292 ANATOMY OF THE RABBIT 



medial side of the lateral malleolus with the lateral and ventral 

 surfaces of the trochlea tali. 



The Bone Marrow 



Before the bones are discarded following the foregoing dissection, 

 an instructive view of the marrowy which is not observed in the 

 study of dried bones, may be obtained by breaking the femur and 

 examining its interior. The marrow is a mass of reticular con- 

 nective tissue with, in long bones such as the femur, a predominance 

 of fat cells. It contains numerous vessels and is one of the principal 

 sites of development of erythrocytes as w^ell as producing white 

 blood cells. The vessels are accompanied by sympathetic (efferent) 

 and afferent nerve fibres. 



X. THE HEAD AND NECK 



This dissection includes the various structures of the region, 

 with the exception of the cervical and occipital musculature and the 

 central nervous system, which are treated in the succeeding parts, 

 and the special musculature of the ear, which has been omitted. 



To begin the dissection, the median ventral incision of the skin 

 should be extended forward to the mandibular symphysis and the 

 skin should be separated from the underlying platysma along the 

 side of the head, and reflected until the surface is clear to a point 

 near the dorsal median line of the skull. The more posterior part 

 of the platysma has already been described on page 257. It is a 

 very thin sheet of muscle originating in the skin over the first two 

 ribs and extending over the neck and the ventral and lateral aspects 

 of the head to be inserted in the skin over the cheek. Closely 

 associated with this muscle and lying immediately beneath it, the 

 depressor conchae posterior (p. 257) and the depressor conchae 

 anterior (parotideoauricularis anticus), the latter originating on 

 the ventrolateral surface of the mandible just in front of the 

 masseter muscle, meet and are inserted together on the outer part 

 of the base of the external ear. 



The platysma proper is composed mainly of longitudinal fibres inserted in 

 the skin about the angle of the mouth and the chin. A more superficial la^^er of 

 roughly vertical and transverse fibres, which nearly always comes away with the 

 skin, is the sphincter superficialis of the head. A specialized portion of the 



