CONNECTIVE TISSUE. 35 



certain of the large organs of the body. From the 

 parablast, a fourth layer, the exact origin of which 

 is still in doubt, are formed the blood-vessels and 

 blood-cells, lymphatic tissues and vessels, endothe- 

 lium and members of the connective-tissue group. 

 Besides the relationship given to them by this com- 

 mon origin, these tissues show their close alliance by 

 the fact that during the process of development one 

 is sometimes formed from another. Finally, certain 

 frequently observed pathological conditions seem to 

 consist chiefly in the transformation of one of these 

 forms of tissue into another of the same group. 



With fibrillar connective tissue or connective tissue 

 proper, or simply connective tissue, as it is often 

 called, we commence our systematic study. This 

 tissue is most widely distributed in the human body, 

 occurring in the greatest diversity of forms, and an 

 exact knowledge of its structure and arrangement is 

 absolutely essential to a correct understanding of 

 most other tissues and organs, since it occurs in 

 one form or another in nearly all of them. Fibrillar 

 connective tissue, under the name of tendons and 

 ligaments, forms bands and cords which bind the 

 muscles to the bones, and bind the bones together. 

 It is spread out in thin layers in the faciae and 

 aponeuroses ; it surrounds the bones and cartilage 

 as periosteum and perichondrium. It divides and 

 encloses muscular bundles and the nerves ; it sup- 

 ports the blood- and lymphatic-vessels, and forms 



