138 



ANIMAL BIOLOGY 



known as involuntary muscles. From the standpoint of their 

 microscopic structure, muscle cells are of three kinds. Voluntary 

 muscles consist of striated muscle cells, and involuntary muscles, 

 except those of the heart, are composed of unstriated muscle 

 cells. The cells of the heart approach somewhat in structure those 

 of voluntary muscles and are known as cardiac muscle cells. 

 (Figs. 7, 32, 98, 99.) 



D. Skeleton 



The form of the Vertebrate body is maintained by a system of 

 supporting and protecting structures, termed the skeleton. 

 Although various outgrowths of the skin, such as scales, feathers, 



"Notochord- 



Notochordal sheath- 



nmsm 



7^ 



Blood vessel 



in connective tissue 



Growing cartilage 

 invading notochord 



Bone developing to 

 form vertebra 



Muscle tissue 



Fig. 100. — Diagram of a longitudinal section through a developing verte- 

 bral column to show the invasion of the notochordal sheath by cartilage to 

 form the centra of the vertebrae. 



and hair, form a part of the skeletal system known as the exo- 

 skeleton which is comparable to the protective coverings of the 

 Invertebrates, it is a bony endoskeleton which is characteristic 

 of the higher animals. This internal skeleton, which is largely mes- 

 odermal in origin, exhibits such great diversity and complexity 



