264 MICROSCOPIC ANATOMY OF THE ORGANS. 



There are in it considerable masses of erectile tissue and firm 

 elastic strands like those of the penis. The glans clitoridis is 

 supplied richly with nerves, and besides the MeissnerV and 

 Pacinian tactile bodies there are also special genital corpuscles 

 (see Nerve-endings). 



VI. LOCOMOTOR SYSTEM. 



Here must be considered the skeleton, and the muscles, 

 and their mode of development. 



1. THE SKELETAL SYSTEM. 



The bones form the essential part of the skeletal system, 

 and in connection with these the cartilages play an important 

 role. The structure of adult bone and cartilage as tissues 

 has been described, but here they must be spoken of as organs. 



A. Bones. 



Bones considered as organs consist of bony tissue, perios- 

 teum, and bone-marrow, with blood-vessels and nerves sup- 

 plying the different parts. Each bone (here the teeth are not 

 considered) is surrounded by a connective-tissue sheath, the 

 periosteum, with the exception of such places as are covered 

 by cartilage. In this firm layer of connective tissue there arc 

 two layers : an outer fibrous layer, in which there are few cells, 

 but numerous nerve plexuses and blood-vessels; and an inner 

 delicate layer, poor in blood-vessels, but especially rich in 

 elastic fibres and connective- tissue cells. 



At the boundary between the periosteum and the bony 

 tissue we find a layer of cubical cells (osteoblasts), which play 

 an important part in the regeneration and development of the 

 bone. A more or less intimate connection is established 

 between bone and periosteum, partly by means of blood-vessels, 

 and partly by bundles of connective-tissue fibres (Sharpey's 

 fibres) which run from the periosteum almost at right angles 

 to its surface and enter the bone. 



