Okajimaslia Anat. Jpn., 67 (1): 53 -58, May, 199()

The Cutaneous Branch of the Mylohyoid Nerve in the

Crab-eating Monkey (Macaca fascicularis)

By

Yutaka TAKAHASHI and Kunihiko KIMURA

Department of Anatomy, National Defense Medical College,

2, Namiki-3, Tokorosawa 359, Japan

-Received for Publication, June 20, 1989-

Key words: Mylohyoid nerve, Crab-eating monkey, Sinus hair

Summary: Cutaneous branches of the mylohyoid nerve were observed in both sides of 25 (83.3%) and in one side of 5

(16.7%) out of 30 crab-eating monkeys (Macaca fascicularis). The branch supply the skin and sinus hairs at the mental

part. These sinus hairs seem to be the intermandibular sinus hairs according to the manner of nerve supply.

Cutaneous branches are not listed up for the mylohyoid nerve (N. mylohyoideus) in Nomina Anatomica (5th ed., 1987) and Nomina Anatomica Veterinaria (3rd ed., 1987). According to anatomical textbooks of man, the mylohyoid nerve comprises muscular branches to the mylohyoid and to the anterior belly of the digastric muscle. "Gray's Anatomy" edited by Clemente (1985) described that, the mylohyoid nerve may communicate with the lingual nerve, and it has been described as sending filaments to the depressor anguli oris, the platysma, the submandibular gland, and the integument below the chin. Koyama (1934) observed the cutaneous branch of mylohyoid nerve (CM) in six sides (30.0%) of four specimens (40.007o) out of ten Japanese. CM is also reported in man as anomalies by Yoshizaki (1961), Kumaki (1980), and Shigemasa et al.,

(1982). Neurologic studies on the macaque monkey (Christensen; 1933, Schwartz and Huelke; 1963, Sadowski et al.; 1970) and the baboon (Gasser and Wise; 1971) did not refer to CM. Out of 41 macaque monkeys, Koizumi (1950) reported CM in 20 sides

(24.4%) of 14 specimens (34.1%). On the other hand, in many domestic animals (Greene; 1963, Miller; 1969, Zietzchmann; 1974, Shigemasa et al., 1985), CM is usual nerve supplying to the skin and sinus hairs at the

iiitcrmandibular space. Bowden, Maharan and Gooding

(MO) observed a communication between CM and the nerN c in the rabbit, dog and cat.

In the present study, the existence and the distribu-tion 01 ( 'M vere examined in 30 crab-eating monkc\-,

1/,/cocir hociciri(lris).

Materials and methods

Fifteen heads and necks of the crab-eating monkeys in each sex were used in the present study. Specimens were obtained from National Institute of Health and

preserved in 10% formalin at least 3 months. The skin accompanied with the platysma and other endermic facial muscles was peeled off from the mandible. The skin flap was washed in the running tap water for few days and stored in deionized water for overnight. The silver impregnation was applied according to the method described previously (Kimura and Takahashi, 1985). The distributing manner of CM was examined under the dissecting microscope.

Results

The cutaneous branch of mylohyoid nerve (CM) was observed in both sides of 25 (83.307o) and in one side of 5 (16.7°7o) out of 30 specimens.

Figure 1 shows the infero-lateral aspect of the mandible and the upper part of the neck of the crab-eating monkey. The skin was peeled off from the mandible together with endermic facial muscles, and turned to the right side. The mental muscle is separated from its insertion at the mandible. The left mental nerve is cut off at the emergence from the mental

foramen. The anterior bellies of the right and left digastric muscles form the inferior floor of the intermandibular space. The intermediate muscular slit between the right and left digastric muscles emends

h, Lac Pi1r 1-0110,0 ;1111,1 (P)09 _Nddress: Dr. Kunihiko Kimura, 1),1),ITtmcili t 1,11,iton1 Dcicn„1/4. )

, 1s,),

53

54 Y. Takahashi and K. Kimura

from the tendinous margin of digastric muscle in front of the hyoid bone to its insertion at the mental spine. CM (white arrows) generally emerges from the slit at its middle third, and intrudes into the platysma from its deep surface. In 4 specimens (13.3°7o), CM appears from different muscular slits as shown in Figures 1, 2.

