80 HISTOLOGY. 



rations made by treatment with strong acids, the cells are 

 usually shrunken away from the walls of the cavities. They 

 are stellate, sending out processes into the bone canaliculi. In 

 the lower animals the processes of neighboring cells anastomose, 

 and also during the development of higher vertebrates the cells 

 join with one another in the canaliculi. It is, however, certain 

 that in adult individuals of the higher animals and man there 

 is no such cell combination. 



Spongy bone substance has a quite similar minute structure, 

 as has been described for compact bone. Its ground substance 

 has a fibrillar structure and contains bone lacuna. There are, 

 however, no Haversian canals and no lamellar systems. The 

 layer masses of this tissue show a lamellar structure, of which 

 the Iamella3 lie parallel to the broad surface of the mass. 



Other subjects in this connection, such as the vascularization 

 and development of bone, are treated of in the section on the 

 Skeletal System. 



m. MUSCLE. 



This tissue is characterized by a marked contractility of the 

 protoplasm. The power of contracting on stimulation from 

 without is possessed by all protoplasm to a certain degree, but 

 in muscle the contraction takes place mainly in one axis of the 

 cell. According as the contraction is under the control of the 

 will or not, we distinguish voluntary and involuntary muscle 

 tissue. This is a physiological classification, but there are 

 structural differences which allow the same division to be used 

 histologically. It is, however, perhaps better to divide the 

 tissues into groups on an entirely histological basis, speaking 

 of : 1, smooth muscle ; 2, heart muscle ; and 3, voluntary striated 

 muscle. 



All muscle cells contain one or more nuclei and protoplasm 

 with a more or less highly differentiated structure. There may 

 or may not be a cell membrane, and the cells are united by 

 only a small quantity of cement substance. In the protoplasm 

 there are usually to be found fibrils which may be regarded 

 as differentiations in one direction of the primitive network of 



