5o6 



HUMAN ANATOMY. 



>i?- 



It has been demonstrated also that the obHque movements of the eyeball are by 

 no means due to the action of the superior and inferior oblique muscles acting alone, 



but that in every such movement there is 

 a coordination of two of the recti muscles 

 with one of the obliques. Thus, in rotations 

 which direct the pupil upward and inward 

 the superior and internal recti cooperate with 

 the inferior oblique, and in the downward 

 and outward movements the inferior and 

 external recti cooperate with the superior 

 oblique. 



A purely outward or inward rotation 

 can be produced by the action of the external 

 or internal rectus, as the case may be. But 

 it is to be noted that the movements of the 

 eyeball are always bilateral, and that the in- 

 ward rotation of the one eye is generally as- 

 sociated with the outward rotation of the 

 other, the combined movements thus re- 

 quiring the cooperation of different muscles. 

 In all movements of the eyeballs there 

 is, accordingly, a coordination of various 

 orbital muscles, and when the combined 

 oblique movements are performed this co- 

 ordination becomes somewhat complicated. 

 The direction of both pupils upward and to 

 the right requires the coordination in the 

 right eye of the inferior oblique and the su- 

 perior and external recti and in the left eye of the inferior oblique and the superior 

 and internal recti. 



Diagram showing action of ocular muscles. S,Si, 

 Q,Q\, sagittal and transverse axes of eyeball; di- 

 rection of pull of muscles is indicated by lines; 

 dotted lines indicate axes around which superior 

 and inferior recti and oblique muscles rotate eye- 

 ball ; vertical axis (O) corresponds to axis of rota- 

 tion of internal and external recti. (Landois.) 



Variations.— But few variations have been observed in the orbital muscles. Absence 

 of the levator palpebrae superioris has been noted, and a slip from this muscle, termed the 

 tensor trochlecs, sometimes passes to the trochlea. 



II. THE HYPOGLOSSAL MUSCLES. 



Genio-glossus. 

 Hyo-glossus. 



3- 



4- 



Stylo-glossus. 

 Lingualis. 



It is well known that the hypoglossal nerve represents the anterior roots of 

 three spinal nerves which have secondarily been taken up into and consolidated with 

 the cranial region. Corresponding to these three nerves are three myotomes which 

 combine to give rise to muscles connected with the tongue. 



I. Genio-Glossus (Fig. 1339). 

 The genio-glossus is described with the tongue (page 1578). 



2. Hyo-Glossus (Fig. 1339). 

 The hyo-glossus is described with the tongue (page 1578). 



Variations. — The fibres which arise from the lesser cornu of the hyoid bone are frequently 

 separate from the rest of the muscle and have been described as the chondro-fflossiis, and the 

 fibres arisinji^ from the body of the hyoid are frequently separated by a distinct interval from 

 those arising from the greater cornu, the former constituting a muscle wiiich has been termed 

 the basio-glossus and the latter the cerato-<s:losstis. A bundle of fibres, forming what has been 

 termed the triticeo-glossus, sometimes arises from the cartilago triticea, situated in the lateral 

 hyo-thyroid ligament, and passes upward and forward to insert along with the posterior fibres 

 of the hyo-glossus. 