Figure 2 shows the origin of CM in the same specimen used in Figure 1. The left anterior belly of the digastric muscle (D1) is separated from its inser-tion and turned downward. At the rostral/anterior border of the medial pterygoid muscle (P), the left mylohyoid nerve (M1) ramifies into the muscular branches to the mylohyoid muscle (open triangle) and the anterior belly of the digastric muscle (filled triangles), and CM (white arrow). M1 branches off from the inferior alveolar nerve just before the entrance to the mandibular foramen deep to the medial pterygoid muscle (P).

Figure 3 shows the skinned surface of the lower lip, mentum and upper part of the neck. The subcutaneous connective tissue is difficult to remove at the lower lip and the mentum, because there are studded by many follicles of the sinus hairs. The inserting fiber of the

platysma extends to the mentum where three follicles (white arrow heads) of sinus hair line up the lower margin of the mental sinus hair. The right and left CM

(Cr and Cl) appear on the outer surface of the platysma and ramify into fine nerve fibers. They run tortously and distribute to the mentum. CM did not distribute to the facial muscle. CM and the facial nerve interlace each other on their course. However, any anastomoses between two nerves were not observed in the present study in any specimens. The cutaneous branch of the cervical nerve (thick arrows) distributes to the skin lower and lateral to the area supplied by CM.

Figure 4 shows the arrangement of sinus hair at the lower lip and mentum. The sinus hair is easily distinguished from the other hairs by their length, cur-vature, pigmentation and brushlike structures. The entire lower lip and mentum contain about 50 to 100

pieces of sinus hairs. Two pieces of sinus hairs (white arrow heads) at the lower part of the mentum are demarcated from the other sinus hairs by their loca- tion and direction of the curvatures.

Figure 5 shows the inner structure of the skin at the lower lip, mentum and upper part of the neck. On its median section, the longitudinal sections of the follicles of the sinus hairs are visible in black spots. The

platysma (filled circle) extends beyond the lower margin of the mandible to the mentum. A fine branch (white arrow) from the right CM (Cr) dips into the sub-cutaneous connective tissue at the upper margin of the inserting muscular fiber of the platysma and the lower

margin ()I the cross suLA lOfl o[ the muscular fiber of the orhicu Iiri U (1111(..(,1

Figttre 6 hov,) 111(2 iliateged r)f the menturn which (J)if,„j),)r)(1' 1 iv 1 I I H L in rLH

5. Two fine nerve bundles (white arrows) from Cr in-trude into the follicle (F) of the sinus hair (white arrow head) at about the middle of body. Only 5 sides out of 30 dissections in the present study, CNIs were able to follow up to the follicles of the sinus hairs. The rest specimens, fine CM fiber lost to view within the sub-cutaneous connective tissue at the mentum and is no longer follow them to the sinus hair.

Discussion

The cutaneous branch was observed in the mylohyoid nerve of every specimen in this series, though Koizumi (1950) observed it in 34.107o of macaque monkeys. It may be due to the detailed observation by the aid of silver impregnation.

The sensory innervation of primate hairy skin at the upper lip was studied by Rice, Mance and Munger

(1986). They focused the morphology of nerve-ending in the hair follicles and did not refer to the origin of nerve fibers. According to them, the sinus hair is usually supplied by two or three nerve bundes. It is consisted with those observed in the present study.

According to Miller (1969) the chin and lower lip sinus hair and the intermandibular sinus hair are supplied by the mental nerve and CM, respectively in the dog. From the manner of nerve supply, a few sinus hairs supplied by CM observed in the crab-eating monkey should be classified into the intermandibular sinus hair for their nerve supply.

The movement of the cranial sinus hair is mainly

produced by the muscular layer of the facial muscles/extrinsic muscles and the capsular muscle of each hair/intrinsic muscle (Huber, 1930a, Wineski, 1985). The intermandibular sinus hair locates at the intermandibular space and the root of their follicles are studded in the sphincter colli profundus in kangaroo, cat and sea-lion (Huber, 1930a). The sphincter colli pro-fundus was not separable from the platysma in the crab-eating monkey. In the crab-eating monkey, inter-mandibular sinus hair is not located at the inter-mandibular space but observed at the lower part of mentum where is the upper margin of the platysma and lower margin of the orbicularis oris muscle, respectively. The intermandibular sinus hair of the crab-eating monkey should be moved by the platysma and orbicularis oris muscles.

