THE BRAIN IN THE EDENTATA. 377 



of the pallivim (probably by differentiation from the basal ganglion) provides a cortical 

 area with a free scope for expansion, in connection with which paths from all the lower 

 sensory centres become established. Thus the primitive jmllium becomes a field in which 

 the retina, the sensory lining of the internal ear, and all the tactile surfaces of the body 

 become represented, so that the impulses of these various senses may meet and mutually 

 react the one upon the rest. Thus tlie primitive pallium is essentially a "sensory area," 

 but it is at the same time a "projection area." Por all of these centres may exercise a 

 combined influence upon the pallial nerve-cells, from which efferent imjiulses proceed to 

 the lower executive centres. Complex groupings of nerve-cells are also formed within 

 the jiallium, representing potential muscular acts of increasing degrees of complexity, 

 which may become kinetic w'hen the activity of the pallium stimulates, through its 

 projection-fibres, the lower executive centres in the brain-stem and spinal cord. 



The pallium, then, is essentially a place where afferent impulses from all parts of the 

 body may meet and combine and exert a direct influence itjion the mechanism which 

 initiates bodily actions. It is for this reason that the mammal is able to execute move- 

 ments of a more complex order, of greater jirecision and variety — in a word, can 

 perform more skilful and purposeful acts than the animal without a pallium. The direct 

 result of this is a marked advancement in tlie development of the limbs in ordei- to 

 execute the movements which the possession of a ^^allium makes possible. The way is 

 opened for all kinds of specialization of movements, and the limbs and the otiier 

 muscular parts of the body rapidly adapt themselves to special kinds of activities. 

 Following in the train of these neuro-muscular modifications and specializations, the 

 skeletal and alimentary organs rapidly accommodate themselves to the changing mode 

 of life of the individual. 



The small early pallium may be regarded as being entirely composed of sensory areas, 

 which rapidly increase in size and complexity, as the intermediate sensory stations, such 

 as the corpora geniculata and optic thalamus, grow, and produce richer and more abundant 

 sensory paths to the pallium With the increasing size and complexity of sensory areas 

 the demand for association-elements wdiicli shall link together in functional unity the 

 various sensory areas and the various parts of each area becomes imperative. Of neces- 

 sity, therefore, a new set of elements springs uj), consisting of nerve-cells whose nerve- 

 fibre processes serve to bring into functional co-operation all the scattered elements of 

 the sensori-projection pallium. A time comes at an early period in the evolution of the 

 mammal when the variotrs sensory areas become fully represented in the paUial growth. 

 Up till this time any variation in the acuteuess of a sense-organ has an imjiortant and 

 obvious effect upon the size of the pallium. 



The diminution or extinction of the visual element in the cortex, such as occurs to a 

 high degree in Notoryctes, Carysochloris, Talpa, and CJilamijdophoi'Hs, causes a most 

 profound difference in the size of the pallium, and any interference with the tactile or 

 auditory acuteuess would doubtless have considerable influence upon the pallial region. 

 But after the sensory areas are fully represented the pallium still continues to increase 

 in extent. 



Inextricably intertwined with the sensori-projection elements, the association-elements 



