716 Comparative Morphology of Chordates 



in the conduct of the business. However, in a large number of minor 

 and routine matters, subordinate employees are authorized to act 

 directly without referring the matter to a superior. 



The brain, like the office of the factory, must receive "informa- 

 tion," not necessarily conscious, as to conditions and events both 

 outside (exteroceptive) and inside (intero- and proprioceptive) the 

 animal. On the basis of the information received, "orders" must go 

 forth to bring about appropriate action of the body. If the action is to 

 be of highest advantage to the animal, there must be some agency 

 competent to act in such a way that we must describe the action as 

 showing discrimination, judgment, or choice of one — the most advan- 

 tageous — among several possible reactions. However, as in the fac- 

 tory, it will tend toward efficiency and economy of effort if routine 

 activities and subordinate details of motor operations are carried on 

 automatically. 



Incoming information arrives at various levels of the brain. To its 

 rear entrance the spinal cord brings up from the body vast tracts of 

 afferent nerves transmitting reports from the skin (exteroceptive), 

 from internal surfaces such as that of the digestive epithelium (intero- 

 ceptive), and from within the tissues of working parts such as muscles 

 and joints (proprioceptive). The group of cranial nerves V, VII, IX, 

 and X, entering at lateral portals of the medulla, contain afferent 

 fibers of various sorts coming in from the head (both its skin and its 

 internal surfaces) and (X) from the digestive and respiratory tracts 

 within the trunk. Information concerning taste enters here (VII, IX, 

 X). Also at the medulla arrive reports from the ear, exteroceptive via 

 the cochlear trunk and proprioceptive via the vestibular trunk of the 

 "auditory" nerve (VIII). Visual reports are received at the midbrain 

 and, to greater or less extent, by the thalami. Consistent with the 

 extreme anterior location of the olfactory receptors, the olfactory 

 messages come in at the anterior extremity of the brain — in fact, the 

 brain pushes forward more or less elongated olfactory lobes to receive 

 them. 



Each incoming tract of afferent fibers of a specific sort is received 

 at an appropriate primary sensory center where the entering fibers 

 come into relation with other neurons. These centers correspond, in a 

 way, to the subordinate secretaries or receiving clerks at the factory. 

 At the opposite or highest level of the controlling agencies of the brain 

 are the nervous centers in the cerebral cortex. Between the primary 

 centers and the ultimate cortical centers is an intricate maze of fiber- 

 tracts and centers which collectively serve to establish the greatest 

 possible number and variety of nervous pathways between the sensory 

 receptors and the effectors (muscles and glands) of the body. The 



