trolled by large efferent tracts from the brain, including cerebello- 

 spinal, vestibulo-spinal, and tecto-spinal pathways. There is no direct 

 tract from the forebrain to the spinal cord, but the influence of the 

 large corpora striata is probably exercised through fibres running to 

 the red nucleus and tegmentum of the midbrain, from which others 

 pass to the cord. 



