382 



MONOTREMATA. 



and with it also the tail and the pelvic extre- 

 mities. 



The psoas ma gnus and iliacus interims form 

 a single muscle, having the usual origins, and 

 inserted by a common tendon into the large 

 internal condyle. 



The glutens externus is larger than is usually 

 the case with quadrupeds; its tendon is inserted 

 into the plantar fascia and the bone which sup- 

 ports the spur. The gluteus medius, gluteus 

 internus, pectineus (45), biceps flexor crurls, 

 grucilis (34), sartorius (35), rectus femoris (36), 

 adductores femoris (46), semitendinosus (47), 

 semimembranosus, vastus externus, otter no 

 notable deviations from the usual structure. A 

 strip of fibres (49) descends from the gracilis to 

 the sphincter cloaca (H). A muscle, called by 

 Meckel ' flexor accessorius a cauda ad tibiam 

 tendens ' (51), arises from the transverse pro- 

 cesses of the anterior caudal vertebrae, and 

 converges to be inserted into the tibia. Another 

 peculiar adductor of the leg, which might be 

 termed ' intertibialis ' (52), is attached by its 

 extremities to both tibia; its fleshy belly passes 

 across the sphincter cloacae (H), and is connected 

 with a strip of the panniculus carnosus (i). 



The gastrocnemius (48) derives its largest 

 origin from the produced and expanded head of 

 the fibula, and its smaller belly from the internal 

 femoral condyle; its tendon is implanted in the 

 calcaneum. The analogy between the gastro- 

 cnemius and ulnaris interims is strikingly illus- 

 trated in the Ornithorhynchus. 



The soleus arises from the head of the fibula 

 and from a large proportion of the tibia ; it is 

 nowhere blended with the gastrocnemius, but 

 is inserted by a thick and short tendon into the 

 astragalus. 



The abductors of the outer digits of both 

 the hand and foot are well developed for the 

 purpose of expanding the web which connects 

 the toes. 



In the figure the following muscles of the 

 leg are shown, viz. 37, tibiatis anticus, 38, ex- 

 tensor hallucis longus, 39, peronaus longus, 

 40, peronteus brevm, 41, extensor digitorum 

 communis profundus, brevi analogus, 42, exten- 

 sor digitorum communis sublimis, 43, a portion 

 of the same muscle corresponding with the 

 indicator of the fore leg, 44, exttnsor digiti 

 quinti accessorius. 



NERVOUS SYSTEM. 



Brain, In the male Ornithorhynchus 

 Meckel found that the brain weighed two 

 drachms, German weight (about four drachms 

 avoirdupoise), and bore a proportion to the 

 weight of the body as 1 to 130. It was in- 

 closed by a pretty strong dura mater, of which 

 the fold corresponding with the bony falx ad- 

 hered but slightly to that process. The cere- 

 brum weighed one drachm and a half, German; 

 neatly the whole of its superficies was smooth ; 

 a few vascular impressions marked the side of 

 the anterior lobe : its shape was triangular, 

 depressed ; the contracted anterior lobes form- 

 ing the obtuse apex of the triangle : the pos- 

 terior lobes are wide and cover the corpora 

 bigemina. The surface of the cerebrum is 

 smooth and unconvoluted (fig. 181). 



F/g. 181. 



Side view and base of brain, Ornithorhynchus. 

 (Meckel.) 



In describing the structure of the cerebral 

 hemispheres Meckel observes, with reference 

 to the most characteristic part of this structure 

 in the Mammalia, " Corpus callosum adest 

 quidem, sed breve, quam haud quatuor lineas 

 longitudine aequet, memorabilius etiam videtur, 

 in dimidia duo lateralia, linea mediana haud 

 confluentia, esse disjunctum. Equidem sal- 

 tern in faciebus sese spectuntibus internis nul- 

 lum dilacerationis vestigium invenire potui." 

 L. c. p. 33. 



During my investigations of the structure of 

 the brain in the Marsupial animals,* I had 

 in memory the apparent exception to the bird- 

 like condition of the corpus callosum, which 

 the Ornithorhynchus, according to the above 

 description, presented, but which each suc- 

 cessive example of the brain of the Marsupial 

 quadruped served to establish more firmly as 

 the rule of structure in the higher order of the 

 Implacental sub-class. It was difficult to 

 believe that in the lower or Monotrematous 

 group, the cerebral organ, which indicates so 

 accurately the true affinities and natural posi- 

 tion of the Vertebrate animal, and which fol- 

 lows so faithfully the degradation of the general 

 organization of each species, should offer so 

 abrupt an ascent to the cerebral condition of 

 the placental Mammalia, as would be indi- 

 cated by a corpus callosum of four lines long 

 in a brain of which the hemispheres measure 

 only fourteen lines in length (German scale). 



The strong suspicion of an error in the cele- 

 biated anatomist's description justified a re- 

 serve in acknowledging this exception until 

 the opportunity of testing it by a dissection 

 of a brain of a Monotrematous quadruped 

 should have presented itself; and my doubts 

 as to the great development of the corpus 

 callosum of the Ornithorhynchus were further 

 justified by the indication of its nearer ap- 

 proach to the Oviparous type afforded by the 

 simple bipartite condition of the tubercules 



Philos. Trans. 1837, p. 87. 



