176 Charles Seariiiff Mead 



The lateral and dorsal portions of the occipital region can be 

 divided into two parts, a ventral third extending upwards as far as 

 the dorsal border of the foramen magnum, and a dorsal two-thirds 

 extending up over the brain and forming tlie tectum posterius. The 

 ventral third forms a nearly flat rectangular plate (pars lateralis, 

 regio occipitalis) with its outer face directed backward and out- 

 ward. Posteriorly it forms the greater part of the lateral border 

 of the foramen magnum ; anteriorly it is separated from the auditory 

 capsules throughout nearly its entire length by a slit, which probably 

 represents merely a place where the capsule has not united with the 

 rest of the cranium. The dorsal two-thirds, the tectum posterius, 

 is shaped somewhat like a pair of saddle bags, curving nearly uniform- 

 ly from side to side. Ventrally it is bounded by a membrane, the mem- 

 brana atlanto-occipitalis dorsalis. The dorsal part, i. e., the band 

 connecting the saddle bags is narrower than the lateral portions and 

 is placed perpendicularly, that is, with its outer face looking pos- 

 teriorly. This is the usual position in the mammals. In man, owing 

 to the great development of the brain, the tectum has been shunted 

 backward and downward, so that its outer face looks ventralward. 

 Its free dorsal border is nearly straight, but in the mid-dorsal line 

 and above the tectum there is a free nodule of cartilage (Pis. I and 

 III, pr. asc. ted, post.). This may possibly be the homolog of 

 the processus ascendens of the tectum posterius of the reptiles. The 

 lateral portions of the tectum are perforated by several small 

 foramina, which serve for the passage of blood-vessels. 



Anteriorly the tectum posterius has three connections: two with 

 the postero-dorsal part of the auditory capsule, between which lies 

 the foramen petroso-occipitale , and one, by which it is continued 

 forward into the parietal plate. 



Regio Otica. 

 In comparison with the simple occipital region, the otic region 

 presents a very complex and highly differentiated portion of the 

 chondrocranium. It extends from the posterior borders of the ear 

 capsules, forward to the sella turcica, and includes the laminsB 

 parietales, the ear-capsules and the anterior three-fifths of the basal 



