8 GUIDES FOR VERTEBRATE DISSECTION 



by inscriptiones tendineae. Each rectus arises from the pubic bone 

 and divides, as it passes forward, into medial and lateral portions. 

 The medial part continues forward to the sternum, the lateral 

 becomes confluent with the portio abdominis of the pectoralis 

 muscle, to be studied later. 



Lateral to the rectus is the external oblique muscle. What 

 is the direction of its fibres? On lifting the external the internal 

 oblique is seen. What is the direction of its fibres? (Beneath 

 this is a third layer, the transversus. These layers are not to be 

 separated until later.) 



The pectoralis muscle, already mentioned, extends from the 

 midventral line to the shoulder. It presents three divisions. 

 Beginning behind, these are (1) the portio abdominis, continuous 

 with the rectus abdominis; (2) portio sternalis, arising from the 

 sternum and xiphisternum, and (3) portio epicoracoidea, arising 

 from the sternum proper and the medial end of the coracoid. 



In front of the pectoralis is the coraco-radialis muscle, arising 

 from the omo- and episternum and from the coracoid. Its 

 fibres run outward and backward and are continued as a tendon 

 passing through the deltoid (infra) to reach the radial side of the 

 radio-ulna. 



The deltoideus muscle arises by two heads, one (clavicularis) 

 from the clavicle and omosternum, the other (scapularis) more 

 external and arising from the clavicle and especially from the 

 scapula. Its fibres are largely inserted on the crista deltoidea of 

 the humerus. 



Sketch the muscles so far made out. 



In front of these muscles in the floor of the lower jaw is a 

 transverse submaxillaris muscle. Can you distinguish in it an 

 anterior and a posterior (subhyoideus) portion? Are the fibres 

 continuous across the median line? 



Add these muscles to your sketch. Then cut the submaxil- 

 laris in the median line and reflect the halves. 



In front, at the symphysis of the jaw, is a small transverse 

 submentalis muscle. Behind this the floor of the mouth is formed 

 by a median hyoglossal and a pair of geniohyoid muscles. The 

 latter are partly double, having two heads which are attached 

 to the lower jaw, while behind two similar heads are attached to 

 the hyoid. The hyoglossal is also divided behind, while in front 

 it extends into the tongue. Open the mouth and follow its course. 



