416 INTRODUCTION TO CRANIATA [CH. 



the enlarged and highly developed representative of the sense 

 vesicle of the Urochorda and of the cerebral vesicle of Amphioxus. 

 In the embryo it is a simple thin- walled vesicle, the lateral walls of 

 which become changed into the retinae or the essential sensory 

 portion of the eyes. The Craniate eye like that of the Ascidian 

 tadpole, has its perceptive surface turned inwards towards the 

 brain cavity. The nerves by which the eyes are connected with the 

 brain are really the narrowed connections of the lateral portions of 

 the fore-brain with the central portion. The roof of the fore-brain 

 remains thin throughout life and from it a stalk arises leading to a 

 third median eye, the so-called pineal body, vestigial in all living 

 forms. From the front wall of the fore-brain an outgrowth takes 

 place, giving rise to a bilobed vesicle termed the cerebrum, each 

 of the two lobes of which is termed a cerebral hemisphere. 

 This in the higher Craniata is the seat of the more complex mental 

 processes, but in the lower it appears to be intimately connected 

 with the organ of smell. The cerebrum in these cases remains 

 thin-roofed, but its base thickens owing to a great develop- 

 ment of nervous matter. In order to distinguish it from the 

 cerebrum the original fore-brain is denoted by the name thal- 

 amencephalon for which American authors often substitute the 

 term 'twixt brain.' Underneath this section of the brain lies a 

 glandular organ termed the pituitary body. This body is com- 

 pounded of a downgrowth of nervous tissue from the fore-brain and 

 of a portion of tissue evaginated and constricted off from the roof 

 of the buccal cavity. It represents the sub-neural gland of the 

 Urochorda, and in the higher Vertebrates produces a substance 

 which is of importance to the normal metabolism of bone and con- 

 nective tissue, and recent research suggests that its secretion also 

 influences the activity of the kidneys. 



The mid-brain acquires thick lateral pouches, the so-called optic 

 lobes: the hind -brain remains thin-roofed, except in front where 

 a transverse nervous band, the cerebellum, is formed. The 

 cerebellum is believed to be the portion of the brain intimately 

 connected with the semicircular canals of the ear (see p. 420) and 

 to have for its function the control of the muscles so as to maintain 

 the equilibrium of the body. The rest of the hind-brain is termed 

 the medulla oblongata or spinal bulb ; it controls the beating of 

 the heart, the respiratory movements and other vital processes. 

 The hinder part of the neural tube is known as the spinal cord, 

 and it develops thick walls, so that its cavity is exceedingly small. 



