248 HEAD-MUSCLES 



inner lamella apparently also contribute cells to the limb-bud. The cleavage of 

 the premuscule sheath into the rudiments of the adult muscles is completed by 

 the seventh week (Lewis). 1 



The premuscule sheath is the rudiment, not only of the muscular tissue proper, but of the 

 connective-tissue framework, fasciae, and tendons of the muscles. There is no distinction at first 

 between the cells which will become muscle-cells and those which will give rise to connective- 

 tissue elements. In quite early stages, according to Bardeen, 1 areas are to be made out which 

 will become muscles, and areas which represent inter muscular spaces. As differentiation 

 proceeds these spaces become more definite ; the premuscular masses become divided up into 

 individual muscle-rudiments, and these again into the fasciculi of the individual muscles by the 

 growth of connective-tissue septa, which are more abundant in embryonic than in fully developed 

 muscles. The main nerve-paths follow the spaces between the rudiments of the muscle-groups, 

 and the larger branches of the nerve supplying a muscle-group lie in the septa, between the 

 members of the group. Differentiation of a muscle-rudiment usually begins at the point of 

 entrance of the nerve into it. According, further, to Bardeen, ' Metameric segmentation in 

 the innervation of the limb-muscles is not due to ingrowth into the limb of myotomes, accom- 

 panied by nerves, but to the fact that a given region in the developing musculature is in the 

 more direct path of fibres extending into the limb from one or two specific spinal nerves. ' 



Muscles of the head. We have already seen that there are three primitive 

 segments in the occipital region, but that in front of this point there is no trace 

 of cleavage of the mesoderm. The tongue-muscles supplied by the hypoglossal 

 (occipito- spinal nerves), and formed in the floor of the primitive mouth, are probably 

 derived from these occipital myotomes, and come into the same category with the 

 trunk-musculature. The remaining head-muscles fall into two groups, the muscles 

 of the eye and the branchial musculature. 



The eye-muscles are developed from a cell-complex which appears between 

 the jugular vein and carotid artery, mesial to the trigeminal ganglion (Eeuter). 

 This mass is sickle-shaped, and has three limbs two anterior which embrace the 

 optic stalk on the inner side, and a posterior. Each limb has its own nerve connected 

 with it, the upper being associated with the trochlear, the inferior with the oculo- 

 motor, and the posterior with the abducens. The rudiment moving forward 

 surrounds the optic stalk, and the two anterior limbs uniting into a ring (Reuter) 

 the whole complex forms a sort of cup embracing the optic vesicle ; out of the walls 

 of this the straight and oblique muscles are developed. 



The eye-muscles in the mammal are developed in the unsegmented head -mesoderm, but in 

 the lowest vertebrates (Cyclostomata and Selachia) they are formed in connexion with the 

 so-called head-cavities, which are supposed to represent primitive segments in the prechordal 

 part of the head. The first of these gives rise to the muscles supplied by the oculo-motor 

 nerve, the second to the superior oblique, and the third to the external rectus. The three 

 limbs of the premuscle-cell-complex in mammals would represent, arguing from their relation 

 to the three nerves, the three head -cavities of lower forms. 



The muscles of the branchial group, which may be termed the visceral 

 musculature, are derived from the unsegmented mesoderm of the branchial 

 region. All are supplied by the lateral motor-roots of the cranial nerves. 



The masticatory muscles develop in the mandibular arch near the angle which 

 it forms with the maxillary process (Reuter), appearing as a cell-complex round 

 the branches of the mandibular division of the fifth nerve. The mass 

 resembles in shape an inverted Y, the limbs embracing the rudiment of the ramus 

 of the mandible. The stem forms the temporal, the outer limb the masseter, 

 and the inner the yterygoids (Reuter for pig). 



The muscles supplied by the facial are derivatives of the hyoid arch. The 

 platysma and all the mimetic muscles wander from this site. There are few develop- 



1 Lewis, Amer. Journ. of Anat. i. Regarding the development of the arm-muscles, see also 

 Griifenberg, Anat. Hefte xxx. ; of the leg-muscles, Bardeen, Amer. Jour, of Anat. vi. 1907. 



