The pia mater, which accompanies the ven tricular artery into the floor of the ventricle, at the base of the hippocampus, spreads over the optic thalami, and sends upwards a smooth fold of membrane, one line and a half broad, between the bippocampus and the corpus striatum. The free margin of this fold is slightly thickened by the choroid artery.
It is necessary to remove the hippocampus and posterior part of the hemisphere, in order to bring into view the optic thalami and bige minal bodies.
The optic thalami ( fig. 182, 1) and nates appear as one convex body slightly contracted laterally, and divided from each other by a sig mold linear fissure: the testes are only half the breadth of the nates, and the median longitudinal line of division, which is very faint in the larger bodies, is not visible in the smaller and pos terior tubercle. The Echidna corresponds in this characteristic modification with the Omi thorhynchus.t The medullary fibres of the optic thalami (fig. 182, t) and bigeminal bodies (r, s) form a thin stratum above a third ventricle of unusual capa city, the relative size of 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' iter ' 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 vertebrx : the cauda equina is very feebly represented.
In the Echidna the form and proportions of the spinal chord (fig. 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 sl igh fly 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 parts more distant from the chord.
It is interesting to find the peculiar structure of so important a part as the spinal chord re peated in two species, which, with the excep tion of the dermal spines, and their common characters as Mammalia, differ in other respects as widely from one another, and occupy such distant places in their' class. Can the short ness of the solid chord, and the great length of the nerves within the spinal canal, have any physiological relation with the habit, common to both the placental and monotre matous hedgehogs, of rolling the body into a ball when torpid or asleep, or when the tegu mentary armour is employed in self-defence?* The olfactory nerves are large in the Orni thorhyuchus (Jig. 181, 1,1). The external root is remarkable for its length and relative size: it arises from the poste rior surface of the cere bral hemisphere imme diately behind the bige minal body ; bends round the crus cerebri to the inferior surface ; and is continued forward to join the internal root which rises from the base of the anterior lobes of the brain.
In the Echidna the olfactory nerves may be described as enormous. The external root (fig. 183, 1 a) arises from nearly the whole anterior part of the natiform pro tuberance, which extends its origin, as in the Or to the pos terior part of the hemi sphere. The internal root (183, 1 b) is also very large: the lateral ventri cle is prolonged forwards into the olfactory nerve, which would appear like a continuation of the en tire hemisphere, were it not that it is overlapped by the anterior convo lution.