NERVOUS SYSTEM. 



119 



are of importance not only as special sense organs (tactile organs), 



but also for the production of various hard structures (scales, teeth). 



The epidermis is composed of several layers of cells, of which the 



upper and older layers are cast off, while the lower layers (stratum 



Malpighi] are actively growing and serve as a matrix for the con- 



tinual renewal of the upper layers, and sometimes contain the 



cutaneous pigments. Some of the appendages of the skin are 



epidermal structures, in which 



case they arise as the result 



of peculiar and independent 



growths of the epidermis (hairs 



and feathers). Some are de- 



rived from ossifications of the 



dermal papillse which sometimes 



may even give rise to a hard 



and complete dermal armour 



(scales of Fishes and Reptiles, 



carapace of Armadillos and 



Tortoises). 



The central nervous system 

 is placed in the dorsal cavity 

 formed by the upper arches of 

 the vertebree ; it consists essen- 

 tially of a cord the spinal cord 

 the anterior enlarged and 

 more differentiated part of 

 which is distinguished as the 

 brain. The spinal cord con- 

 tains a narrow central canal, 



which is Continued into the 



brain, where it widens out and 



forms the Ventricles of the 



, . rp-i i i i 



brain. The brain and spinal 

 cord are, therefore, parts of the same organ. The brain seems 

 to be the seat of the intellectual faculties and the central organ of 

 the sensory apparatuses ; while the spinal cord conducts the impulses 

 to and from the brain, and in particular is the centre of reflex move- 

 ments, but it also contains the centres of certain automatic actions. 

 The mass of the brain and that of the spinal cord increase as might 

 be expected, as the grade of life is higher. They increase, however, 

 in an unequal ratio, for the brain soon preponderates over the spinal 



iG. 576. Embryo chick at end of second day 

 (after Kolliker). Vh, Fore-brain; 3/A, mid- 



**', 



vertebral plates of the mesoderm ; SP, lateral 

 mesoblastic plates ; H t heart. 



