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 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- 

 thorhynchus (fig. 181,1,1). The external root 



dant aclmis, dans les figures relatives a 1'encephale 

 de 1'ornithorhynque 1'existence du corps calleux; 

 mais, en ttudiant avec soin 1'encephale de notre 

 echidne, nous nous avons reconnu que les descrip- 

 tions de M. R. Owen sont plus exactes que celles 

 de Meckel, et que les determinations de 1'anato- 

 miste Anglais doivent etre adoptees." Voyage de 

 la Favorite, p. 166. 



* " Get echidne passait la majeure partie de son 

 temps dans une espece d'engourdissement, blotti, 

 enroule a la manierc des herissons." Voyage de la 

 Favorite, p. 159. 



VOL. nr. 



Fig. 185. 



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- 

 minalbodv; 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 ) arises from 

 nearly the whole anterior 

 part of the natiform pro- 

 tuberance, which extends 

 its origin, as in the Or- 

 nithorhynchus, to the pos- 

 terior part of the hemi- 

 sphere. The internal root 

 (183, \ 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. 



The extent and com- 

 plications of the olfactory 

 cavity are proportionate 

 in the two Monotremes 

 to the size of their respec- 

 tive nerves. 



The optic nerve (fig. 

 181, 183, 2) is small in 

 both Monotremes, in ac- 

 cordance with the dimi- 

 nutive size of the eye : 

 the two nerves are joined 

 by a transversely oblong 

 chiasma. 



The eye is protected, 

 in the Ornithorhynchus, 

 by a cartilaginous plate 

 continued from the upper 

 part, of the orbit, which 

 Meckel compares with 

 the bony palpebral plates 

 Brain and spinal chord, in the Crocodile. Both 

 Echidna, Half natural Monotremes have a well 

 size. (Original.) developed membrananic- 

 titans : there are also an upper and a lower 

 eyelid, each of which has its proper apertor 

 muscle. 



In the Ornithorhynchus the sclerotic is carti- 

 laginous, the cornea flabby, the retina very 

 thick : there is no trace of pecten or marsu- 

 pium : the lens is very small, two lines in 

 vertical and transverse diameter, one line in 

 antero-posterior diameter; the anterior smfarp 



2 i: 



I 



