Book - Handbook of Pathological Anatomy 2.5
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Section II. Of the Muscles of the Head
Â§ 1095. The muscles of the head comprise those of the skull and those of the face.
The muscles of the skull are the occipito-frontalis, the auricular muscles, and one muscle of the lower jaw.
The muscles of the face are those of the eyes, the nose, the lips, the other muscles of the lower jaw, and the hyoid muscles.
Of these muscles we shall here examine only the occipito-frontalis, and those of the lower jaw, both because the parts which must necessarily be known to understand the descriptions of the others are not yet mentioned, and also because it is more convenient to examine them in connection with the other constituent parts of the organs which they assist to form.
Â§ 1096. The occipi to- frontalis or epicranial muscle (JVt. epicranius , s. cranii cutaneus , s. occipito frontalis ) is a flat, digastric muscle, situated directly under the skin, to which it is intimately attached, and covers the anterior, upper, and posterior parts of the skull, and also the central and upper part of the face.
Its posterior belly, which is also described as a separate muscle, termed the occipitalis , has an oblong square, or triangular form. It arises by tendinous fibres from the root of the mastoid process, and from the upper occipital ridge of the basilar bone, where it unites with the sterno-cleido-mastoideus and trapezius muscles, soon becomes fleshy, ascends on the squamous portion of the occipital bone, and terminates by a concave edge which unites with the median tendon.
This tendon is called the skull-cap ( galea capitis ), and is formed of very distinct longitudinal fibres. It extends all along the skull to the frontal bone, where it is attached to the anterior belly or the frontalis muscle.
The anterior belly or the frontalis muscle is much more extensive than the posterior. It begins by an upper convex edge, then descends along the squamous portion of the temporal bone, goes straight to its inner part, which is the thickest, and obliquely forward to the outer, which is thinner, and terminates as follows : at its inner part, it is continuous by several slightly tendinous slips with the pyramidalis nasi and the levator labii superioris alÅ“que nasi muscles ; in the region of the inner angle of the eye, it is attached to the nasal process of the upper maxillary bone and to the lower portion of the frontal bone ; finally, at its outer part, it blends with the corrugator supercilii and the orbicularis palpebrarum muscles.
The occipito-frontalis muscle corresponds in situation and attachment to the interspinales muscles of the vertebrÃ¦. It resembles one of these muscles which is enlarged, rounded, and divided in its centre, from before backward, into two parts, united by an intermediate tendon.
Considered as a whole, this muscle moves the skin of the top of the head. Its two bellies wrinkle in a transverse direction the skin above them, and extends that near them when they contract from the side of their tendon. Thus the frontal muscle raises that of the upper part of the neck.
II. MUSCLES OF TIIE LOWER MAXILLARY BONE.
Â§ 1097. The muscles of the lower maxillary bone comprise those which raise it, those which move it to the side, and those which depress it.
I. LEVATORS OF THE LOWER JAW.
Â§ 1098. The lower jaw is moved by three levators, the action of which is to bring it towards the upper jaw, which is fixed. These are the temporalis, the masseter, and the pterygoideus internus muscles.
Â§ 1099. The temporalis muscle, Temporo-maxillaire, Ch., the largest and strongest of all the muscles of the lower jaw, is broad and triangular. It occupies all the lower region of the central part of the lateral face of the skull, for it fills the temporal fossa, and covers the plain semicircular surface.
It arises by very short tendinous fibres and by a convex edge from the semicircular line which bounds the lower part of the outer face of the frontal bone, from the large wing of the sphenoid bone, from the parietal bone, and the squamous portion of the temporal bone, and by fleshy fibres from the parts of these same bones situated below this line. Its posterior fibres go from above downward and from behind forward, the central are almost perpendicular, the upper go from above downward and from before backward ; all converge to unite in the temporal fossa.
As they leave the circumference the muscle becomes narrower and thicker, and terminates in a short but very strong tendon, which is attached to the coronoid process of the lower maxillary bone.
The entire muscle is covered externally by a tendinous expansion, formed of descending fibres which arise immediately over it, serves for the attachment of its fibres above, is separated below, on its outer face, by a greater or less quantity of fat, and is very loosely united to it in this place by cellular tissue and vessels, and is attached to the posterior edge of the malar bone and also to the upper edge of the zygomatic arch.
This muscle draws the lower jaw forward and upward.
Â§ 1100. The masseter muscle, Zygomato-maxillaire , Ch. (JVI. masse ter } s. mandibularis externus ), has an oblong square form. Its length exceeds its breadth, and it is formed of fibres which go upward. It covers the outer face of the ascending branch of the lower maxillary bone, and fills the space between the posterior part of the lower edge of this bone and the zygomatic arch.
It is very evidently formed of two layers entirely separate from each other, which differ also in the direction of their fibres, and which may be regarded as two distinct muscles.
