THE FROG 39 



Besides the hyaline cartilage already described, two other 

 varieties, white fibre-cartilage and elastic fibre-cartilage, are met 

 with in 'tke mammals. As their names imply, one or other sort of 

 fibre is present in addition to the hyaline matrix. Membrane bone 

 is laid down in connective tissue, and, as has been pointed out, during 

 the formation of cartilage bone the cartilage is destroyed and the 

 bone actually deposited in a sort of connective tissue. Hence it is 

 that some writers regard both cartilage and bone as highly modified 

 forms of connective tissue. 



The corpuscles of connective tissue may take up fat, and 

 so the tissue becomes somewhat fatty. This must not be confused 

 with true fat or adipose tissue. In this we find numbers of cells, 

 each containing a relatively enormous globule of fat, and the proto- 

 plasm is reduced to a very thin enclosing layer, thickened at one 

 point where the nucleus is situated. These fat cells are bound 

 together in groups or lobules by connective tissue. The fat is 

 formed gradually as a number of minute globules within the cell, 

 which ultimately run together and form the large globule. 



We have thus seen that the skeleton forms a framework of 

 firm supports of characteristic structure, articulating one with 

 another by means of joints. The soft parts of the body are connected 

 with the skeleton, and the various parts of this with one another by 

 means of connective tissue. 



Muscular System and Integument. 



It has already been seen that the flesh or muscle is situated 

 beneath the skin and forms the mass of the limbs and the body wall. 

 Muscular tissue is the tissue that is specially modified in order to 

 bring about movements of all kinds. The property of contractility 

 is common to all protoplasm to a certain limited extent, but in 

 muscle it is much more highly developed than elsewhere. Any 

 change that is capable of bringing into activity the whole or any 

 part of an animal, however small, is spoken of as a stimulus. 

 Stimuli may come from inside the organism, i.e. be internal, or from 

 its surroundings, i.e. be external. The stimulus that causes muscle 

 to contract is usually nervous, taking the form of a message from a 

 nerve cell in the brain or elsewhere, although, when a muscle is 

 removed from the body, the same response may be obtained by 

 mechanical, chemical or electrical means, or by the application of 

 heat. When muscle contracts in reply to a stimulus it shortens in the 

 direction of its long axis and increases correspondingly in thickness. 

 The energy necessary for such activity is obtained by the oxidation 

 of the substances of the protoplasm itself, i.e. by katabolism. 



