COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 135 



parts, which have received separate names. Some of these will be seen later, 

 but the study of the parts of the platysma is almost impossible in any but freshly 

 killed specimens. The platysma muscle is inserted on the skin of the ears, eyelids, 

 lips, etc., and serves to move them. In man it constitutes the muscles of facial 

 expression. The platysma is a visceral muscle, derived from the muscles of the 

 hyoid arch by extension. 



In females on the under surface of the skin of the ventral side the mam- 

 mary or milk glands will be noted spread out as a thin irregular layer. 



The skin having been removed and discarded, clean away fascia and fat 

 from the surface of the muscles. There is generally a large mass of fat at the 

 base of the hind legs. It will now be seen that the exposed surface in part 

 consists of muscles, pinkish masses composed of parallel fibers, and in part of the 

 deep fascia which forms very strong white sheets. The posterior half of the back 

 is covered by such a sheet, known as the lumbodorsal fascia. In the median 

 ventral line is the linea alba. The angle between the base of the thigh and 

 the abdominal wall is known as the inguinal region. At the bottom of this will 

 be found in the rabbit a stout white shining cord, the inguinal ligament, which 

 stretches from the pubic symphysis to the crest of the ilium. It is absent in the 

 cat. The angle between the upper arm and chest is called the axilla, or axillary 

 fossa. 



In studying the muscles it is necessary to separate each muscle from its 

 neighbors. This is done by searching carefully for the white lines of connective 

 tissue which mark the boundaries of muscles and slitting along these lines with 

 the point of the scalpel. Observing the direction in which the fibers run will 

 also aid in separating muscles, since the fibers in one muscle generally run in 

 the same direction, which is usually different from that of the neighboring muscles. 

 After freeing the margins of a muscle the fingers should be worked under the muscle 

 until it is separated from its fellows. As each muscle is inclosed in a connective 

 tissue sheath, it will separate smoothly from its neighbors; the presence of rough 

 edges indicates that the muscle itself has been cut. Avoid the use of sharp 

 instruments in freeing the muscles. In case it is necessary to cut through a 

 muscle in order to reveal another muscle beneath it, always cut through the center 

 of the muscle. 



2. The parts of a muscle. We shall first study the parts of some convenient 

 muscle in order to become acquainted with the terminology applied to muscles. 

 For this purpose we shall select the external oblique. The external oblique is 

 the large muscle covering the sides of the abdomen. The superficial fascia should 

 oe cleaned away from its surface. On closely inspecting the cleaned surface of 

 the external oblique, the muscle will be observed to consist of numerous pinkish 

 stripes directed obliquely ventrad and caudad. 



The pinkish stripes are bundles of muscle fibers, called fasciculi. The white 

 material between the fasciculi which binds them together into a muscle and 



