THE SKIN. 199 



throat ganglion," situated above the throat (Fig. 211, g ; Plate 

 V. Fig. 11, ni). The complex central nervous system of all 

 higher animals has developed from tliis simple rudiment. 

 In the higher Worms, e.g., the Earth -worms, according to 

 Kowalevsky, the earliest rudiment of the central nervous 

 system (Fig 210, to) is a local thickening of the skin- 

 sensory layer (As), which afterwards becomes entirely 

 detached from the horn-plate. Even the medullary tube of 

 Vertebrates has the same origin. From the germ-history 

 of Man, we already know that this medullary tube, the 

 commencement of the central nervous system, originally 

 develops from the outer skin-covering. 



Let us now turn aside from these very interesting 

 features in evolution, and examine the development of the 

 later human skin-covering, with its hairs, sweat-glands, etc. 

 Physiologically, this outer covering {derma, or tegumentum) 

 plays a double part. The skin, in the first place, forms the 

 general protective covering (integv/mentv/m com/mune) which 

 covers the whole surface of the body, and protects all other 

 parts. As such, it, at the same time, effects a certain ex- 

 change of matter between the body and the surrounding 

 atmospheric air (perspiration or skin-breathing). In the 

 second place, the skin is the oldest and most primitive 

 sense-organ, the organ of touch, which effects the sensation 

 of the surrounding temperature and of the pressure or re- 

 sistance of bodies with which it comes in contact. 



The human skin, hke that of all higher animals, consists 

 essentially of two distinct parts ; of the outer-skin, and of 

 the underlying leather-skin. The outer-skin (epidermis) 

 consists only of simple cells, and contains no blood-vessels 

 (Fig. 212, ah). It develops from the first of the secondary 

 32 



