118 COMPARATIVE ANATOMY. 



with as far as the Primates, always arising from the anterior border 

 of the pubis, right and left of the middle line. 



The external and internal oblique muscles are to be met with 

 in the thoracic region in Mammals, as in Sauropsida, in the form 

 of external and internal intercostals. 



What has been said above as to the differentiation of the 

 dorsal portion of the trunk-muscles in Reptiles, applies also in 

 its essentials for Mammals. 



MUSCLES OF THE VISCERAL SKELETON AND HEAD. 



The common origin of the muscles of the visceral skeleton and 

 the ventral trunk musculature may be much more easily proved than 

 that of the cranial muscles with the latter system. The muscles 

 of the head may be divided into two sections, viz. facial muscles 

 and muscles of the jaws. 



Fishes. Leaving out of consideration the Cyclostomes, which 

 show a remarkable transformation of the cranio-visceral muscula- 

 ture in correspondence with their peculiar cranial skeleton (sucto- 

 rial apparatus) and branchial basket, these muscles in Elasmo- 

 branchs may all be regarded from the same standpoint. They 

 may be divided into the following four groups or systems : 



1. Superficial circular muscles. 



2. Upper adductors of the arches. 



3. Middle flexors of the arches. 



4. Ventral longitudinal muscles. 



The latter group occupies a more independent position than 

 the other three, which are more closely connected together. 



The superficial circular muscle, receiving its nerve-supply from 

 the vagus, glossopharyngeal, facial, and trigeminal (third division) 

 acts essentially as a constrictor; it narrows the entire oral and 

 pharyngeal cavities, closes the gill-clefts, and elevates the whole 

 visceral skeleton, together with the floor of the mouth and 

 pharynx. 



The main mass of the upper and middle flexors is supplied by 

 the vagus and glossopharyngeal, and their muscles act essentially 

 as adductors of the branchial arches, bringing them nearer 

 to one another. 



The ventral longitudinal muscles, supplied by the first 

 and second spinal nerves, are to be looked upon as a direct con- 

 tinuation of the ventral portion of the trunk-muscles, that is, 

 of the rectus- system, which is to a certain extent undifferentiated 

 in Fishes. Like the rectus, these muscles possess tendinous inter- 

 sections, and they extend from the anterbr border of the pectoral 

 arch forwards to .the lower jaw, or only to the hyoid arch (coraco- 

 inandibular and coraco-hyoid muscles). 



