324 THE MUSCULATURE 



organs of special sense, the brain is developed. In most animals the head also 

 is the chief organ for the prehension of food and for attack and defense. The 

 neck is a part of the trunk differentiated to give freedom to the movements of the 

 head. The forelimbs, relatively unimportant as the forefins in the fishes, become 

 important organs of locomotion in the land animals. In the fishes there is no 

 true neck, but the forefins are developed at the sides of the cervical part of the 

 trunk. In the higher vertebrates the forehmbs are also first differentiated at the 

 sides of the cervical region'(fig. 340) but, as embryonic development goes on, they 

 shift caudalward to the sides of the cranial (anterior) part of the thorax. The 

 cervical region is thus left free for movement but the musculature and nerves of 

 the upper extremit}^ remain intimately related to it. 



In man, "with the assumption of the erect posture, the head no longer has to 

 bear the brunt of the new surroundings as the body moves forward There is, 

 however, a distinct advantage in having those organs of special sense, which put 

 the individual into touch with the more distant parts of the environment, situated 

 high above the ground, and a motile neck is of great value in directing the 

 organs of special sense toward various parts of the environment. The develop- 

 ment of the superior extremities as organs for the prehension of food and as 

 organs of attack and defense reduces the value of the head for these purposes, but 

 still leaves it the important functions of the reception of food and air and the 

 preparation of food for gastric and intestinal digestion. The head, furthermore, 

 assumes a new and most important function as an organ for the expression of the 

 emotions and of speech. 



The expression of the emotions, such as anger, fear, affection and the like,* 

 is brought about largely through the action of flat, subcutaneous "facialis" 

 muscles which underlie most of the skin of the face and head and extend down 

 under that of the neck (figs. 341 and 344). They also line the mucous membrane 

 of the lips and cheeks. Most of them arise from the surface of the skull and are 

 inserted into the skin, which they pull in various directions causing it to become 

 smooth or wrinkled, according to the direction of the pull. The various muscles 

 are grouped about the buccal, nasal and aural orifices and about the orbit of the 

 eye. Some of the fibre-bundles are arranged so as to constrict the orifices, others 

 radiate out so as to dilate them. 



The chief groups of muscles of the head and neck, in addition to the facialis 

 group just mentioned, are the muscles of the orbit and middle ear, the muscles 

 used in mastication and swallowing (cranio-mandibular, supra- and infrahyoid 

 groups, muscles of the tongue, soft palate and pharynx), the muscles of the 

 larynx, and the ventral and dorsal groups of muscles which lie in the region of 

 neck, extend over the thorax and move the head, neck and shoulder girdle. A 

 brief summary of these groups will be given before proceeding to a more 

 detailed account. 



Facialis group. — The muscles are especially well developed about the mouth, a 

 sphincter muscle {orbicularis oris) serving to close, the radiating muscles to open the lips 

 {quadratus labii superioris and inferioris), to pull the corners of the mouth in various directions, 

 as, for instance, upward to express bitterness {caninus) or pleasure (zygomaticus), or lateral- 

 ward and downward to express grief or pain (risorius, triangularis, platysma) or to protrude the 

 lips as in pouting (mentalis and incisive muscles). The buccinator, which radiates out from the 

 corner of the mouth and lines the mucous membrane of the cheek, is used in mastication and 

 whistling. 



About the orbit and in the eyelids a circular musculature {orbicularis oculi) is broadly 

 developed. It is used.to close the eyes, and to contract the skin about the orbit. Associated 

 with the orbicularis are muscles which produce perpendicular furrows in the skin of the fore- 

 head above the nose {procerus, corrugator). The skin is drawn upward from the orbit and 

 horizontal furrows are caused in the skin of the forehead by muscles attached to the scalp 

 (epicranius). Two of these muscles, the occipilales, arise one on each side from the occipital 

 bone and are attached to an aponeurosis which lies beneath the scalp to which it is firmly 

 united. Two of the muscles, the frontales, extend one on each side from this aponeurosis to the 

 skin above the eyebrows. 



About the nasal orifices Iherc are weak constrictors {alar part of the nasalis, depressor alee 

 nasi) and dilators {dilator naris anterior and posterior, transverse part of the nasalis, angular 

 head of the quadratus laldi superioris). From the ear (auricle) three flat muscles radiate, 

 one backward {auricularis posterior), one upward {auricularis superior) and one forward 

 {auricularis anterior). These muscles are seldom functionally dcveloi)cd. They pull the 

 auricle in their respective directions. They may be looked ui)oti as (primitively) dilators 

 of the aural orifice. On the cartilage of the auricle are several rudimentary " intrinsic " muscles 

 which may be looked upon as remnants of a constrictor of this orifice. 



*See Darwin: The Expression of the Emotions in Man and Animals. 