Doerfl (1980) and Wineski (1985) depictcd the micro architecture of the upper lip sinus hair and ihe intrin-sic and extrinsic facial muscles at this rest in the rodents. According to their (1(sscription thc infraorbital nerve is a (ruc iii tributL:,,( ) hc ()I I i ii i I) ,1! H t-t-oniO,,,

k ii;Hid akt) i() thc I()HickI (),;111()kHr 0i( H, tril( ^,1 1.

Cutaneous branch 01 N.11ohyoid Tier-se in crab-eating \lonk y 55

and facial nerve observed in the present study are con-sisted with their conclusions.

Horn (1970) reported 70 to 80 sinus hairs on the mandibular skin in the rhesus monkey. The number of the sinus hair on this site in the crab-eating monkey observed in the present study is similar to those reported by Horn (1970). The position and muscular arrange-ment make few clue to distinguish the intermandibular sinus hair from the chin and lower lip sinus hair in the crab-eating monkey. The facial sinus hair of the macaque monkey was studied by Frederick (1905), Huber (1930a, 1930b) and Horn (1970). They paid little attention on the nerve supply, and failed to distinguish the intermandibular sinus hair from the chin and lower lip sinus hair.

It is uncertain how many sinus hairs should be classified into the intermandibular sinus hair in each specimen, because the connection of the follicle and CM was defined in only several sinus hairs in 5 specimens. The deficiency of CM in some specimens observed in the present study indicates that, the reduction of the cutaneous field of the mylohyoid nerve is apt to begin at the lower part of mentum and intermandibular region in the crab-eating monkey.

References

1) Bowden, R.E., Maharan, Z.Y. and Gooding, M.R.: Corn- munications between the facial and trigeminal nerves in cer-

tain mammals. Proc. Zool. Soc. London, 135: 587-611 (1960). 2) Christensen, K.: The cranial nerves. in The Anatomy of the

Rhesus Monkey. Hartman, G.C. and Strauss, W.L. Jr. (eds.), Williams and Wilkins Co., N.Y. (1933).

3) Clemente, C.G.: Gray's Anatomy of the Human Body. 30th Amer. Ed. Lea and Febiger, Philadelphia (1985).

4) Doelfl, J.: The innervation of the mystacial region of the white mouse. A topographical study. J. Anat., 142: 173-184 (1980).

5) Frederick, J.: Untersuchungen uber die Sinushaare der Affen, nebst Bemerkungen uber die Angengbrauen und den Schnurr-

hart des Menschen. Z. Morph. Anthrop., 8: 239-275 (1905). 6) Gasser, R.F. and Wise, D.M.: The trigeminal nerve in Baboon.

Anat. Rec., 172: 511-522 (1971). 7) Greene, E.C.: Anatomy of the Rat. Hafner Pub., N.Y. (1963). 8) Horn, R.N. van,: Vibrissae structure in the rhesus monkey.

Folia Primatol., 13: 241-285 (1970). 9) Huber, E.: Evolution of facial musculature and cutaneous field

of trigeminus. Part I. Quar. Res. Biol., 5: 133-18 (1930aj. 10) Huber, L.: Evolution ol facial musculature and cutaneous field

of trigeminus. Part II. Quar. Res. Biol., 5. .:89-437 (1930b).

11) Kimura, K. and Takahashi, Y.: Application of impregnation

for peripheral nerve fiber analysis in topographic anatomy. — A communicating branch of the lateral sural cutaneous Tiers e

to the sural nerve in the crab-eating monkey (,l.lacaca

fascicularis). Acta Anatomica Nipponica, 60: 8-10 (1985) (in Japanese with English summary).

12) Koizumi, K. Comparative study of the N. mandibularis. I. N. mandibularis in the macaque monkeys. Acta Anatomica

Niigata'ensia, Sectionis Anatomicae Universitatis Nii2ata'ensis, 60: 67-108 (1950) (in Japanese).

