146 THE VOYAGE OF H.M.S. CHALLENGEE. 



the following are given off, — the occipital, the internal maxillary, the inferior orbital, and 

 the superior orbital. 



(a) The Occipital Artery passes backwards from the internal carotid, and gaining the 

 lateral aspect of the neck immediately behind the occiput terminates by breaking up into 

 branches for the supply of the extensor muscles of the neck, as well as for that of the 

 digastric muscle. 



In Pygosceles this branch is given off from the anastomotic artery, which in that 

 species is derived from the internal and not from the external carotid artery, as in the 

 other species examined. 



(b) The Internal Maxillary Artery passes forwards from the internal carotid previous 

 to the passage of the latter through the carotid canal. It wdnds round the outer side of 

 the quadrate bone, lying under cover of the digastric muscle, and then passes horizontally 

 forwards, parallel with and to the inner side of the jugal arch, to terminate by supplying 

 the pterygoid muscle together with the soft parts which occupy the anterior j^ortion of 

 the floor of the orbit immediately behind the angle of the gape. 



(c) The Inferior Orbital Artery also arises from the internal carotid previous to the 

 passage of the latter into the cranium. It passes obliquely forwards and downwards, 

 lying internal to the quadrate bone, which thus separates it from the internal maxillary 

 artery. It is concealed by the digastric muscle, in front of which it rests upon the upper 

 or orbital surface of the pterygoid muscle, and terminates by supplying these two 

 structures. Previous to its termination, it lies upon the outer surface of the insertion of 

 the temporal muscle immediately below the jugal arch. 



(d) Tlie Superior Orbital Artery arises from the internal carotid, close to the origin 

 of the inferior orbital branch. It passes obliquely upwards and forwards, and enters a 

 bony canal, which surrounds the upper half of the fenestra ovaUs. Escaped from this 

 canal into the orbit, it breaks up into a number of small branches, which, along with 

 others supplied by neighbouring branches, constitute a close arterial rete mirabile situated 

 between the posterior wall of the orbit and the lachrymal gland. From this rete a branch 

 of considerable size, which may be named the temporal artery, passes vertically upwards 

 under cover of the digastric muscle, and supplies the latter as well as the temporal muscle. 

 After forming the rete mirabile, the superior orbital artery passes forwards, and divides 

 into two branches. Of these the first and larger runs downwards and forwards, lying in 

 contact with the roof of the orbit, and terminates at the base of the maxiUary bone by 

 passing into the nasal region, where it is distributed. The second and smaller branch 

 passes downwards to reach the entrance of the optic nerve into the eyeball, where it 

 breaks up into branches which in part contribute to the formation of the orbital plexus, 

 and in part supply the muscles of the eyeball. 



From the preceding description of the arteries of the head, it wiU be observed that 

 there are two distinct arterial plexuses in each orbit. One of these, the orbital plexus 



