FIBROUS ENVELOPES. 155 



The periosteum covers the bones, and is commensurate 

 in extent with the osseous system, enveloping every part 

 of it except its articular surfaces. In infancy it is thick, 

 and easily separated, while in the adult it is more com- 

 pact and adheres strongly to the hones, sending processes 

 into their substance. The periosteum protects the bones, 

 conducts the vessels into their structure, connects the 

 epiphysis with the general shaft of the bone, during infan- 

 cy, and gives insertion to ligaments and muscles. It is 

 restored when injured or destroyed. 



The perichondrium is the periosteum of the non-articular 

 cartilages, and has the same properties and uses. 



The fibrous-envelope of the brain and nervous system 

 is the dura mater, which lines the whole interior sur- 

 face of the cranium, sending down prolongations into the 

 brain, which divides it into several parts, and passing 

 into the vertebral canal to surround the spinal marrow 

 and the nerves. Several other organs have fibrous capsules ; 

 thus the eye has its sclerotica, the testicle its tunica albu- 

 ginea, and the kidneys, liver, and ovaries their special 

 fibrous envelopes. 



There is another class, called the compound fibrous mem- 

 brane, which consists of serous or mucous membrane, 

 associated with the fibrous, as the pericardium and tunica- 

 vaginalis, which are fibro-serous ; and the larynx and 

 trachea, which are fibro-mucous. 



The yellow elastic fibrous system derives its name from 

 the yellowness of its color and the elasticity of its fibre. It 

 is found in those parts where there is much resistance to 

 overcome. The yellow ligaments of the vertebne are of 

 this class. The elastic coat of the arteries, of the excretory 

 ducts, the coverings of the spleen, and corpora cavernosa, 

 are also viewed as belonging to this division. It has less 

 tenacity, but more extensibility, than the other fibrous tis- 

 sues. -Its chief physical property, however, is elasticity, 

 which it possesses in consequence of the water it contains, 

 for, when deprived of its water, it loses this property, and, 

 when this is restored to it, it again recovers. 



