THE SKIN. 199 



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

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

 higher animals has developed from this simple rudiment. 

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

 Kowalevsky, the earliest rudiment of the central nervous 

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

 sensory layer (hs), 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 (integumentum commune') 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, like 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, a&). It develops from the first of the secondary 



