OF SIMPLE ISOLATED CELLS FORMING TISSUES. 33 



vessel. They are, however, surrounded by numerous blood-vessels, 

 which form large amimll<x^ or dilatations at their edges, or on their 

 surfaces, from which they derive their nourishment by imbibition. 



Cartilage is insensible ; neither nerves nor lymphatics can be 

 traced into its substance, and it is doubtful whether it is ever replaced 

 by a similar structure when once destroyed. 



Fibro-cartilage is a compound of white fibrous tissue and carti- 

 lage in varying proportions. When the intercellular substance as- 

 sumes a fibrous arrangement, surrounding the cells, it is known by 

 this name. In some instances the fibrous structure is developed so 

 much at the expense of the cells, that the latter disappear altogether, 

 and the whole structure becomes fibrous. This structure is seen in 

 all those cartilages which unite the bones by synchondrosis, as in 

 the vertebral column and pelvis. The reticular structure is seen in 

 the concha auris and in the epiglottis. 



The cornea, according to the researches of Messrs. Todd and 

 Bowman, is a peculiar modification of the white fibrous tissue, in 

 which the fibres, which in the sclerotic have been densely interlaced, 

 flatten out into a membranous form, so as to follow the curvatures 

 of the cornea, and constitute a series of more than sixty lamella 

 united to one another by delicate processes extending from one to 

 the other. The 



resulting areolse '^' ' 



lie in superposed 

 planes, the conti- 

 guous ones of the 

 same plane be- 

 ing for the most 

 part parallel, but 

 crossing those of 

 the neighbouring 

 planes at an an- 

 gle, and seldom communicating with them. (Fig. 8.) 



The crystalline lens has long been known to be fibrous. The 

 fibres are united into laminae by means of numerous teeth or sinuosi- 

 ties at their edges which lock into one another. They originate in 

 cells, several of which coalesce to form one. After the lens is fully 

 formed it is not permeated by blood-vessels ; these being confined to 

 the capsule. It consists chiefly of albumen in its soluble form, and 

 is coagulated by heat. The latest analyses represent the substance 

 of the lens as consisting of that modification of albumen called 

 globuline. The vitreous humour is an example of a very loose form 

 of cellular tissue. The cells have no open communication with each 

 other, and contain a fluid holding a small quantity of albumen and 

 saline matter in solution. It is nourished by the vessels which are 

 minutely distributed upon its general surface, there being none dis- 

 tributed through its substance for this purpose. 



