426 ZOOLOGY FOR MEDICAL STUDENTS chap. 



Arch III persists on both sides and supplies blood to the portion of 

 aortic root lying in front of it (isolated except in most Lizards by the 

 disappearance of the portion of aortic root lying between arches III and 

 IV) and this in turn is continued forwards to supply the brain as the 

 dorsal or internal carotid artery (Fig. 183, d.c). As development goes 

 on the arch as a rule becomes straightened out and in the adult there is 

 nothing to show that the proximal portion of this artery was originally 

 an aortic arch. 



Arches II and I disappear entirely. 



The anterior paired portions of the ventral aorta are continued for- 

 wards into the head as the ventral or external carotid arteries (v.c). 

 The portion immediately behind arch III becomes the common carotid 

 artery {r.c and l.c) while the portion on the right side behind Arch IV 

 becomes the innominate artery (i). 



The skeleton in the reptiles is well ossified in the adult. Its most 

 striking peculiarities occur in the Chelonia where the trunk is enclosed 

 in a rigid box composed of plates of dermal bone and attached to the 

 deeper parts of the skeleton. The dome-like dorsal portion is the cara- 

 pace, the flatter ventral portion the plastron. This bony box is overlaid 

 by horny epidermal plates (" tortoise-shell ") which do not, however, 

 agree in form or number with the underlying plates of bone. These are 

 not shed — as is the usual fate of the horny layer of the epidermis — but 

 grow during the life of the animal both in area and in thickness. The 

 ribs are firmly fixed to the inner surface of the carapace. 



The brain in the reptiles is still small in size and simple in structure 

 although it shows an advance on the condition in Amphibia. In the 

 hemisphere region the pallium is thinner than in amphibians but shows 

 greater complexity in its minute structure, distinctly foreshadowing the 

 cortex of the mammalian brain. The most interesting feature occurring 

 in the reptilian brain is seen in Sphenodon and in various lizards, in which 

 the pineal body forms an eye, provided with a retina and a more or less 

 distinctly developed lens. This pineal eye lies immediately beneath a 

 transparent patch of skin, within a small hole in the roof of the skull 

 (parietal foramen). As there are indications of eye-structure in the 

 pineal organs of other vertebrates (Lampreys), and as various extinct 

 amphibians possessed a very large parietal foramen presumably for the 

 accommodation of a well-developed pineal eye, some zoologists believe 

 that the pineal organ of vertebrates in general is to be regarded as 

 representing the degenerate remains of an ancestral eye. Others, in- 

 cluding the writer of this book, taking into account that many cases are 

 known in the animal kingdom of nervous structures exposed to the 



