556 HUMAN ANATOMY. 



posterior belly run the brachial plexus, which can often be felt and sometimes seen, 

 and, near the clavicle, the subclavian artery. 



Farther out the anterior border of the trapezius may be seen passing from the 

 occiput to its insertion at the outer end of the middle third of the clavicle. The 

 triangular interval between it and the posterior border of the sterno-mastoid is filled 

 — from below upward — by the scalenus medius, the levator anguli scapulae, and the 

 splenius, but none of them is recognizable through the deep fascia. 



In the mid-line behind, in addition to the bony points already given (pages 

 146-148), the line of origin of the trapezii can be seen as a slight elongated de- 

 pression. None of the deeper muscles can be seen or felt upon the surface. 



In the mid-line in front the hyoid bone and its cornua can be felt in the angle 

 between the under surface of the chin and the front of the neck. From the hyoid 

 bone on either side the anterior bellies of the digastric run up towards the symf)hysis 

 and with the subcutaneous fat give convexity to the submental region. Farther out 

 on this level the submaxillary salivary glands can be felt and often seen. 



The thyro-hyoid depression, the prominence of the thyroid cartilage {poviunt 

 Adami), the crico-thyroid space, the cricoid cartilage, and sometimes the upper 

 rings of the trachea may be felt from above downward. The relations of these parts 

 to important vascular and nervous structures will be considered later. 



The sterno-thyroid and sterno-hyoid muscles, while not visible, cover over and 

 obscure the outlines of the trachea, as does also the thyroid isthmus. The thyroid 

 lobes may be felt on each side of the larynx. The average distance from the 

 cricoid to the upper edge of the manubrium is about one and a half inches when 

 the head is erect. In full extension three-quarters of an inch additional can be 

 gained. 



The trachea recedes as it approaches the sternum, so that it is fully an inch and 

 a half behind the upper border of the latter. In this position between the two 

 sternal heads of the sterno-mastoid is the deep, V-shaped suprasternal notch (fossa 

 jugularis), the depth of which is noticeably affected by forced respiration, being 

 much increased in obstructive dyspnoea. 



All the surface appearances above described differ in different individuals, and 

 vary in the same person in accord with many conditions, as the amount of subcu- 

 taneous fat, the muscular vigor and development, the pulmonary capacity, the state 

 of repose or of violent exertion, etc. This should be remembered in looking for 

 landmarks in this region, which is in that respect one of the most variable of the 

 body, and most unlike that of the cranium, which perhaps typifies the other extreme 

 of unchangeability. 



DiAPHRAGMA (Fig. 549). 



The diaphragm is a dome-shaped muscular sheet which separates the thoracic 

 and abdominal cavities. Notwithstanding its position in the adult, it is a derivative 

 of the cervical myotomes. It represents the upper portion of a structure which is 

 termed in embryology the septum transversum (page i70i),a connective-tissue 

 partition which extends between the ventral and lateral walls of the body and the 

 heart, and serves to convey venous trunks to that organ. Like the heart, when 

 first formed it lies far forward in the uppermost part of the cervical region, but later 

 it descends with the heart until it reaches its final position. As it passes the third 

 and fourth cervical myotomes in its descent, it receives from them some muscle-tissue 

 which eventually forms all the muscle-tissue of the diaphragm, that structure, so far 

 as it is to be regarded as a muscle, being a derivative of the cervical myotomes 

 named. 



The diaphragm is a muscular sheet composed of fibres radiating from the lower 

 border of the thorax and from the upper lumbar vertebrae towards a central tendi- 

 nous area, termed the centrum tcndincum. According to their origin, the muscle- 

 fibres may be grouped into three portions. The stomal portion consists of, usually, 

 two bands which arise from the posterior surface of the xiphoid process of the 

 sternum and are separated from one another by a narrow interval filled with con- 

 nective tissue. Laterally they are separated by a similar interval, through which 

 the superior epigastric artery enters the sheath of the rectus abdominis, from the 



