668 Comparative Morphology of Chordates 



The brain is analogous to the offices of a factory. To say that cells 

 of the brain correspond to the human occupants (each having a brain 

 within himself!) of the offices would precipitate some perplexing ques- 

 tions which, for present purposes, need not be discussed. The cerebral 

 "office" is connected with all parts of the body by nerves. Sense- 

 organs, some at the surface of the body, others internal, report respec- 

 tively the external and internal conditions to the "office," which 

 thereupon sends out appropriate orders to the working parts of the 

 body. But the nervous mechanism directly involved in the running of 

 a greyhound differs in no conspicuous way from that which effects the 

 awkward and laborious crawling of a sloth. The significant anatomic 

 differences are in the structure of the appendages (part of the "fac- 

 tory"), not in the nervous structures ("office" and "telephones") 

 concerned in the locomotion, and the facility with which the function 

 is performed depends largely upon the efficiency of the cerebral "of- 

 fice." However, increase in size of a motor structure and in its com- 

 plexity of differentiation necessitates increase in the number of related 

 nerves, and this may be reflected by increase in the gross bulk of 

 nervous tissue in certain transmitting regions and correlating centers 

 of the spinal cord and brain. Comparing brains of mammal and reptile, 

 there are differences in the relative bulk of the various regions, but 

 there are no differences in the basic plan of the organ. These differences 

 in mere volume of corresponding parts of the central nervous organs 

 of different animals are indicative of the degree of differentiation of 

 the motor organs concerned and the degree of complexity of their 

 activities, but give no clue to the nature and pattern of those activities. 



Sense-organs 



sense-organs of skin 



Cutaneous (integumentary) sense-organs in mammals are 

 more numerous and structurally more diversified than in other verte- 

 brates. The hard external horny layer and thick scales make reptilian 

 skin an unfavorable site for delicate sensory structures, although they 

 do occur beneath thinner regions of the horn or at perforations in the 

 scales. The thick fur of mammals must greatly reduce the direct impact 

 of external objects and agencies on the surface of the skin but, in- 

 directly, the hair may, to an important degree, contribute to the 

 sensitiveness of the skin. The epidermal wall of the follicle which 

 lodges the root of a hair (Fig. 443) is richly beset with free nerve- 

 endings. The surface of the hair is usually minutely scaly or even 

 spiny — a texture which increases the susceptibility of the hair to 

 mechanical disturbance by contact with an external object. Slight 



