316 VERTEBRATE ZOOLOGY 



The triceps brachii forms the back of the arm ; the extensor anti- 

 brachii is a slender muscle on its inner surface, inserted in the 

 olecranon and acting with the triceps; the biceps brachii forms 

 its anterior surface and is a large muscle which bends the forearm, 

 having its origin in the glenoid fossa of the scapula, and its inser- 

 tion in the radius and ulna. 



Exercise 38. Draw the ventral aspect of the trunk and place in it 

 outlines of these muscles, showing the directions of their fibers. 

 Carefully label all. 



Dissect out each of the muscles that have been observed ; com- 

 pletely free it from the abutting organs and find its origin and 

 insertion. Do not, however, disarticulate or injure any of the 

 bones. While studying the external oblique muscle note that the 

 abdominal wall is composed of the following muscles : on the out- 

 side the external oblique ; just beneath it the internal oblique, the 

 fibers of which run at right angles to those of the external oblique ; 

 in the midventral area the rectus abdominis, which lies between 

 the aponeurosis of the two obliques anteriorly and dorsal to both 

 posteriorly; and, next to the abdominal cavity, the transverse 

 muscle, the fibers of which have a transverse direction. 



The Skeletal System. This system is made up of two portions, 

 the exoskeleton and the endoskeleton. The former consists of cer- 

 tain special integumental structures,— the hair, claws, and teeth. 

 The hairs are epidermal structures which form a protective cover- 

 ing against cold and external injuries. They are also to a certain 

 extent tactile organs. The whiskers, or vibrissa?, are enlarged 

 hairs the principal function of which is sensory. The claws are 

 horny, epidermal structures and are closely allied to hairs. The 

 teeth have a twofold origin: the dentine is a dermal structure, 

 being modified dermal bone, and the enamel an epidermal struc- 

 ture, arising from epidermal cells. 



The endoskeleton forms the bony and cartilaginous framework 

 of the body. It is divided into the axial skeleton, which includes 

 the skull and the vertebral column, together with the ribs and the 

 sternum, and the appendicular skeleton, which includes the frame- 

 work of the extremities. 



