_amb_, ambiens; _cap_, _ext_, _gastr_, external head of gastrocnemius; _cap_, _int_, _gastr_, internal head of gastrocnemius; _cd_, _fm_, caudali-femoralis; _cd_, _il_, _fem_, caudi-ilio-femoralis; _ex_, _il_, _tb_, extensor ilio-tibialis; _ext_, _l_ (_long_), _dig_, extensor longus digitorum; _fl_, _tb_, _ext_, flexor tibialis externus; _fl_, _tb_, _int_, flexor tibialis internus; _fm_, _tb_, femoro-tibialis; _il_, _cd_, ilio-caudalis; _il_, _cost_, ilio-costalis; _il_, _fib_, ilio-fibularis; _il_, _fm_ (_il_, _f_), ilio-femoralis; _il_, _s_, _cd_, ilio-sacro-caudalis; _is_, _cd_, ischio-caudalis; _is_, _f_, ischio-femoralis; _ob_, _ext_, obliquus externus; _pb_, _cd_, pubi-caudalis; _pb_, _is_, _tb_, pubi-ischio-tibialis; _pb_, _tb_, pubi-tibialis; _peron_, _ant_, peroneus anterior; _peron_, _post_, peroneus posterior; _p_, _is_, _f_, _ext_, pubi-ischio-femoralis externus; _p_, _is_, _f_, _int_, pubi-ischio-femoralis internus; _p_, _is_, _f_, _post_, pubi-ischio-femoralis posterior; _qudr_, _lb_, quadratus lumborum; _rect_, rectus abdominis; _tib_, _ant_, tibialis anticus; _tib_, _post_, tibialis posticus; _trans_, transversus abdominis; _tr_, _per_, transversus perinei; _m_, _post_, _il_, posterior border of ilium; _ob_, foramen in pubis for the obturator nerve; _o_, _il_, ilium; _o_, _is_, ischium; _o_, _pb_, pubis; _o_, _cl_, cloacal bone; _pr_, _l_, _pb_, lateral process of pubis; _pr_, _tr_, transverse process; _sp_, _ant_, _il_, anterior spine of ilium; _Sy_, _p_, symphysis pubis; _Sy_, _is_, symphysis of ischium; _tb_, _is_, tubercle of ischium. [Illustration] CHAPTER IV THE NERVOUS SYSTEM SPINAL CORD The spinal cord extends the whole length of the vertebral canal and ends near the end of the tail as a thin, round thread. It varies in thickness and shape in cross section, being nearly always elliptical, but at places approaching a circle. Large, spindle-formed thickenings of about equal diameter are present in the cervical and lumbar regions. A cauda equina is absent in the alligator, the nerves of the large tail leaving the cord like the intercostals. On its ventral surface the cord has a deep perpendicular fissure, the fissura ventralis, that extends almost to the center; it extends even along the reduced region in the tail. A vascular membrane extends into this fissure. A shallow but distinct furrow extends along the dorsal side of the cord, parallel to which, on either side, is a fine, linear furrow. The first two spinal nerves have no dorsal roots. BRAIN The cervical cord passes insensibly into the medulla, the dorsal furrow becoming wider and more shallow as it merges into the fourth ventricle.