74 PRACTICAL ANATOMY. 



respiration, and to supply the serous membranes associated with the respiratory 

 movement. 



5 . How is the cervical plexus formed / 



It is formed by the union of the anterior primary divisions of the first, 

 second, third, and fourth cervical nerves. The communications of the upper 

 two nerves of this plexus are with nerves whose function has to do with 

 respiration, with expenditure of air, or with movement of the tongue the 

 sympathetic, the pneumogastric, the hypoglossal. The communications of the 

 lower two nerves are with the brachial plexus. 



6. Explain the distribution of tlie brandies of the cervical plexus. 



This is rational, and must be studied in a philosophical light to be remem- 

 bered and appreciated. Remember, it is one thing to commit to memory a 

 statistical table of the branches of distribution of this plexus, as is often done ; 

 and quite another thing to understand the rationale of a distribution. I desire 

 you to learn the latter first. 



1. This plexus has muscular branches which supply muscles that, without a 

 single exception, have two well-defined actions. 



2. This plexus has branches that supply the skin or serous membrane, as 

 the case may be, covering these muscles. This is in accordance with Hilton's 

 law, and applies to the articulations, and also to the skin covering the most 

 extensive origin or insertion of the muscle. 



3. This plexus has communicating branches, like any other plexus. One 

 set of communicating branches is to those muscles which act synergistically to 

 produce the muscular movements of respiration directly. Another set is to 

 communicate with nerves that supply parts dependent on respiration. 



The muscles supplied are as follows : 



1. The platysma myoides acts toward clavicle or toward mandible. 



2. The sterno-cleido-mastoid acts on the head or the thorax. 



3. The scaleni act on the ribs or on the neck. 



4. The levator anguli scapulae acts on scapula or neck. 



5. The recti capitis antici major and minor act on head or neck. 



6. The trapezius acts on head or shoulder girdle. 



7. The diaphragm acts on ribs and on cervical fascia. 



8. Sterno-hyoid acts toward sternum or toward hyoid bone. 



9. Omo-hyoid acts toward scapula or to the hyoid bone. 



10. Thyro-hyoid and sterno-thyroid toward sternum or hyoid. 



1 1. Genio-hyoid acts toward mandible or toward hyoid. 



12. Genio-hyo-glossus acts forward and backward. 



13. Stylo-glossus acts to or from the tongue. 



14. Hyo-glossus acts to or from the hyoid. 



The serous membranes and integument supplied in these areas : 



1. The pleurae, pericardium, peritoneum. 



2. Skin over the pectoralis major and acromion and clavicle. 



3. Skin over the depressors of the hyoid bone. 



4. Skin over the genial muscles. 



5. Skin over the upper ends of sterno-mastoid, trapezius, and scaleni. 



The phrenic supplies all the serous membranes. The descending branches of 

 the plexus supply the skin over the anterior part of the upper thorax and over 

 the shoulder. The lesser and second occipital supply the skin over the sterno- 

 mastoid and trapezius. (Fig. 43.) The superficial cervical supplies the whole 

 front of the neck and the region under the chin. 



The ascending and descending brandies, then, of the cutaneous, sensory, or 

 superficial nerves of the plexus are thus accounted for. The supraclavicular, 

 suprasternal, and supraacromial are the descending ; the superficial cervical is a 