The anterior external layer is the longest and strongest, and covers most of the lower. It is formed of fibres which are oblique from above downward and from before backward, and become a little narrower from below upward. It arises by short tendinous fibres from the lower edge of the malar bone, and is attached to the lower half of the ascending branch of the lower maxillary bone, as far as its inferior edge and its angle.
The inferior or posterior layer is much smaller and feebler than the preceding ; its form is also square, and it is composed of fibres which go backward. It becomes thicker from below upward, is loose posteriorly, and is covered anteriorly by the preceding layer. It arises by fleshy fibres from the lower edge of the posterior part of the zygomatic arch, and is attached by short tendinous fibres above the upper end of the insertion of the external layer in the centre of the outer face of the ascending branch of the lower maxillary bone.
The two layers unite and draw the lower jaw upward, the external brings it forward, and the internal backward.
C. PTERYGOIDEUS INTERNUS.
Â§ 1101. The pterygoideus intermis muscle, Grand ptereygo-mariltaire , Ch., an oblong quadrangular muscle, arises by its upper thick edge from all the pterygoid fossa of the pterygoid process of the sphenoid bone, goes obliquely downward and outward and is attached, opposite the preceding, but to a much less extent than it, to the lower part of the inner face of the ascending branch of the lower maxillary bone, as far as the angle.
It draws the jaw upward and inward; but if the muscles of both sides act, the jaw is moved directly upward.
II. PTERYGOIDEUS EXTERNUS.
Â§ 1102. The lower jaw is moved laterally by a single muscle, the pterygoideus externus muscle, Petit pterygo-maxillaire, Ch., situated between the lower maxillary bone and the pterygoid process. This is the smallest muscle of the lower jaw, and differs from the others in the transverse direction of its fibres. It arises by short tendinous fibres from all the outer face of the outer layer of the middle sphenoid or pterygoid process, then goes directly backward and outward, and is inserted by short tendinous fibres in the inner face of the neck, and of (he condyle of the lower maxillary bone.
It draws the lower jaw from the opposite side, that is inward and forward, when it acts alone : but forward only when it contracts with its synonymous muscle of the other side.
Â§ 1103. Although several muscles contribute more or less directly to depress the lower jaw,(l) there is however but one appropriated specially to this function ; this is the digastricus muscle, JVLastoidogenien , Ch., ( J\I . biventer maxillÅ“ inferior is), so named because formed of two oblong bellies united by a central tendon.
The posterior longer and stronger belly is more rounded than the other and arises from the mastoid fissure of the temporal bone, and is covered hi this part by the upper end of the sterno-cleido-mastoideus muscle. Leaving this point, it goes downward, forward, and inward, and gradually becomes a thinner oblong median tendon, which is attached by a small tendinous expansion on its anterior extremity to the lateral end of the central piece of the hyoid bone ; so that it is situated between the temporal and hyoid bones.
The anterior belly is shorter and flatter than the posterior, and arises behind the median tendon ; goes forward and inward, and is attached dnectly at the side of the synonymous belly of the opposite side to the centre of the inner lip of the lower edge of the jaw.
This muscle draws the lower jaw downward and backward. If the posterior acts alone, it raises the hyoid bone and draws it backward. If the anterior acts singly, this bone is also raised but earned forward. When the posterior belly contracts behind and the anterior before, it draws the skull and the face, except the lower jaw, backward, and thus by its action on the skull raises the upper jaw, separates it from the lower, and opens the mouth.
Â§ 1104. A very common anomaly of this muscle consists in the union of the anterior bellies of the two sides with each other and with the pterygoideus externus muscle, by the formation of a larger or smaller fleshy portion. (2) Sometimes too we find between it and the skin a special transverse fasciculus, which is extended between the branches of the lower maxillary bone. (3)
These anomalies are evidently imitations of what is seen in several mammalia, where the anteriÃ¶r bellies even blend with each other.
More rarely the anterior belly does not extend to the chin, but is attached to the centre of the horizontal branch of the lower maxillary bone, as is also the case in many mammalia, where the single muscle, with which it is provided is also inserted more posteriorly than in man
(1) A Monro, Remarks on the articulation, muscles, arid luxation of the lower jaw, in ihe Edinburgh medical Essays, v ol. i. p. 103-129. â€” j. C. Platncr, De musculo digastrico maxillÅ“ inferioris, Leipsic, 1737. â€” Winslow, Observ. par V anatomie comparÃ©e sur V usage des muscles digastriques de la mÃ¢choire infÃ©rieure dans l'homme , in the MÃ©m. de Paris , anno 1742, p. 236.
(2) G. Meckel, De duplic. monstr., p. 42.
(3) Fleischnrann, in the Erlanger Abhandl., vol. i.