13) Koyama, K.: The distribution of the cutaneous nerve on the head of Japanese. Acta Anatomica Nipponica, 7: 909-1074

(1934) (in Japanese). 14) Kumaki, K.: Kaibo-gaku Shiryo Shu. 2nd Dept. of Anatomy,

Univ. Kanazawa, Kanazawa, p. 376 (1980) (in Japanese). 15) Miller, M.E.: Anatomy of the Dog. Saunder, Philadelphia

(1969). 16) Nomina Anatomica, 5th ed., Williams and Wilkins, Baltimore,

London (1987). 17) Nomina Anatomica Veterinaria, 3rd ed. Adolf Holzhausens,

Successors, Vienna (1987). 18) Rice, F.L., Mance, A. and Munger, B.L.: A comparative light

microscopic analysis of the sensory innervation of the mystacial

pad. I. Innervation of Vibrissal follicle-sinus complexes. J. Comp. Neurol., 252: 154-174 (1986).

19) Sadowski, T., Wojtowicz, Z., Kurylcio, L. and Pawlega, T.: The trigeminal nerve in macacus rhesus. Folia Morphol.,

(Warsz.), 29: 381-390 (1970). 20) Schwartz, D.J. and Huelke, D.F.: Morphology of the head

and neck of the macaque monkey — The muscles of mastica-

tion and the mandibular division of the trigeminal nerve. J. Den. Res., 42: 1222-1233 (1963).

21) Shigemasa, K., Kobayashi, M., Nomoto, A., Koyano, T., Kinosita, T., Kitagawa, T., Tezuka, M. and Takemoto, R.:

Double innervation of the anterior belly of the digastric muscle. Nihon Univ. Dent. J., 56: 636-641 (1982) (in Japanese).

22) Shigemasa, K., Muramatsu, H., Kobayashi, M., Nomoto, A. , Koyano, T., Nakayama, T., Kitagawa, T. and Tezuka, M.:

The digastric muscles of cat. Nihon Univ. Dent. J., 59: 759-768

(1985) (in Japanese). 23) Yosizaki, F.: A case of double innervation in the anterior belly

of the M. digastricus. Acta Medica Okayama, 73: 173-175

(1961) (in Japanese). 24) Wineski, L.: Facial morphologN and vibrissal men ensent in the

golden hamster. J. Morphol., 183: 199-217 (1955). 25) Zietzschmann, 0.: Die allgemeine Decke. in Ellenberger Baum

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56 Y. Takahashi and K. Kimura

Plate I

Explanation of Figures

Plate I

Fig. 1. Infero-lateral aspect of the mandible and the upper part of the neck. The skin is peeled off from mandible together with the

platysma (P1) and other endermic muscles. The right and left cutaneous branch of the mylohyoid nerve (white arrows) appear from the anterior belly of the digastric muscle and intrude into the platysma.

Fig. 2. The anterior belly of the left digastric muscle (D1) is turned down. The left mylohyoid nerve (M1) ramifies into the muscular

branches to the mylohyoid (open arrow head) and to the anterior belly of the digastric (filled arrow heads) muscles, and-CM

(white arrow), respectively.

Cutaneous branch of Mylohyoid nerve in crab-eating Monkey 57

Plate 11

Plate H

Fig. 3. The inferior aspect of the skinned lower lip and upper part of the neck. The cutaneous branch of the cervical nerve (thick arrow) and CM are impregnated in black. The right (Cr) and left (C1) CM appear on the inferior surface of the platysma and ramify

into fine fibers. They dip into the subcutaneous connective tissue succeeding to three pieces of the sinus hairs (white arrov, heads)

at the lower part of the mentum. There are 50 to 100 cross sections of the sinus hair follicles at the lower lip and mentum.

Fig. 4. The inferior aspect of the lower lip and upper part of the neck. Black, long and brushlike sinus hairs are distinguishable from

other hairs Two sinus hairs (white arrow heads) are demarcated from other sinus hairs.

58 Y. Takahashi and K. Kimura

Plate III

Plate III

Fig. 5. The inner structure of the skin at the lower lip, mentum and upper part of the neck . The sagittal section of the follicles of the sinus hairs are visible in the subcutaneous connective tissue in black. A branch (white arrow) from the right CM (Cr) runs on

the inferior surface of the platysma (filled circle). One sinus hair (white arrow head) at the marginal region of the platysma and the orbicularis oris (filled square) is correspond to the sinus hair indicated by the same symbol in Figure 4.

Fig. 6. Magnified view of the mentum represented in Figure 5. Two nerve fibers (white arrows) from Cr intrude into the hair follicle

(F) of the sinus hair (white arrow head).