. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MONOTREMATA. 385 The medullary fibres of the optic thai-ami (fig. 182, t) and bigeminal bodies (r, .s) form a thin stratum above a third ventricle of unusual capa- city, the relative sizeof which appeared somewhat larger than was natural from the decomposition of the medullary matter of the soft commis- sure. The principal commissure of the he- mispheres is the anterior one, which is sub- cylindrical, and measured two lines thick verti- cally, and one and a half horizontally. The pos- terior commissure is a narrow strip
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MONOTREMATA. 385 The medullary fibres of the optic thai-ami (fig. 182, t) and bigeminal bodies (r, .s) form a thin stratum above a third ventricle of unusual capa- city, the relative sizeof which appeared somewhat larger than was natural from the decomposition of the medullary matter of the soft commis- sure. The principal commissure of the he- mispheres is the anterior one, which is sub- cylindrical, and measured two lines thick verti- cally, and one and a half horizontally. The pos- terior commissure is a narrow strip of medullary matter, which thickens the upper part of the valvula Vieussenii. The ' her' or canal from the third to the fourth ventricle is proportion- ally wide. The arbor vita?, as displayed by a vertical section of the vermiform process, sends off four principal and some minor medullary branches. The spinal chord in the Ornithorhynchus is long and slender, but fills closely the spinal canal : it is thickest at its commencement and at the lower two-thirds of the cervical region; it is more slender in the dorsal region, espe- cially near the loins; it is slightly enlarged in the lumbar region, and gradually terminates in a point in the canal of the sacral vertebrae: the cauda equina is very feebly represented. In the Echidna the form and proportions of the spinal chord (Jig. 185) are strikingly dif- ferent: it is here nearly as short and thick, relatively, as in the hedge-hog, and terminates in a point, at d,before it has reached the middle of the dorsal region. Nevertheless, in this short tract the two usual enlargements, giving origins respectively to the nerves of the pec- toral and pelvic extremities, are clearly marked ; the slightly contracted intermediate portion being extremely short: the cauda equina is remark- able for its length. The nerves escape, as usual, from the intervertebral foramina, and have a longer course in the spinal canal, in proportion as they supply
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